Chapter 50. Aortic Dissection

Thoracic aortic dissection occurs when an intimal tear allows redirection of blood flow from the aorta (true lumen) through the intimal defect into the media of the aortic wall (false lumen). A dissection plane that separates the intima from the overlying adventitia forms within the media. The acute form of aortic dissection is often rapidly lethal, whereas those surviving the initial event go on to develop a chronic dissection with more protean manifestations. The purpose of this chapter is to review the etiology and pathogenesis of aortic dissection, examine current diagnostic algorithms, and provide detailed descriptions of contemporary surgical techniques for treatment. Additional information regarding follow-up and the subsequent management of these patients is presented to provide a comprehensive understanding of a clinical entity that has challenged physicians and surgeons for centuries.

Sennertus is credited with the first description of the dissection process, but the earliest detailed descriptions of the clinical entity appeared in the seventeenth and eighteenth centuries, during which time Maunoir named the process aortic "dissection." Laennec defined the propensity of the chronically dissected aorta to become aneurysmal. Aortic dissection was exclusively a postmortem diagnosis until the first part of the twentieth century, but in 1935 Gurin attempted surgical intervention with the first aortic fenestration procedure to treat malperfusion syndrome.1 In 1949, Abbott and Paulin advanced surgical treatment by theoretically preventing aortic rupture by wrapping the aorta with cellophane. Other attempts at surgical treatment over the years met with limited clinical success, although certain concepts regarding surgical management are still in use today.2 With the advent of cardiopulmonary bypass, DeBakey and Cooley forever altered the natural history of aortic dissection by successfully performing primary surgical repair using techniques not remarkably different from contemporary procedures.3 Investigators such as Wheat made substantial contributions by defining physiologically based medical management algorithms to complement surgical correction.4 There is still considerable controversy regarding surgical versus medical treatment of certain forms of acute thoracic aortic dissection.

The classification systems used for aortic dissection are based on the location and extent of dissection. The particular type is then subclassified based on the timing of dissection. Acute dissection has traditionally been used to describe presentation within the first 2 weeks, whereas the term chronic is reserved for those patients presenting at more than 2 months after the initial event. The more recently added subacute designation is sometimes used to describe the period between 2 weeks and 2 months.

Two classification systems are most frequently used in clinical practice: the DeBakey and the Stanford systems (Fig. 50-1). The DeBakey system differentiates patients based on the location and extent of aortic dissection.5 The advantage of this system is that four different groups of patients with different forms of aortic dissection emerge. This structure provides the greatest opportunity for subsequent comparative research. In contrast, the Stanford system proposed by Daily et al. is a functional classification system.6 All dissections that involve the ascending aorta are grouped together as type A, regardless of the position of the primary tear or the distal extent of the dissection. Proponents of the simpler Stanford system contend that the clinical behavior of patients with aortic dissection is essentially determined by involvement of the ascending aorta. Critics, however, suggest that individual patients in the type A classification may be quite different from one another depending on the distal extent. Drawing clinical conclusions from such a potentially heterogeneous patient population has inherent limitations. However, because of its simplicity, practicality, and widespread use, the Stanford system is used throughout this chapter.

Aortic dissection is the most frequently diagnosed lethal condition of the aorta. Dissection occurs nearly three times as frequently as rupture of abdominal aortic aneurysm in the United States.7 There is an estimated worldwide prevalence of 0.5 to 2.95 per 100,000 per year; the prevalence ranges from 0.2 to 0.8 per 100,000 per year in the United States, resulting in roughly 2000 new cases per year.8 These figures are only an estimate. In one autopsy series, the antemortem diagnosis was made in only 15% of patients, revealing that many immediately fatal events go undiagnosed.9 Clinically, type A dissections occur with an overall greater frequency (Table 50-1).

Several hypotheses exist regarding the etiology of the intimal disruption (primary tear) that permits aortic blood flow to create a cleavage plane within the media of the aortic wall. This was originally viewed as a consequence of a biochemical abnormality within the media on which normal mechanical forces in the aorta acted to create an intimal tear. The link between the abnormal media, termed cystic medial necrosis or degeneration, and the primary tear has not been scientifically established. In fact, medial degeneration is found in only a minority of patients with acute aortic dissection, and most are children.10 This theory has lost support over the years.

Alternatively, there are data supporting a relationship between aortic dissections and intramural hematoma. Advocates of this theory suggest that bleeding from vasa vasorum into the media creates a mass that results in localized areas of increased stress in the intima during diastole. These areas then permit intimal disruption. In fact, between 10 and 20% of patients thought to have acute aortic dissection are found to have intramural hematoma, suggesting that it may be a precursor to dissection.11 Penetrating atherosclerotic ulcers have been implicated as the source of intimal disruption in some cases; thus many centers treat penetrating ulcers of the ascending aorta similarly to true dissections.12 Although it may apply to certain patients, enthusiasm for the penetrating ulcer mechanism causing all dissections has waned. The pattern of atherosclerotic involvement of the thoracic aorta resulting in penetrating ulcer and the frequency of dissection throughout the aorta do not support this theory.

Although no single disorder is responsible for aortic dissection, several risk factors have been identified that can damage the aortic wall and lead to dissection (Table 50-2). These include direct mechanical forces on the aortic wall (ie, hypertension, hypervolemia, derangements of aortic flow) and forces that affect the composition of the aortic wall (ie, connective tissue disorders or direct chemical destruction). Hypertension is the mechanical force most often associated with dissection and is found in greater than 75% of cases.8 Although the role of increased strain on the aortic wall is intuitive, the mechanism by which hypertension actually leads to dissection is unclear. Similarly, hypervolemia, high cardiac output, and an abnormal hormonal milieu certainly contribute to the increased incidence of dissection in pregnancy, but the mechanism is unclear. Atherosclerosis is not a risk factor for aortic dissection except in preexisting aneurysms or in the case of atherosclerotic ulceration. Iatrogenic trauma to the aortic intima may result in dissection. Catheterization procedures, aortic root and femoral artery cannulation for cardiopulmonary bypass, aortic cross-clamping, surgical procedures performed on the aorta (aortic valve replacement and aorto-coronary bypass grafting), and placement of intra-aortic balloon pumps have all been reported to result in dissection. Aortic transection as a result of trauma rarely results in excessive dissection and deserves differentiation from the process of aortic dissection. This process is usually limited to the aortic isthmus and in addition to the risk of rupture may present as a circular prolapse of the intima and media producing aortic obstruction referred to as "pseudo-coarctation" (Fig. 50-2).

Once a cleavage plane exists in the media, the aortic wall floating within the lumen is termed the dissection flap and is composed of the aortic intima and partial-thickness media. The primary tear is usually greater than 50% of the circumference of the aorta. Full aortic circumference is rarely involved, but may carry a worse prognosis. The primary tear in type A dissection is usually located on the right anterior aspect of the ascending aorta and follows a somewhat predictable course, spiraling around the arch and into the descending thoracic and abdominal aorta on the left and posteriorly. The dissection may propagate in a retrograde fashion for a variable distance as well to involve the coronary ostia; this occurs in roughly 11% of all dissections.11 Myocardial ischemia or aortic rupture into the pericardium are the causes of death in as many as 80% of mortalities from acute dissection. Often the distal false lumen communicates with the true lumen through one or more fenestrations within the dissection flap. The false lumen may also end blindly in as many as 4 to 12% of patients, in which case blood in the false lumen frequently thromboses. The false lumen may also penetrate the adventitia, causing rupture and death. Regardless of whether the true and false lumens communicate, perfusion of aortic side branches may be compromised by the dissection, resulting in end-organ ischemia (Fig. 50-3). If these acute complications are avoided, the weakened outer aortic wall, composed of partial media and the adventitia, may dilate over time, resulting in aneurysm formation. This evolving dilatation is the reason for operation in the majority of chronic dissections regardless of type.

The remaining adventitia provides most of the tensile strength of the aortic wall with minimal contribution from the media. The media is composed of concentrically arranged smooth muscle interposed with connective tissue proteins such as collagen, elastin, and fibrillin within the ground substance. Abnormal constituents of the media, as in certain connective tissue disorders such as Marfan's disease and Ehlers-Danlos syndrome, are associated with aortic dissection. Marfan's syndrome is an autosomal dominant inherited disorder in which a point mutation in the fibrillin-1 gene (FBN1) located on the long arm of chromosome 15 results in an abnormal media. The incidence of Marfan's syndrome is approximately 1 per 5000 live births.13 There are, however, many incomplete forms of the disease, and as many as 25% may be sporadic in which no known fibrillin abnormalities are observed. Type IV Ehlers-Danlos syndrome is a connective tissue disorder of the proα1(III) chain of type III collagen with an incidence of 1 in 5000.14 The structurally abnormal media is susceptible to dissection. Of note, there are also familial aggregations of dissection without discernable biochemical or genetic abnormalities.7

Signs and Symptoms

As many as 40% of patients suffering from acute aortic dissection die immediately. Those surviving the initial event may be stabilized with medical management, and it is these patients in whom subsequent therapeutic intervention on aortic dissection has altered the natural history of the disease. The clinical outcome is eventually determined by dissection type and timing of presentation, patient-related factors, and the quality and experience of the individuals and institution providing care.

The initial evaluation of a stable patient with suspected aortic dissection includes a detailed history and physical examination focusing on those elements likely to rule in the diagnosis. Most importantly, the diagnosis of aortic dissection requires a high level of suspicion. Up to 30% of patients ultimately diagnosed with acute dissection are first thought to have another diagnosis. Aortic dissection should always be considered in the setting of severe, unrelenting chest pain, which is present in most patients. Patients usually have no previous episodes of similar pain, which often causes anxiety. Pain is usually located in the midsternum for ascending aortic dissection, while in the interscapular region for descending thoracic aortic dissection (see Table 50-1). The location of maximum pain tends to change as the dissection extends in an antegrade or retrograde direction. Such "migratory pain" should arouse clinical suspicion. The character of the pain is often described as "ripping" or "tearing." The pain is constant with greatest intensity at the onset. Although painless dissection has been described, it usually occurs in the setting of an existing aneurysm in which the pain of a new dissection may not be differentiated from chronic aneurysm pain. Patients may also present with signs or symptoms related to malperfusion of the brain, limbs, or visceral organs. These findings confuse the true diagnosis, as the obvious signs of ischemia distract the historian from a less apparent initial episode of pain.

Elements of the past medical history such as primary hypertension, presence of aneurysmal disease of the aorta, or familial connective tissue disorders are useful as risk factors to help establish the diagnosis. Illicit drug use is an increasingly important predisposition to ascertain during the initial evaluation. The differential diagnosis of chest pain as a result of aortic dissection includes diagnoses such as myocardial ischemia, aortic aneurysm, acute aortic regurgitation, pericarditis, musculoskeletal pain, and pulmonary embolus. It is essential to consider aortic dissection in each case, as specific therapy (eg, thrombolytic therapy for acute myocardial infarction) may impact the survivability of acute dissection.

Patients suffering acute dissection appear ill. Tachycardia is usually accompanied by hypertension in the setting of baseline essential hypertension and increased catecholamine levels from pain and anxiety. Hypotension and tachycardia may result from aortic rupture, pericardial tamponade, acute aortic valve regurgitation, or even acute myocardial ischemia with involvement of the coronary ostia. An abnormal peripheral vascular examination is present in a minority of patients with acute aortic dissection, but when present an abnormal pulse exam may indicate the type of dissection. Absence of pulses in the upper extremity suggests ascending aortic involvement, whereas pulse deficits in the lower extremities speak to involvement of the distal aorta. These findings are subject to change as the dissection progresses or reentry into the true lumen occurs. Auscultation of the heart may reveal a diastolic murmur consistent with acute aortic regurgitation or an S3, indicating left heart volume overload. Physical exam findings such as jugular venous distention and a pulsus paradoxus are signs of pericardial tamponade that should be identified in any unstable patient to initiate the correct diagnostic and treatment algorithms. Unilateral loss of breath sounds, usually the left, may indicate hemothorax as a result of aortic leak or rupture with hemothorax. Alternatively, a pleural effusion may exist secondary to pleural inflammation related to the dissection. This finding requires additional evaluation before treatment.

A complete central and peripheral neurologic exam is critical in that abnormalities are present in up to 40% of acute type A dissections. Involvement of the brachiocephalic vessels with loss of brain perfusion may result in transient syncope or stroke. Syncope may also result from rupture into the pericardium and is an ominous sign. Stroke rarely improves with restoration of blood flow and may even cause hemorrhage and brain death, yet surgery is indicated in such patients. Fortunately, stroke is a presenting feature in fewer than 5% of patients with acute type A dissection. Loss of perfusion to intercostals or lumbar arteries may result in spinal cord ischemia and paraplegia. Peripheral nerve ischemia as a result of malperfusion may yield findings similar to spinal cord malperfusion and should be discerned as these patients often improve with restoration of blood flow. Acute aortic dissection may also cause superior vena cava syndrome, vocal cord paralysis, hematemesis, Horner's syndrome, hemoptysis, and airway compression as a result of local compression and mass effect.

Malperfusion of aortic branch vessels may occur from the coronary ostia to the aortic bifurcation and may dominate the presentation of certain patients. Although autopsy series yield a greater percentage of patients with evidence of malperfusion, clinical series reveal that dissection is not infrequently complicated by malperfusion of at least one organ system (Table 50-3).15 Compression of the true lumen by the false lumen is the mechanism by which aortic branch vessel occlusion occurs in the majority of cases. Branch vessels may also be completely sheared off the true lumen and perfused to various degrees by the false lumen.

Chronic aortic dissection is usually asymptomatic. It may be incidentally discovered following an asymptomatic acute dissection, most often in patients with a preexisting aortic aneurysm. Some patients eventually require surgical treatment for chronic dissection and most do so as a result of aneurysmal dilatation of a chronically dissected aortic segment. Presenting complaints often include intermittent, dull chest pain, or even severe skeletal pain from erosion into the bony thorax with large or rapidly expanding aneurysms. Aortic insufficiency may develop with chronic type A dissection and present with typical features of congestive failure, including fatigue, dyspnea, and mild, dull chest pain. Infrequently, chronic dissection may result in paralysis/paraplegia from loss of vital intercostal arteries or even distal embolization of thrombus or atheroma from the false lumen. Malperfusion syndrome is an uncommon presentation for patients with chronic dissection given the likelihood that the true and false lumens communicate.

Diagnostic Studies

Routine diagnostic studies including blood tests, chest x-ray, and electrocardiogram (ECG) should be obtained, but are often not sufficient to establish the diagnosis of acute aortic dissection. ECG often reveals no ischemic changes. Obvious ischemic changes are present in up to 20% of acute type A dissections, whereas only nonspecific repolarization abnormalities are present in nearly one third of patients with coronary ostial involvement. The ECG may also reveal left ventricular hypertrophy in those patients with long-standing hypertension. The chest x-ray is abnormal in 60% to 90% of patients with acute dissection (Fig. 50-4). Although most patients have at least one, if not several abnormal findings, a normal chest x-ray does not rule out the diagnosis. Blood should be drawn and sent for complete blood count, serum and electrolytes, creatine kinase with myocardial isoenzymes, troponin, and blood type and screen. These tests obtained at the time of initial observation are usually unremarkable. There is frequently a mild to moderate leukocytosis. Anemia may result from sequestration of blood or hemolysis. Liver function tests, serum creatinine, myoglobin, and lactic acid may all be abnormal in the setting of certain malperfusion syndromes.

Diagnostic Imaging

Diagnostic imaging is essential to clarify the anatomy of an acute aortic dissection, regardless of clinical certainty of diagnosis or the acuity of the patient. The diagnosis should be made rapidly with minimal distress for the patient. Two imaging modalities currently meet these criteria and are used to diagnose acute aortic dissection: computed tomography (CT) and echocardiography. Magnetic resonance imaging (MRI) and aortography, with or without intravascular ultrasound (IVUS), are used to diagnose acute aortic dissection but are second-line modalities for various reasons. The benefits, disadvantages, and diagnostic accuracy of each are useful when choosing the most appropriate study for a particular clinical situation (Table 50-4). Each test provides disruption, reentry points, whether there is flow or thrombus in the false lumen, status of the aortic valve, presence and nature of myocardial ischemia, and brachiocephalic and aortic branch vessel involvement. Specific data may be necessary for operative planning and subsequent management to define the imaging study most appropriate for a particular patient.

Helical CT scanning is widely available and is now the most frequently used test to diagnose acute aortic dissection. It requires intravenous contrast medium that may limit its use in certain clinical situations but generates images familiar to most practitioners and has a high sensitivity and specificity. This technique can be performed quickly, fulfilling the requirements for use in the early management of acute dissection. Additional structures such as the pleural and pericardial spaces are imaged. When performed and formatted as an arteriogram, aortic branch vessels may also be evaluated: Involvement of the brachiocephalic vessels is identified with nearly 96% accuracy. The diagnosis of dissection requires two or more channels separated by a dissection flap (Fig. 50-5). Transaxial two-dimensional images can be reconstructed to display three-dimensional images of the aorta that not only aid in diagnosis but also are useful for operative planning.

Transesophageal echocardiography (TEE) is currently the second most frequently used study for making the diagnosis of acute aortic dissection. It is widely available, requires no intravenous contrast or radiation, and generates dynamic images of the aorta from which the diagnosis can be made (Fig. 50-6). It requires operator expertise both to acquire the necessary images and to conduct the examination safely. Although the safest setting in which to perform TEE is the operating room under general anesthesia, it can be performed in a monitored setting using topical anesthesia and light sedation. Patient comfort is paramount in this situation as rupture has been reported during difficult studies and a complete examination of the entire aorta is necessary to exclude the diagnosis of acute dissection. Absolute contraindications to TEE include esophageal abnormalities such as varices, stricture, or tumor. A full stomach or recent meal are relative contraindications, but recognition of these conditions permits safe examination with few complications in the vast majority of patients. Criteria for making the diagnosis of acute aortic dissection include visualization of an echogenic surface separating two distinct lumens, repeatedly, in more than one view, and that can be differentiated from normal surrounding cardiac structures. The true lumen is identified by expansion during systole and collapse during diastole. Communication of the false lumen is found by identifying distal tears in the flap and flow in the false lumen with the addition of color Doppler. Similarly, the absence of flow indicates false lumen thrombosis. TEE additionally may provide high-quality images of the aortic valve and pericardial space. The coronary ostia are directly evaluated and regional left ventricular function may be assessed to identify myocardial ischemia indirectly. Color flow Doppler reliably quantifies aortic regurgitation and may be used to assess for additional valvular abnormalities. The pericardium and pleural space are also visualized and therefore effusions may be identified.

Transthoracic echocardiography (TTE) provides images of the ascending aorta and sections of the aortic arch that may yield the diagnosis but with much less sensitivity than transesophageal imaging. As such, transthoracic imaging may prove useful but is generally insufficient to reliably establish the diagnosis. Transthoracic evaluation is additionally limited by patient-related factors including body habitus, emphysema, and mechanical ventilation. A negative transthoracic study should be complemented by a transesophageal study, which provides greater detail of the entire aorta.

Aortography was the first study used to diagnose acute dissection in 1939 and until recently was considered the gold standard for diagnosis. It is an invasive test requiring nephrotoxic contrast media in which the aorta is visualized in multiple two-dimensional projections. The diagnosis of dissection depends on visualization of the intimal flap, two distinct lumens, or compression of the true lumen by flow through an adjacent false lumen (Fig. 50-7). Indirect signs of dissection include the presence of branch vessel abnormalities and abnormal intimal contour on injection of the false lumen. The status of the aortic valve may be evaluated and coronary angiography in the setting of type A dissections is possible only with this diagnostic test. However, coronary angiography is not recommended given that the coronary ostia are involved in 10 to 20% of acute type A dissections and are easily evaluated at the time of surgery. Coronary atherosclerosis is present in 25% of all patients with acute aortic dissection, but even in those patients repair of the dissection should take precedence. Aortography is sometimes useful in acute type B dissections with evidence of mesenteric ischemia or oliguria and in type A dissections with signs of malperfusion because catheter-based intervention may be possible. Aortography can have a high false-negative rate secondary to thrombosis of one lumen or when contrast equally opacifies each lumen, impairing distinction of a separate true and false lumen.9 The diagnosis of intramural hematoma may also be difficult given the absence of intimal disruption, whereas penetrating atherosclerotic ulcer is usually easily visualized. Visualization of the dissection variants is best accomplished with either CT scanning or MRI (Figs. 50-8 and 50-9). One major limitation to the use of aortography in the acute setting is the need for skilled personnel. The time required to assemble this team varies with each institution, rendering aortography less useful when compared with other immediately available diagnostic tests. Aortography also requires arterial access, which can be painful and precipitate rupture or dissection extension.

IVUS is a catheter-based imaging tool that provides dynamic imaging of the aortic wall and an intimal flap in patients with aortic dissection. It is particularly useful in delineating the proximal and distal extent of dissection and for identifying the true and false lumens in questionable cases during aortography. High-resolution images of the normal three-layered aortic wall are differentiated to identify the abnormally thin wall adjacent to the false lumen. Because the aortic wall itself is imaged, intramural hematoma and penetrating atherosclerotic ulcers may also be identified. Currently, as an isolated imaging study, it is time consuming and requires skilled personnel, as with aortography, and generally is not useful as an initial study in the acute setting. It may be most useful in combination with aortography when the initial imaging studies are negative, yet there remains a high clinical suspicion of dissection.

MRI and the newer contrast-enhanced magnetic resonance angiography (MRA) generate superior images reliably demonstrating aortic dissection (Fig. 50-10). In fact, some consider this the gold standard imaging study given the published diagnostic accuracy. Dissection is identified as an intraluminal membrane separating two or more channels (Fig. 50-11). MRI provides detailed images of the entire aorta, the pericardium, and pleural spaces similar to those obtained with CT. Cine imaging may also be used to evaluate left ventricular function, the status of the aortic valve, and flow in aortic branch vessels as well as flow in the false lumen. It is, however, not widely available and the presence of ferromagnetic metal contraindicates its use. Another disadvantage of MRI is that artifact is identified in up to 64% of studies, which underscores the need for expert radiologic interpretation of the images. These factors account for its infrequent use in the acute setting.

Diagnostic Strategy

The evaluation of suspected acute aortic dissection begins with a determination of the clinical likelihood that the diagnosis is correct and an evaluation of the hemodynamic stability of the patient. The unstable patient should undergo ECG to rule out acute coronary syndrome and be transferred immediately to the operating room. Medical management may be initiated as soon as the diagnosis is suspected. It is our practice to intubate and mechanically ventilate such patients while essential monitoring lines are placed. A TEE is then performed. If TEE fails to reveal acute aortic dissection, a hemodynamically unstable patient will then have a protected airway and invasive monitoring lines for subsequent evaluation of alternative diagnoses and continued resuscitation. If acute dissection is suspected despite a negative TEE, CT arteriogram (CTA) or aortography (potentially with IVUS) is the next study of choice.

Clinically and hemodynamically stable patients permit a more detailed history and physical examination with imaging decisions tailored to specific aspects of the presentation. At the University of Virginia, such patients are first evaluated with a CTA. A CT scanner is located in the emergency room and these data may be obtained in less than 15 minutes. If that study is negative yet the diagnosis still entertained, TEE is performed. In a recent review, an average of 1.8 imaging studies was used to correctly diagnose acute aortic dissection.9 Although TTE is a relatively insensitive study (especially in the descending thoracic aorta), patients with suspected acute type A dissection may first undergo that study. If positive, subsequent confirmation using TEE may be performed in the operating room to expedite surgical management; if negative, either CT scanning or TEE performed in the ICU is appropriate.

Diagnostic imaging of chronic aortic dissection is usually performed for surveillance, but may also be necessary in patients with symptoms attributable to dissection and for operative planning. Routine follow-up for acute dissection occurs on a scheduled basis and is usually done with either CT or MRI. We prefer CT scanning for patients with normal renal function and no contrast allergy because CT is usually the original imaging study obtained during the acute dissection. The improved accuracy that comes with comparing similar studies combined with the availability, cost, and patient satisfaction make CT favorable for this purpose. MRI is used mostly as a follow-up study for patients with renal insufficiency, but is the study of choice to provide precise anatomical detail for operative planning. TTE is useful to follow chronic type A dissection when there is aortic insufficiency. It can provide cross-sectional images of the ascending aorta, but generating images useful for comparison to previous studies is highly dependent on the skill of the operator. For that reason, we use echocardiography to follow patients with aortic insufficiency but also obtain a CT scan to assess ascending aortic diameter. Aortography is used primarily for operative planning. Patients older than 50 years and those with risk factors for coronary artery disease routinely undergo coronary arteriography before operation, and images of the aorta are obtained at that time. Aortography is especially useful to determine the origin of aortic branch vessels for operative planning when noninvasive imaging is inadequate (Fig. 50-12).

Natural History

Acute type A aortic dissection is impressively morbid. Fifty percent of patients suffering acute type A aortic dissection are dead within 48 hours if untreated.16 Data such as these suggests that acute type A dissection carries a "1% per hour" mortality for missed diagnoses. More contemporary data reveal a different prognosis such that medical management may be considered in certain high-risk groups. In one such study in octogenarians, type A dissection was managed medically in 28% of patients for various reasons with a 58% in-hospital mortality.8 Regardless, because of the extreme mortality with medical management, patients surviving acute type A aortic dissections must be aggressively diagnosed and treated with surgical intervention.

Initial Medical Management

The high morbidity of acute type A aortic dissection dictates that management should precede confirmation of diagnosis in highly suspicious cases. The initial patient encounter centers on making the diagnosis while identifying factors that require immediate treatment. The site of this initial evaluation and resuscitation is determined primarily by the hemodynamic stability of the patient. The unstable patient belongs in the operating room, whereas a more detailed diagnostic approach and subsequent management can be undertaken in stable patients. Therefore, the hypotensive patient, whether from hemorrhagic shock or tamponade, requires the aforementioned evaluation and resuscitation on transfer to the operating room. It is preferable to avoid procedures such as TEE or central line placement on an awake patient outside the operating room because hypertension resulting from patient discomfort may precipitate aortic rupture or propagation of dissection. However, as in any patient with potential aortic rupture, anesthetic induction remains dangerous in patients compensating for impaired preload, whether from pericardial fluid or hypovolemia. The operating room must be prepared for prompt decompressive pericardiotomy and/or initiation of cardiopulmonary bypass.

In the hemodynamically stable patient, blood pressure is measured in both arms and both legs. These dissections can propagate in either direction, but proximal propagation can quickly destabilize the situation. In general, the goals of hypertension management in acute aortic dissection, regardless of anatomy, are twofold.4 First, transmural aortic wall stress is diminished by decreasing the systolic blood pressure, which reduces the possibility of rupture. Second, shear stress on the aorta is decreased by minimizing the rate of rise of aortic pressure to decrease the likelihood of dissection propagation, so-called anti-impulse therapy. Specifically, the immediate goal for this situation remains to achieve a target systolic blood pressure between 90 and 110 mm Hg with a target heart rate of less than 60 beats per minute.

Pain control is important to reduce catecholamine release and decrease the risk of rupture. Therefore, therapy begins with pain control using narcotic analgesics. The drugs most commonly used for anti-impulse therapy are beta-blockers and peripheral vasodilators. In most cases, beta-blockers such as esmolol should be used first because adequate heart rate control may be difficult if the blood pressure is controlled by peripheral vasodilators first and because vasodilators may increase ventricular ejection and aortic shear stress if used unopposed. Short-acting beta-blockers should be titrated to a heart rate less than 60 beats per minute. After beta-blocker treatment has been initiated, vasodilators such as sodium nitroprusside are used for further blood pressure control. Sodium nitroprusside is a direct arterial vasodilator with a short onset and duration of action, which makes it ideal to rapidly achieve the target systolic blood pressure. Loading doses for esmolol and sodium nitroprusside should be avoided to prevent hypotension. Alternative beta-1 blocking drugs such as propranolol or metoprolol, and the combined alpha- and beta-blocker labetalol are appropriate in the subacute phase. Calcium channel blockers may be necessary to reduce systolic blood pressure in those patients with a contraindication to beta-blocker use. A commonly used alternative to nitroprusside is nicardipine. This is also a calcium channel-blocker devoid of any cardiac effects which is easily titrated to a goal blood pressure.

Operative Indications

The goals of surgery in acute type A dissection are to prevent or treat an aortic catastrophe while restoring blood flow to the true lumen of the aorta. Aortic catastrophe includes aortic rupture into the pericardium or pleural space, dissection and occlusion of the coronary ostia, and progression to aortic valvular incompetence. The presence of ascending aortic involvement is therefore an indication for operative management in all but the highest-risk patients (Table 50-5). The difficulty arises in determining which patients are high risk and which additional factors should affect the management algorithm. Patient age, for example, is not regarded as an absolute contraindication to surgery. However, this factor should be considered given the relative worse outcomes of operative treatment for acute type A dissection for patients greater than 80 years of age. Neurologic status at the time of presentation can also affect the decision to operate. Although most agree that obtunded or comatose patients are unlikely to improve with surgical repair, complications such as stroke or paraplegia at the time of presentation are not contraindications to surgical correction. It must be acknowledged that dissection repair will most likely not improve neurologic condition, and may even make it worse. Neither the distal extent nor the thrombosis of the false lumen obviates the need for surgical repair because the risk of developing an aortic catastrophe remains. Similarly, patients with subacute type A dissection who present or are referred after 2 weeks of dissection onset require operation. Scholl et al demonstrated that these patients have avoided the early complications of dissection and may safely undergo elective operation rather than emergency repair.17

Surgical Therapy

Anesthesia and Monitoring

Anesthesia used during the repair of aortic dissections is often narcotic based with inhalational agents for maintenance. Single-lumen endotracheal tubes are used for procedures performed through a median sternotomy, whereas double-lumen endotracheal tubes are useful but not mandatory for procedures performed through a left thoracotomy. Monitoring lines often include central venous access with a pulmonary artery catheter and one or more arterial pressure monitoring lines specific to the operation performed. Preparation must be made for all possibilities in these cases, most importantly for the possible need for hypothermic circulatory arrest. Arterial monitoring should be tailored to both the anatomy of the dissection and the method of cannulation. One or two radial arterial lines and at least one femoral line may be required to ensure adequate perfusion of the upper and lower body. All patients require a TEE probe. Core body temperature is monitored in the bladder using a Foley catheter and in the esophagus using a nasopharyngeal probe. A wide skin preparation to include the axillary and femoral arteries is essential to provide all possible cannulation options.

Neurologic monitoring is available, but its utility remains controversial even in elective cases. Advocates of both cerebral and spinal cord monitoring argue that these monitors are able to detect injury to neurons before irreversible cellular injury.18 Thus, this warning allows for detection of imminent injury and subsequent evasion of injury. Opponents argue that there is a significant learning curve and that the injury has already occurred once these monitors can identify ischemic neurologic changes. The optimal type of monitoring depends on the location of the dissection and the resultant details of required vascular control. Manipulation of the ascending aorta and arch can affect cerebral perfusion. In these cases, transcranial Doppler (TCD) or near-infrared spectroscopy (NIRS) have been used. Intraoperative TCD monitoring is used to identify malpositioned cannulas or document the need for adjustment of retrograde perfusion.19 Opponents of TCD argue difficulty with low baseline flow and poor signal in patients with thick temporal bones, which confuses interpretation of the results and the response to them. Continuous noninvasive NIRS can be used to monitor cerebral oxygenation, marker of cerebral blood flow. Although the role of NIRS in aortic dissection has not been elucidated, advocates extrapolate use from studies on carotid endarterectomy and coronary bypass. NIRS can be used for identification of regional oxygenation changes during the case, which may be particularly useful during normothermic periods of these cases.18 Somatosensory evoked potentials (SSEP) is argued to be useful in identifying neurologic injury ranging anywhere from the peripheral nerve to the brain. These studies may even identify ischemic cerebral injury during hypothermic circulatory arrest earlier than electroencephalography (EEG).20 SSEP can also be useful in the detection of spinal cord ischemia to identify crucial spinal cord vasculature requiring reimplantation. The use of SSEP at some centers has led to reduced intraoperative and postoperative paraplegia in retrospective studies.21 Neurologic monitoring remains a relatively new technology that is operator dependent, but probably is useful in experienced hands.

Hemostasis

Open aortic dissection repair is commonly associated with significant blood loss caused by weak tissues and coagulopathy from bleeding or hypothermia. Strict blood conservation is an important aspect of the operation. At least one cell-saver device should be available. Packed red blood cells, platelets, and fresh-frozen plasma should be in the operating room at the start of the operation. Coagulopathy as a result of the preoperative status of the patient, cardiopulmonary bypass, and deep hypothermic circulatory arrest contribute to excessive blood loss. Antifibrinolytic drugs can be useful hemostatic adjuncts. Patients will often require transfusion of fresh-frozen plasma, platelets, and possibly cryoprecipitate. Fibrin glues and hemostatic materials such as Surgicel and Gelfoam are useful as systemic coagulopathy is corrected.

Cardiopulmonary Bypass

Cannulation for type A dissection repair requires thoughtful evaluation of the dissection anatomy while taking into consideration the extent of the repair to be undertaken. The crucial point is to provide arterial flow into the true lumen of the aorta with proof of sufficient end-organ perfusion, in particular as dynamic flaps may alter perfusion. Some flexibility regarding arterial access should be exercised. In certain situations, a patient may require multiple cannulation sites to adequately perfuse the entire body. Various options for cannulation exist, but the optimal choice of cannulation in aortic dissection requires tailoring to the combination of surgeon preference with dissection anatomy.

Venous cannulation remains relatively straightforward. Venous cannulation is obtained commonly through the right atrium using a two-stage venous cannula, whereas bicaval cannulation is used for certain cases in which retrograde cerebral perfusion is used during hypothermic circulatory arrest. A left ventricular vent is necessary in the setting of aortic valve incompetence and is easily placed through the right superior pulmonary vein or rarely through the left ventricular apex wall. Cardioplegia is administered in a retrograde fashion through a coronary sinus catheter with additional protection via direct cannulation of the undissected coronary ostia.

Arterial cannulation requires a much more thoughtful process. Historically, femoral cannulation was the site of choice for arterial cannulation for type A dissection. However, the optimal site of cannulation should be tailored based on the combined goals of surgery and the specific anatomy of the patient, with contingency plans for evidence of malperfusion.

The goals of surgery, in terms of distal extent of the repair, depend to a certain extent on surgeon's preference. Many surgeons feel that optimal repair should be limited to the ascending aorta. This scenario can be accomplished by cannulation at most sites with flow directed into the true lumen of the aorta. Femoral cannulation has been a traditional mainstay in dissection repair. The most favorable side of femoral cannulation has been debated in the past, but as long as there is perfusion into the true lumen, the side most likely does not matter. Reports from the University of Virginia, among others, have also shown that the dissection itself can be cannulated safely with echocardiographic guidance.22 This technique involves confirming access to the true lumen of the aorta with echocardiography of the descending aorta. Then, using the Seldinger technique, a percutaneous cannula can be properly positioned. Direct cannulation should be avoided through areas with evidence of hematoma. Proper perfusion of the true lumen must be confirmed with ultrasound or echocardiography. A potential salvage maneuver involves cannulation of the ventricular apex with advancement of the cannula though the aortic valve and into the true lumen of the aorta. This technique also requires confirmation of true lumen perfusion. Aortic cannulation of either the dissection area or the apex mandates cannulation of the graft after repair in most cases.

Other surgeons believe that resection of the maximal amount of abnormal aorta possible is ideal. Because this resection commonly involves the aortic arch, many choose to cannulate in a way that allows for antegrade cerebral perfusion. Although cannulation of the innominate and the left common carotid artery have been described, the most common cannulation site for arch cases has become right axillary cannulation.23 The right axillary artery provides direct access to the right carotid artery for selective antegrade perfusion. This can be done by sewing a graft to the artery or directly cannulating it. However, direct axillary cannulation appears to cause more morbidity than graft cannulation, including further dissection, brachial plexus injury, and limb ischemia. Axillary cannulation may be suboptimal in cases in which the axillary, right common carotid, or innominate artery are dissected. Similar techniques have been described in cannulation approaches to the innominate and left carotid arteries. Finally, the open aorta allows direct access to the lumen of the innominate and the left common carotid, which can be cannulated during hypothermic circulatory arrest for selective antegrade perfusion. No matter the preferred site of arterial cannulation, the surgeon must be cognizant of whole body perfusion. Patients that are not cooling properly or show other signs of malperfusion may require more than one arterial cannulation sites for cardiopulmonary bypass. Routine confirmation of blood flow in the carotids as well as the descending aorta can be critical to avoid malperfusion.

Cerebral Protection

Type A dissection repair involving the arch disrupts blood flow to the brachiocephalic arteries during a period of circulatory arrest. Cerebral protection during that period is critical to neurologic outcome. Cerebral protection is optimized through deep hypothermia with or without potential neuroprotective adjuncts. Straight hypothermia during circulatory arrest was the first method used to perform operations on the aortic arch and remains an effective method for shorter procedures. Two alternative primary end points for cooling are employed: goal temperature or EEG silence.

The published temperature goals vary widely, namely anywhere from 14 to 32°C. The ischemic tolerance of the brain improves with colder temperatures. However, cooling to a temperature below 14°C can result in a form of nonischemic brain injury and is therefore not recommended. The neurologic protection from straight hypothermic circulatory arrest can be very good, especially for short ischemic times. Most data suggest that straight hypothermic circulatory arrest up to 20 minutes is safe. However, increased ischemic time is directly related to increased incidence of neurologic deficits.24 Proponents of straight hypothermic circulatory arrest have suggested in elective patients that longer times can be used safely without significant adverse cognitive outcomes but circulatory arrest should be limited to as short a time as possible.25

For these cases, temperature is being used as a proxy for metabolic function. Unfortunately, nasopharyngeal and tympanic temperature may be imperfect estimates of brain temperature. Moreover, temperature does not directly relate to neurologic activity. For these reasons, some groups use EEG silence to determine the appropriate point at which to discontinue cooling and perfusion. The patients are cooled until EEG silence is obtained. After 5 minutes at this temperature, the circulatory arrest period can be initiated, usually at a temperature between 15 and 22°C. Using this technique, the group from the University of Pennsylvania achieved EEG silence in 90% of patients after 45 minutes of cooling and had a postoperative stroke rate of less than 5%.26 As a result, in the absence of EEG monitoring, they cool for at least 45 minutes in almost all cases to optimize brain protection. Although EEG is attractive in theory, it is not always available when patients with dissections are taken to the operating room.

Continued cerebral perfusion during the period of circulatory arrest is an alternative technique for cerebral protection, especially for circulatory arrest times greater than 20 minutes. Cerebral blood flow may be delivered in either a retrograde or antegrade fashion. The technique for retrograde cerebral perfusion depends on the venous cannulation strategy. If bicaval cannulation is used, reversing flow through the superior vena caval cannula with a proximally placed tourniquet is simple and effective. Use of retrograde flow with dual-stage venous cannulation requires placement of a retrograde "coronary sinus" catheter into the superior vena cava through a pursestring suture. The superior vena cava is then occluded with an umbilical tape to direct flow toward the head. Retrograde cerebral perfusion has the added benefit of flushing atherosclerotic material and air from the brachiocephalic vessels. A flow rate necessary to produce a superior vena caval pressure of 15 to 25 mm Hg is considered optimal.

Selective antegrade cerebral perfusion has recently gained popularity. Once the aortic arch is open, the innominate artery and the left common carotid artery are encircled with vessel occluders and each lumen cannulated with a retrograde "coronary sinus" cannula. With the left subclavian artery occluded, flow rates are slowly increased to achieve perfusion pressures of 50 to 70 mm Hg at the desired circulatory arrest temperature. These cannulae are then removed just before completing the anastomosis of the brachiocephalic vessels to the vascular graft, at which time cardiopulmonary bypass may be reinstituted.

A few basic principles apply to all approaches. During cooling on cardiopulmonary bypass, a maximum temperature gradient between perfusate and patient of less than 10°C is ideal. The head is then packed in ice to maintain a low brain temperature. To ensure maximal protection, the goal temperature should be maintained for 5 minutes before initiation of hypothermic circulatory arrest. Similarly, the body should be reperfused for 5 minutes at the colder temperature before beginning the rewarming process. Rewarming too early can exacerbate neurologic injury. Rewarming proceeds without exceeding a 10°C perfusate-patient temperature gradient to at least 37°C as core body temperature often falls briefly after cessation of active warming and separation from cardiopulmonary bypass.

Pharmacologic adjuncts are believed by some to decrease metabolic rate with hopes of reducing injury. Although methylprednisolone continues to be used in these cases by many, barbiturate administration during cooling has fallen mostly out of favor. If used, methylprednisolone should be given early as the steroid effects require incorporation into the cell nucleus. Others give lidocaine and magnesium before the arrest period to stabilize the neuronal cell membrane. Furosemide and mannitol can be administered to initiate diuresis and promote free radical scavenging after circulatory arrest. The results of all these techniques are not fully substantiated yet.

Operative Technique

The exposure for procedures performed on the ascending aorta and proximal arch is through a median sternotomy. This can be modified with supraclavicular, cervical, or trapdoor incisions to gain exposure to the brachiocephalic vessels or descending thoracic aorta. When dissecting the distal arch, it is important to identify and protect both the left vagus nerve with its recurrent branch and the left phrenic nerve. Replacement of the ascending aorta in type A dissections is best performed by an open distal anastomosis technique if the arch is involved (30%) or if arch involvement is unknown. The open distal anastomotic technique requires clamping the mid ascending aorta and producing cardiac arrest via administration of antegrade and/or retrograde cardioplegic solution. The dissected ascending aorta proximal to the clamp is then opened. Evaluation and surgical correction of the aortic valve is ideally performed at this time while systemic cooling continues. If the dissection does not involve the aortic root, the aorta is transected 5 to 10 mm distal to the sinotubular ridge. When the dissection involves the sinotubular ridge, the proximal aorta is reconstructed by reuniting the dissected aortic layers between one or two strips of Teflon felt using either 3-0 or 4-0 Prolene suture. Safi et al use a technique of interrupted pledgeted horizontal mattress sutures as compared with the felt sandwich technique.27 In their experience, this provides superior stabilization and decreases the potential for subsequent aortic stenosis. The University of Pennsylvania has described aortic reconstruction using felt as a neomedia giving a stable platform to sew the graft to otherwise friable tissue.26

Once the temperature reaches 18 to 20°C, perfusion is discontinued during a brief period of circulatory arrest. When using antegrade or retrograde cerebral perfusion, the selected perfusion is initiated at this time. The aortic clamp is released and the intima of the aortic arch is inspected and repaired accordingly (Fig. 50-13). If the intima is intact, the distal anastomosis is performed and the graft is cannulated, deaired, and clamped for resumption of cardiopulmonary bypass with systemic warming. If the intima of the arch is violated, then a hemiarch reconstruction is performed (Fig. 50-14). We have only rarely found it necessary to perform a complete arch resection for an acute dissection. If a complex aortic root procedure is required, it is often useful to repair the aortic root with one vascular graft and use a separate graft to create the distal aortic anastomosis. The two grafts are then measured, cut, and anastomosed to provide the correct length and orientation for aortic replacement.

If the ascending aorta cannot be cross-clamped, the patient is cooled to 20°C with subsequent circulatory arrest. The distal aortic reconstruction is performed first in this circumstance, at which time the graft is cannulated and proximally clamped with resumption of cardiopulmonary bypass and systemic rewarming. Cannulation of the graft for antegrade systemic perfusion and rewarming is associated with improved neurologic outcomes compared with retrograde perfusion and should be performed whenever possible. Because a cross-clamp is not applied, the left ventricle must be decompressed once fibrillation starts during systemic cooling (approximately 20°C) to prevent distention and irreversible myocardial injury. Proximal ascending aortic repair is completed during the period of rewarming.

An alternative to the open distal technique is possible when the dissection is limited to the ascending aorta or the proximal arch away from the origin of the brachiocephalic vessels. Antegrade arterial perfusion is achieved through distal arch or right subclavian artery cannulation; retrograde perfusion via cannulation of a femoral artery has traditionally provided acceptable results. An aortic cross-clamp is applied tangentially just proximal to the innominate artery. The ascending aorta is resected to include the inferior aspect of the arch. The layers of the dissected aorta proximal to the clamp are then reunited if necessary and the ascending aorta replaced with an appropriately sized, beveled vascular graft. The proximal reconstruction and anastomosis may then be created and the entire procedure performed without requiring deep hypothermia and circulatory arrest.

Isolated dissection of the aortic arch is rare. Classified as a type A dissection, it requires resection of the arch at the site of intimal disruption and aortic replacement. Surgical management of the brachiocephalic vessels is determined by the integrity of the adjacent intima. If intact, the brachiocephalic vessels are reimplanted as a Carrel patch into a vascular graft after repair (Fig. 50-15). If the dissection involves individual vessels, each may require repair and reimplantation individually into the graft used for arch replacement (Fig. 50-16).

Aortic root dissection often fails to violate the intima of the coronary ostia. Repair of the ascending aorta at the sinotubular junction is therefore sufficient to reunite the aortic root layers and provide uninterrupted coronary blood flow. Minimal disruption of the coronary ostial intima should be repaired primarily with 5-0 or 6-0 Prolene suture. If, however, the ostium is circumferentially dissected and an aortic root replacement is necessary, an aortic button should be excised and the layers reunited with running 5-0 Prolene suture, glue, or both. Coronary buttons are then reimplanted into the vascular graft or to a separate 8-mm vascular graft as part of a Cabrol repair (Fig. 50-17). Aortocoronary bypass grafting is performed only when the coronary ostium is not reconstructable and as a last resort.

Acute type A dissection is complicated by aortic valve insufficiency in up to 75% of patients. Fortunately, preservation of the native valve is successful nearly 85% of the time. The mechanism of aortic insufficiency in most cases is the loss of commissural support of the valve leaflets. This is repaired using pledgeted 4-0 Prolene sutures to reposition each of the commissures at the sinotubular ridge (Fig. 50-18). The dissected aortic root layers are then reunited using 3-0 Prolene suture and either one or two strips of Teflon felt to recreate the sinotubular junction and reform the sinuses of Valsalva. Aortic valve preservation must always be performed using intraoperative TEE to assess the valve postoperatively. No more than mild aortic insufficiency should be present. In addition to commissural resuspension, techniques exist to spare the aortic valve and replace the aortic root in acute type A dissection, but the experience is early and the number of patients few. This topic is covered in greater detail in the section on surgical techniques for chronic type A dissection.

If the aortic valve cannot be spared, replacement of the ascending aorta and valve should be performed using a composite valve graft or homograft. The composite valve graft is implanted using horizontal mattress 2-0 Tycron sutures to encircle the annulus and to seat the valved conduit (Fig. 50-19). The previously excised and reconstituted coronary buttons are reimplanted into the vascular graft with running 5-0 Prolene suture (Fig. 50-20). The left coronary button is implanted first, at which time the graft is clamped and placed under pressure to define the proper orientation and position of the right coronary button. The aortic homograft is similarly implanted using horizontal mattress 2-0 Tycron sutures, except that a generous margin of aortic root below the coronary buttons is retained for a second hemostatic suture line of running 4-0 Prolene. This is an ideal solution for individuals who have a contraindication to anticoagulation or for young females. The Ross procedure (pulmonary autograft) is not applicable in those patients with connective tissue disorders and not recommended in acute dissection.

Postoperative Management

Invasive hemodynamic monitoring is used to ensure adequate end-organ perfusion with a target systolic blood pressure between 90 and 110 mm Hg. Early postoperative blood pressure control begins with adequate analgesia and sedation using narcotics and sedative/hypnotic agents. The patient should, however, be allowed to emerge from general anesthesia briefly for a gross neurologic examination. The patient is then sedated for a period to ensure continued hemodynamic stability and facilitate hemostasis. Coagulopathy is aggressively treated with blood products and antifibrinolytic agents as necessary, and by warming the patient. Hematocrit, platelet count, coagulation studies, and serum electrolytes are obtained and corrected as necessary. An ECG and chest radiograph are used to assess for abnormalities and to serve as baseline studies. A full physical exam, including complete peripheral vascular exam, is performed on arrival. Despite adequate repair of the dissection, perfusion of the false lumen may persist; therefore, malperfusion syndrome remains possible. If an abdominal malperfusion syndrome is suspected postoperatively, this should be aggressively evaluated with ultrasound and subsequent angiography if positive. A strong clinical suspicion is enough to warrant this evaluation given the consequences of failed recognition. The patient can be extubated once extubation criteria are met if the patient has been hemodynamically stable without excessive bleeding and the results of a neurologic exam are normal. Management is routine from that point forward.

Long-Term Management

Surviving the operation for acute dissection represents the beginning of a lifelong requirement for meticulous medical management and continued close observation. It has been estimated that replacement of the ascending aorta for type A dissection obliterates flow in the distal false lumen in fewer than 10% of patients. As a result, the natural history of repaired dissection may involve dilatation and potential rupture of the chronically dissected distal aorta. This was the reason for the late death in nearly 30% of DeBakey's original series in 1982 and is currently the leading cause of late death following surgical repair.28 Often a multidrug antihypertensive regimen including beta-blocking agents is required to maintain systolic blood pressure below 120 mm Hg. There are some data indicating that blood pressure control within a narrow range may alter the natural history of chronic dissection by diminishing the rate of aneurysmal dilatation. The long-term durability of the aortic valve after supracoronary reconstruction is quite good with freedom from aortic valve replacement of 80 to 90% at 10 years. Progressive aortic insufficiency of the native valve is possible and should be followed with TEE in some patients.

Follow-up diagnostic imaging is required to monitor aortic diameter in patients after repair. Spiral CT arteriogram and MRI are the imaging studies of choice. MRI and ultrasound are useful in patients with renal insufficiency and those requiring only imaging of the abdominal aorta. Echocardiography is useful for imaging the ascending aorta and provides additional information regarding the aortic valve. It is important to recognize the resolution limitations of each imaging modality and inherent imprecision of comparing different imaging modalities to evaluate changes. In general, measurements should be made at the same anatomical level with respect to reproducible anatomical structures (ie, the sinotubular ridge, proximal to the innominate or left subclavian arteries or at the diaphragmatic hiatus). It is important to recognize that the false lumen should be included in measurements of aortic diameter whether it is perfused or not. Three-dimensional reconstruction of spiral CT and MRI scans minimizes the error introduced by aortic eccentricity when comparing imaging studies and has simplified following this patient population. The current recommendations are to obtain a baseline study before hospital discharge and at 6-month intervals during the first year. If the aortic diameter remains unchanged at 1 year, studies are obtained yearly. Aortic enlargement of more than 0.5 cm within a 6-month period and greater eccentricity on comparison of 3D reconstruction images are high-risk changes for which the interval is decreased to 3 months if surgery is not indicated.

Results

The operative mortality for repair of acute type A aortic dissection has fallen since DeBakey's original 40% mortality was reported in 1965. Improved ICU and floor care of these patients, earlier recognition of dissection through improved imaging modalities, development of hemostatic vascular graft material, more effective hemostatic agents, and improvements in the safety of cardiopulmonary bypass are likely responsible. In the last two decades, most centers consistently report an operative mortality for acute type A dissection of between 10 and 30%. The high early mortality in acute dissection parallels the number of patients who present profoundly hypotensive and in shock. The mode of death is stroke, myocardial ischemia/heart failure, aortic rupture, or malperfusion in most cases.

The International Registry of Acute Aortic Dissections (IRAD) recently reported on the results of 526 patients with acute type A aortic dissection who underwent surgical treatment in 18 large tertiary centers.29 Surgery in these patients included replacement of the ascending aorta in 92%, aortic root in 32%, partial arch in 23%, complete arch in 12%, and descending aorta in 4%. Overall in-hospital mortality was 25%; 31% for hemodynamically unstable patients and 17% for stable patients. Causes of death were aortic rupture (33%), neurologic complications (14%), visceral ischemia (12%), tamponade (3%), or nonspecified (42%).

Age is not an absolute contraindication to surgical treatment of type A aortic dissections. However, operative mortality increases with age. Retrospective series show that operative mortality increases from 20 to 30% for patients younger than 75 years of age to greater than 45 to 50% for those 80 years or older.30

The published results for long-term survival following surgically treated acute type A aortic dissection over the last decade is roughly 71 to 89% at 5 years and between 54 and 66% at 10 years.31–33 Survival for patients who are discharged alive from the hospital after surgical repair of type A aortic dissection carries a survival rate of 96% at 1 year and 91% at 3 years.34

Natural History

Type B aortic dissections account for approximately 40% of all acute aortic dissections.8 Their natural course is more benign than that of acute type A dissections.

Most patients with acute type B aortic dissections survive the acute and subacute phases with medical management alone. Approximately 20 to 30% of patients present with complicated type B dissection, which require urgent operative (surgical or endovascular) intervention. Complicated dissection can be defined as imminent or actual aortic rupture, aortic expansion, hemodynamic instability, persistent pain despite medical management, drug-resistant hypertension, and malperfusion syndrome.8,35 The most frequent causes of death in acute type B dissection are aortic rupture and visceral malperfusion.

The relative success with medical management has relegated surgical treatment for acute type B dissections to patients with complicated dissections or those with progression of the disease (see Table 50-5). Medical therapy of uncomplicated type B aortic dissection confers a relatively good short-term prognosis with in-hospital survival of approximately 90%.35 Medical management in these patients has survival rates of approximately 85% at 1 year and 71% at 5 years.36

On the contrary, patients with complicated dissections that require open surgical intervention have a 30-day mortality of approximately 30%.8 More recently though, endovascular techniques have been used in these complicated dissections with reduced perioperative mortality and morbidity. Endovascular treatment of acute type B dissections was first described in 1999 by Dake et al.37 and Nienaber et al.38 As endovascular treatment of complicated dissections has expanded, the results have improved, with studies now reporting a mortality of around 5%.39

Medical Management

Historically, the mortality of open surgical approaches for type B aortic dissections exceeded 50%, whereas medical management carried a mortality risk of 30% or less. Therefore, medical management has played a pivotal role in the management of these patients. Medical management is identical to that described in the initial management of the acute type A aortic dissection. The goals are control of the heart rate and blood pressure to decrease the shear stress on the aorta and limit expansion of the false lumen and propagation of the dissection.

Medical management initially requires an adequate airway and intravenous access. Patients with suspected or confirmed type B aortic dissection should be admitted to the intensive care unit for close monitoring. Pain control is important to reduce catecholamine release and is best achieved with narcotic medications, in particular morphine sulfate. Management is initiated with beta-blockers such as esmolol or propranolol.9 These medications should be titrated to achieve optimal blood pressure (ie, systolic pressure of 100 to 120 mm Hg) and heart rate while allowing for adequate renal, gut, and brain perfusion. Heart rate control to a goal of less than 60 bpm has been shown to decrease the risk of secondary adverse events such as aortic expansion, recurrent dissection, and aortic rupture.40

Vasodilators such as sodium nitroprusside can be used as an adjunct to beta-blockers if blood pressure is still not adequately controlled. Vasodilators can increase the force of ventricular ejection and aortic shear stress and should therefore be only used concurrently with beta-blockade. Calcium channel blockers can also be used to control blood pressure, especially in patients intolerant to beta-blockers. However, their use has not been appropriately studied in patients with aortic dissection. Patients with normal or decreased low blood pressure at presentation, without evidence of cardiac tamponade or heart failure, may benefit from judicious intravenous volume administration.

Once the patient with uncomplicated type B aortic dissection has been stabilized, blood pressure medications should be transitioned to an oral regimen. The patient can then be discharged home with close follow-up, including imaging and clinical assessment in 3 months and every 6 months thereafter.

Operative Indications

Operative management of acute type B aortic dissection, either by endovascular or open surgical techniques, is currently limited to the prevention or relief of life-threatening complications.9,41 Operative treatment is indicated for patients with persistent or recurrent pain, medically uncontrolled hypertension, rapidly expanding aortic diameter, progression of dissection despite maximal medical management, signs of impending or actual aortic rupture (ie, periaortic or mediastinal hematoma), or malperfusion to limbs, kidneys, or gut (see Table 50-5).

Endovascular Therapy

General Considerations

Given the poor outcomes of open surgical approaches, endovascular therapy has transitioned to the frontline of therapy for complicated type B dissections. Endovascular treatments can include placement of thoracic endovascular graft prostheses, endovascular creation of flap fenestrations, and/or placement of uncovered stents in affected branch vessels to treat malperfusion. Because of the relative simplicity and recent improvements in outcomes, endovascular grafts have become the preferred endovascular technique for treatment of aortic dissections.

The goals of endovascular graft therapy in complicated acute aortic dissection are to restore flow to the true lumen and perfusion to the distal aorta and branch vessels. Coverage of the primary tear is frequently necessary to achieve these goals, especially if the tear is in the proximal descending aorta. Obliteration of the false lumen is desirable as well because it improves the prognosis, but it is not always possible. Endoluminal treatment of these cases usually requires coverage of the entire descending thoracic aorta because of the presence of multiple tears in the intima. The optimal outcome occurs when the prosthesis covers the primary tear, reapposes the dissected layers of the aorta, and prevents blood flow into the false lumen, therefore leading to thrombosis of the false lumen, expansion of the true lumen, and restoration of branch vessel patency (Fig. 50-21).

An alternative technique used to treat branch malperfusion from aortic dissection is percutaneous fenestration with or without stenting of the malperfused branch vessels. According to this technique, a communication is created between the true and the false aortic lumens so as to provide flow to both of them. Percutaneous fenestration is performed by pulling an inflated balloon or a fenestration knife through the dissection flap to create communication between both lumens. An uncovered stent can then be placed in the true lumen at the region of the affected branch vessel or vessels to alleviate dynamic obstruction (occlusion of the branch by prolapse of the dissection flap into the branch vessel).42 If static obstruction (extension of the dissection into the branch vessel with reduction of the true lumen) is also present, a stent is deployed directly into the branch vessel (Fig. 50-22). The major limitation of percutaneous fenestration for treatment of aortic dissection when compared with endovascular grafting, is that it does not induce thrombosis of the false lumen. False lumen thrombosis has been shown to induce aortic remodeling and decrease the long-term risk of aortic dilatation and rupture.43

Percutaneous fenestration and stenting can also be used as an adjunct to endovascular grafting when closure of the primary tear fails to improve distal malperfusion. Another technique that can be used to improve malperfusion after endovascular grafting is the PETTICOAT (provisional extension to induce complete attachment) technique.44 According to this technique, a bare metal stent is placed as a distal extension of the previously implanted stent graft in order to expand the true lumen.

Preoperative Planning

The endovascular treatment of aortic dissections requires meticulous preoperative planning. The anatomy of the aorta and the actual dissection should be studied in detail with the use of imaging technology.

Preoperative imaging for endovascular repair of aortic dissections is accomplished with either computed tomographic angiography (CTA) or magnetic resonance angiography (MRA). Three-dimensional sagittal and coronal reconstructions are useful to assess details of the anatomy of the aorta. Preoperative imaging allows the surgeon to size the aorta for selection of the device, ascertain the presence of adequate proximal and distal landing zones for adequate apposition of the graft, and assess the femoral and iliac vessels to plan the delivery of the device.

The left subclavian artery may be occluded in certain situations without the need for revascularization. Exclusion of the left subclavian artery without concomitant revascularization should be avoided in patients with a patent left internal mammary artery coronary bypass, those with an incomplete posterior cerebral circulation, a dominant left vertebral vessel, or a stenosed or occluded right vertebral artery.45 Left subclavian revascularization should also be considered in those patients in whom a long segment of the aorta is being covered or those with a history of prior aortic surgeries because of the increased risk of spinal ischemia. Left subclavian revascularization is usually accomplished with the creation of a left carotid to subclavian bypass, or less commonly, left subclavian artery transposition. To assess the cerebral circulation, a head and neck CTA should be performed preoperatively in every patient in whom left subclavian exclusion is a possibility. In our experience, the left subclavian artery is covered in approximately 50% of the cases, with 25% of these patients requiring revascularization because of concerns of cerebral circulation or arm claudication.46

Adequate vascular access for delivery of the device is an important requirement for endovascular therapy of the aorta. Both iliofemoral systems are evaluated preoperatively with CTA or MRA with particular attention to size, tortuosity, and presence of calcification that may preclude safe delivery of the device. The minimum size required for the access vessels is determined by the outer diameter of the introducer sheath. Current devices require insertion through a vessel of at least 6 to 8 mm in diameter (equivalent to ~20 French delivery device). When the femoral vessels have an adequate caliber, no tortuosity, and minimal calcification, total percutaneous access is possible, using automated percutaneous closure devices to seal the entry point. Alternatively, an open approach to control the common femoral arteries may be used. If the caliber of the femoral vessels is not appropriate for insertion of the device, retroperitoneal exposure of the iliac artery with placement of a 10-mm tube graft may be necessary as a surgical conduit for insertion of the device.

Endovascular stenting of the aorta is associated with a risk of paraplegia in recent series of 0 to 3.4%.35,47 Preoperative insertion of a lumbar catheter for drainage of cerebrospinal fluid (CSF) may decrease the risk of permanent paraplegia. In our institutions, we perform selective preoperative placement of lumbar catheters at the discretion of the surgeon, taking into account the region and length of planned aortic exclusion, and previous history of aortic surgery.45 If there is no evidence of paraplegia postoperatively, these drains are usually discontinued 48 to 72 hours after surgery.

Operative Technique

The endovascular stenting procedure can be performed either in the angiography suite or an operating room with advanced imaging capabilities. At our institutions, the procedure is performed in the angiography suite by a team of cardiovascular surgeons and interventional radiologists. The procedure is usually performed under general anesthesia, although local or epidural anesthesia can also be used, depending on the comorbidities and clinical status of the patient. Monitoring lines are placed, including a radial arterial line in the right upper extremity. A lumbar drain for CSF drainage is placed preoperatively at the discretion of the surgeon, as stated. Antibiotic prophylaxis is administered.

In most cases, bilateral iliofemoral arterial access is used for the procedure. The larger side with less calcification and tortuosity is chosen for the delivery of the device. The other side is used for percutaneous insertion of a 5 French pigtail catheter for diagnostic contrast injection. If only one side is appropriate for use, the diagnostic pigtail catheter may be inserted through one of the brachial arteries.

If there is significant calcification of the vessel or the surgeon does not feel comfortable with the use of a percutaneous vascular closure device, the femoral artery is surgically exposed and controlled. In our experience, approximately 20% of patients have femoral vessels of an inadequate caliber to accommodate the delivery device. In these patients, a flank incision is performed and the retroperitoneal common iliac artery is exposed. A 10-mm polyester surgical conduit is then anastomosed to the common iliac artery and used as access for delivery of the device.

The pigtail catheter is inserted percutaneously and advanced to the arch of the aorta. Aortography or IVUS is then used to confirm the location of the catheter within the true lumen, define the anatomy, localize the entry tear, and create a roadmap for the procedure. The confirmation of the device being in the true lumen cannot be overemphasized. Many centers use only IVUS for these procedures. From these images, the decision is made regarding the site of deployment. The left subclavian artery may need to be covered to ensure an adequate proximal landing zone and completely exclude the primary tear. The proximal landing zone is usually visualized best at a fluoroscopic angle of approximately 45 to 75 degrees left anterior oblique (LAO).

A super-stiff wire is then inserted through a sheath placed into the iliofemoral vessel previously chosen for device delivery, and advanced into the aortic arch. The patient is heparinized to an ACT of greater than or equal to 200 seconds. The appropriate sheath is advanced into the abdominal aorta under direct fluoroscopic visualization. The device is then advanced to the selected point of delivery. Placement of the sheath and the device into the aorta are probably the most dangerous parts of this procedure. Before the deployment of the device, blood pressure and heart rate should be pharmacologically controlled so as to avoid undue strain on the heart and migration of the endograft on deployment. The device is then deployed. The optimal placement of the prosthesis is confirmed by angiography or IVUS. Although the device may need to be ballooned for full opening and apposition to the aortic wall, this maneuver has inherent risks given the weakness of the aortic wall and the pressure required to expand the stent grafts.

It is important to confirm the correct placement of the device, the absence of endoleaks, and the resolution of malperfusion to branch vessels at the end of the procedure. For this purpose, an angiogram is performed through the diagnostic pigtail catheter. If there is evidence of persistent malperfusion, adjunctive therapies should be considered such as percutaneous fenestration or deployment of additional bare-metal stents on the distal aspect of the prosthesis to expand the true lumen (PETTICOAT technique). Branch vessel stenting may also be necessary to relieve static obstruction.42 In case of a ruptured dissection, the endovascular prosthesis should cover both the site of the primary tear and the site of rupture. Furthermore, in patients with significant hemothorax the dissection should be treated before draining the chest, as this may be tamponading the rupture.

Surgical Therapy

General Considerations

Open surgical intervention for type B aortic dissections is performed under general anesthesia with narcotic and inhalational agents. Double-lumen endotracheal intubation allows for lung isolation, which is critical in exposure of the thoracic aorta. Central venous access, right radial and femoral arterial lines, and in selected cases a pulmonary artery catheter, are inserted before the procedure. Core body temperature is usually measured with the use of a temperature probe in the Foley catheter and/or an esophageal probe. Antibiotic prophylaxis is administered.

Spinal cord ischemia resulting in paraplegia or paraparesis is a recognized complication of acute dissection repair that may be partially preventable and even reversible. The incidence of spinal cord ischemia is between 19 and 36% after repair of acute type B dissection.48,49 Whereas various strategies exist to prevent spinal cord ischemia during repair of a chronic dissection, very few are feasible in the acute setting. Pharmacologic agents such as steroids, free radical scavengers, vasodilators, and adenosine are promising adjuncts to prevent spinal cord ischemia but presently have little to no proven clinical utility. We presently use left atrial to femoral artery bypass and reimplant key intercostals arteries and selectively use cerebrospinal fluid drainage as outlined by Safi et al.50

Operative Technique

The patient is positioned in right lateral decubitus. The pelvis is canted posteriorly to allow access to both femoral vessels. A posterolateral thoracotomy in the fourth intercostal space provides sufficient access to the aorta; notching the fifth and sixth ribs posteriorly can facilitate wider exposure of the thorax. A thoracoabdominal incision may be required to access the abdominal aorta in the case of visceral malperfusion. This may be performed through either a transperitoneal or a retroperitoneal approach. The left hemidiaphragm is divided in a radial fashion while marking adjacent sites on each side of the division with metal clips for later reapproximation.

The ideal open acute type B aortic dissection repair involves replacement of as little of the descending thoracic aorta as necessary. The extent of replacement rarely exceeds the proximal third, which includes the primary tear in most cases. Such a strategy optimizes perfusion of the spinal cord by preserving more intercostals arteries.48 This point is controversial, however, and some groups advocate replacement of the entire thoracic aorta. Any less extensive aortic replacement leaves dissected aorta with the potential for late aneurysmal dilation when there is perfusion of the false lumen. The ideal strategy to minimize spinal cord malperfusion yet resect all involved aorta has not been proved.

Once the thoracic aorta has been exposed, the operation continues with division of the mediastinal pleura between the left subclavian and the left common carotid arteries. It is essential that the left vagus and recurrent laryngeal nerves are identified and preserved during the course of the dissection. The left subclavian artery is encircled with an umbilical tape and Rummel tourniquet. Ultimately, the entire distal arch must be free enough to place an aortic clamp between the left common carotid and left subclavian arteries. Next, the proximal descending thoracic aorta is circumferentially mobilized, dividing intercostal arteries in the segment to be excised.

The formerly popular "clamp and sew" technique used for repair of acute type B dissection has largely been replaced by the use of partial left heart bypass. To institute left heart bypass, the left inferior pulmonary vein is dissected and a 4-0 Prolene pursestring suture placed posteriorly for cannulation. Arterial cannulation sites for this technique include the distal thoracic aorta for limited dissections of the proximal descending thoracic aorta or the femoral artery for those extending into the abdomen. It is important to assure that distal perfusion during cannulation is directed into the true lumen. Following the administration of 100 U/kg of intravenous heparin, 14 French cannulae are inserted into the left inferior pulmonary vein and either a normal-appearing area of descending thoracic aorta or percutaneously into either femoral artery. Bypass is then initiated with flow rates between 1 and 2 L/min.

The left subclavian artery is controlled and vascular clamps are placed on the aorta proximal to the left subclavian artery and distally on the mid-thoracic aorta. Right radial artery pressure is measured to maintain proximal aortic systolic pressure between 100 and 140 mm Hg and mean femoral artery pressure greater than 60 mm Hg.49 The aorta is then opened longitudinally and bleeding from intercostals arteries is controlled by suture ligation. Transection of the aorta distal to the origin of the left subclavian artery provides a site for the proximal anastomosis. This is performed using 3-0 Prolene suture and may require external reinforcement with Teflon felt strips.

The graft inclusion technique is another procedure in which the posterior aspect of the proximal aorta is not fully transected. The proximal anastomosis is then created to the intact posterior aspect of the aorta. We do not recommend this technique because one cannot be certain of including all layers of the aorta in the anastomosis.

The size of the vascular graft is based on the diameter of the distal aorta and beveled to match the aorta proximally. This anastomosis may include the origin of the left subclavian to treat dissection in this vessel. A separate 6- to 8-mm Dacron graft can be used if there is intimal disruption involving the proximal segment of the left subclavian artery. Once the proximal anastomosis is completed, the proximal clamp is released and repositioned on the vascular graft to inspect the anastomosis. The distal anastomosis is then completed, the clamps are released, and partial left heart bypass is terminated. Percutaneously placed femoral artery cannulae 14 French or smaller may be removed without direct repair of the vessel. When cannulae 15 French or larger are required, open surgical repair of the femoral arteriotomy is indicated.

Rupture of the thoracic aorta before or during repair is a catastrophic event often leading to operative death. Successful management requires immediate cannulation of the femoral artery and vein for cardiopulmonary bypass and eventual deep hypothermic circulatory arrest. Assisted venous drainage through the femoral vein is often adequate, but direct cannulation of the right ventricle through the pulmonary artery may also be performed. A left atrial vent is placed through the left inferior pulmonary vein once the heart begins to fibrillate; the left ventricle may be vented as well directly through the apex. Once the nasopharyngeal temperature reaches 15°C, the vent is occluded and cardiopulmonary bypass is stopped. The head is placed down and the aorta opened for repair under circulatory arrest. The distal aorta should be clamped to minimize blood loss. Once the proximal anastomosis is performed, the proximal clamp is moved onto the graft and the graft cannulated to resume cardiopulmonary bypass.

Malperfusion of intra-abdominal viscera or lower extremities may be apparent at the time of presentation or may follow surgical repair of aortic dissections. Proximal repair of the dissection is standard treatment and may be sufficient to treat the malperfusion syndrome. However, if malperfusion presents or persists after surgical repair, percutaneous or surgical fenestration may be necessary. As specified, percutaneous fenestration is accomplished by using a balloon or a percutaneous knife to create a communication between the false and the true lumens. Surgical fenestration is performed through a midline laparotomy or left flank incision to provide exposure of the infrarenal aorta (Fig. 50-23). Occasionally, fenestration of intra-abdominal aortic branch vessels may be required if the intima is violated beyond the ostia. If the dissection flap cannot be completely excised, the distal vessel layers must be reunited. Consideration should be given to patch angioplasty to prevent narrowing when closing smaller vessels. In the event that perfusion is not reestablished, extra-anatomic bypass may be required.

Obstruction of the terminal aorta or malperfusion of the lower extremities following operative repair is best treated with percutaneous fenestration. Surgical fenestration remains an option if percutaneous techniques fail to reestablish blood flow. In the event that surgical fenestration fails, the best solution is femoral-femoral bypass grafting in the setting of unilateral malperfusion or axilla-femoral and femoral-femoral bypass if bilateral lower extremity malperfusion exists.

Results

Medical management remains the mainstay of treatment for acute uncomplicated type B aortic dissections. Early medical management alone is an adequate treatment in approximately 68 to 85% of patients presenting with acute type B dissection and leads to a 30-day survival rate of 89 to 93%.35,51 The long-term outcomes of medical management are not as favorable. In the IRAD series, only 78% of 189 patients discharged alive from the hospital after medical management of type B dissections were alive at 3 years.34 Predictors of follow-up mortality were female gender, history of prior aneurysm, history of atherosclerosis, in-hospital renal failure, pleural effusion, and in-hospital hypotension or shock. In a separate series of patients, 87% of patients managed medically were alive at 5 years, with 25% of patients requiring aortic-related interventions.52 Similarly, in a series of 122 patients with type B aortic dissections managed medically over 36 years, Umaña et al reported survival rates of 85% at 1 year and 71% at 5 years, with reoperation rates of 14% at 5 years.36

These outcomes have prompted some authors to consider operative treatment of uncomplicated dissections in an attempt to prevent long-term aortic-related complications. The rationale for this consideration also comes from the fact that endovascular stenting induces thrombosis of the false lumen in 75% of cases39 and false lumen patency is associated with long-term aortic-related complications and mortality.53

Xu et al recently reported a series of 63 patients with type B aortic dissections (59 uncomplicated) treated with endovascular grafting.54 The authors delayed the intervention in uncomplicated patients until 2 weeks after presentation so as to allow fibrosis and increased stability of the intimal flap. The perioperative mortality was only 3% with morbidity including stroke in one patient, renal failure in two, and retrograde aortic dissection in two. No patients developed paraplegia. Complete thrombosis of the false lumen was achieved in 98% of patients at 1 year, and the 4-year survival was nearly 90%.

Two European randomized clinical trials have been designed to answer the question of whether uncomplicated type B dissections should be treated with endovascular grafting or with medical management alone. The results of the first trial, the INSTEAD (INvestigation of STEnt Grafts in Aortic Dissection) trial, have been recently published.55 This trial randomized 140 patients with subacute or chronic type B aortic dissection (>14 days but <1 year) to either elective stent graft placement or optimal medical management alone. Survival was not significantly different between both groups with 2-year survival rates of 96 and 89% after optimal medical therapy and endovascular stenting, respectively. There were also no differences in the rates of a combined end point, including aortic-related deaths or reinterventions. Complete thrombosis of the false lumen was more common among patients with endovascular stenting (91% with stenting versus 19% with medical management). Aortic expansion to greater than 6 cm was also more common in the group with medical management, requiring crossover to stent grafting in 16% of cases and conversion to open surgery in 4%. All crossover patients had uneventful outcomes and no deaths. Therefore, the trial supported the use of optimal medical management for treatment of uncomplicated type B aortic dissections and reserving endovascular interventions for those patients that develop complications or other indications for intervention. The second trial, the ADSORB (Acute Uncomplicated Aortic Dissection Type B) trial has been designed to study patients with acute uncomplicated dissection (<14 days) by randomizing them to optimal medical management or endovascular stenting.56 The trial started enrollment in 2007 and is underway. No results are currently available.

Based on these results, medical management remains the standard of care for uncomplicated type B aortic dissection.

Approximately 20% of patients with acute type B aortic dissection present with complications requiring operative intervention.35 In the past, surgical intervention was the only therapy available for these patients. The development of endovascular techniques has provided an additional treatment option that is now preferred in the majority of cases.

Surgical treatment of aortic dissection was traditionally poised with dismal outcomes. Even though morbidity and mortality remain high, outcomes from surgical management have improved over the last few decades, from as high as 50% in the 1960s to as low as 13% in more recent times.57 In a series of 76 patients treated emergently with surgery for acute complicated type B aortic dissection including 22% patients with aortic rupture, Bozinovski et al reported an in-hospital mortality of 22% with a risk of stroke of 7%, paraplegia of 7%, and renal failure of 20%.58 Similarly, as part of the IRAD series, 82 patients surgically treated for acute type B aortic dissection had an in-hospital mortality of 29% with a risk of stroke of 9%, paraplegia of 5%, and acute renal failure of 8%.59

Because of the unfavorable outcomes of open surgical therapy for acute complicated type B aortic dissection, endovascular therapies have been increasingly used in this patient population. Table 50-6 summarizes the current experience with endovascular therapy for treatment of type B aortic dissections. 35,37,38,42,46,47,54,60–68

A recent meta-analysis of 39 studies involving 609 patients using endovascular repair of aortic dissections between 1999 and 2004 showed a procedural success of 98% with emergency surgical conversion of 1%. In-hospital complications occurred in 14% of patients including neurological complications in 3%, retrograde dissection in 2%, and a perioperative mortality of 5%.39 Survival rates at 2 years were 89% with a risk of aortic rupture at follow-up of 2%.

Most of these studies have used endografts, rather than percutaneous fenestrations, as the endovascular treatment of choice. In one of the largest series of percutaneous fenestration for treatment of type B aortic dissection, Patel et al reports the experience from the University of Michigan from 1997 to 2008.42 During this period, 69 patients with type B aortic dissection and malperfusion by angiography were treated with endovascular flap fenestration and/or branch vessel stenting. Overall technical success for flow restoration was 96% with an early mortality of 17%. Complications included stroke in 4%, acute renal failure requiring dialysis in 14%, and permanent spinal cord ischemia in two patients that presented initially with paraplegia from their dissection. However, despite these encouraging outcomes all-cause mortality at follow-up was as high as 36% with 7% risk of aortic rupture, likely related to the obligate persistence of the false lumen as part of the fenestration procedure.

Despite the multiple isolated series reporting outcomes after different treatment strategies for acute type B aortic dissection, there are currently no randomized controlled trials addressing this issue. The most recent retrospective report by IRAD comparing the different strategies included 571 patients with acute complicated or uncomplicated type B aortic dissection between 1996 and 2005. Of these, 390 patients (68%) were treated medically and 125 required intervention for complicated dissection; 59 (10%) underwent open surgical procedures and 66 (11%) had endovascular treatment with either stent grafts or percutaneous fenestration. In-hospital mortality was significantly higher for patients that underwent surgery (34%) when compared to those that had endovascular procedures (11%) (Fig. 50-24). In-hospital complications occurred in 40% of patients undergoing surgical treatment and in 21% of those undergoing endovascular therapy. These findings remained true even after comparing patients with similar comorbidities. Interestingly, in-hospital mortality was similar for those patients that underwent endovascular therapy and those that were treated medically, and significantly better than in those that underwent surgery. The low mortality risk of patients treated medically is obviously related to the fact that most of these patients presented with uncomplicated disease and were deemed eligible for medical management. However, the data suggest that endovascular therapies may improve outcomes of patients with complicated type B aortic dissection to the rate of those with uncomplicated dissections managed medically. The data have to be interpreted with caution though, because part of these findings may be associated to selection bias in treatment modality based on patient characteristics not accounted for, or in differing treatment strategies in different institutions.

Natural History

Chronic type A dissection develops in patients who fail to undergo immediate surgical treatment of the acute dissection. Patients with chronic type B aortic dissection include those that have been successfully managed medically after an acute dissection and those with repaired type A aortic dissections that have retained segments of dissected descending thoracic aorta.

Patients with a history of acute aortic dissection, especially those with retained dissected segments, require close surveillance indefinitely. Our preference for follow-up imaging in patients with normal renal function and no contrast allergy is CTA. CTA provides good imaging, is cost effective, and is usually the technique used for the original acute dissection, making it ideal for longitudinal comparison of studies. MRA is utilized mostly as a follow-up study for patients with renal insufficiency but is the study of choice to provide precise anatomical detail for operative planning.

Many patients treated for aortic dissection, especially those with persistent communication between true and false lumens, can progress to develop aneurysmal dilatation of the aorta. They carry similar risk for aortic rupture that that of atherosclerotic aneurysms. Chronic dissections have an annual rate of expansion of 0.9 to 7.2 mm per year.69–71 Despite appropriate medical management and close follow-up, approximately 20 to 40% of patients have aortic enlargement during follow up.69,72 This number is probably even higher in those patients with connective tissue disorders. In one study of 50 patients over a period of 40 months, 18% had fatal rupture and another 20% underwent surgical repair because of symptoms or aneurysm enlargement, emphasizing the need for diligent follow-up care.69 Risk factors for rupture of chronic type B dissection include older age, COPD, and hypertension. Chronic beta-blocker treatment reduces the rate of aortic dilatation as well as the incidence of dissection-related hospital admissions and procedures.72

The presence of a patent false lumen is a significant predictor of aortic enlargement and is associated with a higher incidence of aortic-related complications and death.53,70 In a study of 101 patients followed after medical management for type B uncomplicated dissections, the most important risk factors for aortic enlargement were a maximum aortic diameter of 4 cm or more and a patent false lumen.73

Operative Indications

The operative indications for chronic type A and B dissection are shown in Table 50-5. Chronic type A dissection is rarely symptomatic yet a minority of patients will present with chest pain as a result of aneurysm expansion or heart failure related to aortic regurgitation. Chronic type B dissection may present with intermittent dull chest or back pain, or infrequently, with malperfusion syndrome. Although each of these findings is an indication for intervention, the most common indications for operative management are aneurysmal dilatation of the aorta, rapid expansion, or aortic rupture. The size criteria for intervention in aortic dissection are controversial, but the one generally used is similar to that for general thoracic aortic aneurysms. Based on these criteria, aortic intervention is indicated at a size of 5.5 cm for type A dissections and 6.5 cm for type B dissections, or slightly less if there is a family history or physical stigmata of a connective tissue disorder.74 Similarly, aortic expansion greater than 1 cm per year should be considered an indication for repair.

Recent retrospective studies from Japan have shown that patients with aortic diameters greater than or equal to 4 cm and a patent false lumen have a high likelihood of aortic-related complications (up to 50% in 6 months).73,75 Based on these findings, it has been suggested that these patients should be considered for early repair depending on their operative risk. This issue is controversial and further studies are required before making more definitive recommendations.

Techniques for Chronic Type A Aortic Dissection

Chronic type A dissection, with or without aneurysmal enlargement, is treated using similar operative techniques described for acute dissection. The particular operation performed depends on the specific pathology involving the aortic root, status of the aortic valve, distal extent of dissection, and brachiocephalic vessel involvement. The pathology of each of these components can be very different in a chronic dissection as compared with the acute process. These differences underlie the need for surgical techniques appropriate to each unique abnormality. In general, the ascending aorta is replaced using a vascular graft to include the entire diseased segment as in acute dissection, but surgical treatment of the aortic valve and creation of the distal anastomosis differ.

Whereas the aortic valve can be repaired in most cases of acute type A dissection by simple commissural resuspension, the rate of aortic valve replacement is much higher in patients with chronic dissection. Preservation of the aortic valve is complicated by morphologic changes in the valvular apparatus such as leaflet elongation and annuloaortic ectasia, which render the valve irreparable in as many as 50% of cases. More severe grades of preoperative aortic regurgitation portend a lower probability of valve preservation. In cases in which the aortic valve cannot be preserved with simple commissural reattachment, three options exist to treat aortic insufficiency: composite root replacement, aortic valve replacement with separate ascending aortic replacement, and finally, valve-sparing aortic root repair. The technical aspects of composite valve-graft repair were covered under the surgical management of acute type A dissection. Separate aortic valve and ascending aortic replacement are appropriate when there is an operative indication to repair the ascending aorta in the setting of a normal aortic root and structural aortic valve disease. Note that this operation is not appropriate for patients with connective tissue disease. In this situation, aortic root replacement is required.

Several methods for aortic valve preservation have been described for cases affecting the aortic root. One such technique is performed by reimplanting the valve commissures into an appropriately sized vascular graft, which is secured to the left ventricular outflow tract using multiple horizontal mattress sutures.76 Another elegant yet time-consuming technique requires resection of the sinuses of Valsalva leaving a 5-mm rim of aorta circumferentially around the leaflets.77 Scallops are then created in the vascular graft to resuspend the commissures and remodel the aortic root. David et al advocate Teflon felt reinforcement of the aortic annulus to prevent late annular dilatation and recurrent aortic insufficiency for the remodeling technique.78 The mid-term outcome of such operations revealed a freedom from reoperation rate of 97 to 99% at 5 years and a 5-year survival for the aortic dissection subgroup of 84%. Cochran et al devised a similar technique to recreate the sinuses of Valsalva, which may be more important than previously recognized and contribute to improved long-term valve durability.79 Such data in patients with chronic dissection are lacking. These techniques appear appropriate for patients with Marfan disease and in those with congenitally bicuspid aortic valves.

Treatment of the distal aorta in chronic type A dissection is somewhat controversial. Some advocate obliteration of flow in the false lumen with distal aortic repair, whereas others purposely maintain flow into both the true and false lumen using distal resection of the intimal flap. Those who reunite the chronically dissected aortic layers to perfuse only the true lumen maintain that false lumen perfusion continues through distal reentry tears in greater than 50% of cases. There is a theoretical concern that important side branches arise exclusively from the false lumen and perfusion may be interrupted with this technique. Our practice at the University of Virginia is to resect the distal chronic dissection flap as far as possible to obviate such concerns. The distal anastomosis is therefore made to the outer wall of the aorta, which has been strengthened over time. Malperfusion of the brachiocephalic vessels as a result of chronic type A dissection is treated with resection of the dissection flap from the arch. Infrequently, the chronic dissection flap extends into more distal branch vessels and may present as transient ischemic attacks or stroke. In such cases it is often necessary to resect the dissection flap into the branch vessel or reunite the layers distally before reimplantation.

Infrequently, chronic type A dissection results in extensive aneurysmal dilatation of the aorta extending from the ascending aorta through the arch and into the descending thoracic aorta. Surgical treatment of such extensive disease has traditionally been performed as a staged procedure in which the ascending aorta and arch are replaced first through a sternotomy. The second stage of the so-called elephant trunk procedure is performed 6 weeks later through a left thoracotomy for replacement of the descending aorta using a second vascular graft. Originally described by Borst et al, this technique has been used extensively with good results.80 In some cases, the aorta distal to the left subclavian artery may be so large as to preclude the use of a two-stage repair. Kouchoukos et al. have described a single-stage repair performed through a bilateral anterior thoracotomy in which the arch is repaired first during a brief period of circulatory arrest. Right subclavian and femoral artery cannulation for cardiopulmonary bypass provide proximal and distal perfusion during the subsequent ascending and descending aortic replacement. The hospital mortality was 6.2% and there were no adverse neurologic outcomes in the small series.81 Postoperative and long-term management of these cases are identical to the acute repairs, but with an emphasis on monitoring for evidence of malperfusion.

Techniques for Chronic Type B Aortic Dissection

Endovascular Therapy

Endovascular therapy has been recently used for management of chronic type B aortic dissections38,46,47,54,63,64 (see Table 50-6). The goal of endovascular therapy is to seal any dissection entry points, promote thrombosis of the false lumen, induce aortic remodeling, and prevent further aneurysmal dilatation, rupture, or malperfusion.

The general considerations, preoperative assessment, and operative technique for endovascular therapies in chronic dissection are similar to the ones described for acute type B dissection. Although percutaneous fenestrations may be used in cases of malperfusion, most chronic dissections require treatment for aneurysmal dilatation and aortic expansion because of the persistence of a false lumen, an obligate consequence of fenestrations. Therefore, the endovascular treatment of choice for most chronic dissections is endovascular grafting. Because of the increased risk of spinal malperfusion, lumbar drainage for spinal protection is used liberally in chronic dissections.

Preoperative planning with CTA or MRA is important to define the anatomy and plan the deployment of the endovascular graft. Multiple connections between the true and the false lumen may be identified in chronic dissections. It is important to seal all entry points with the endovascular graft to depressurize the false lumen. Follow-up imaging and close surveillance after the endovascular procedure are necessary.

Surgical Therapy

The purpose of surgical intervention in chronic aortic dissection is to replace all segments of dissected aorta at risk for rupture and prevent the possibility of subsequent malperfusion syndrome. The conduct of the operation including surgical approach, monitoring lines required, anesthetic technique, and cardiopulmonary bypass is similar to that described for acute dissections. Greater emphasis is placed on methods of spinal cord protection.

The incidence of paraplegia after repair of thoracoabdominal aneurysms resulting from aortic dissection is reportedly as high as 25%.82 Both mechanical and pharmacologic interventions have been advocated over the last decade to reduce this risk. Partial left heart bypass alone for replacement of the thoracic aorta above the level of T9 can reduce paraplegia rate to between 5 and 8%.83 We routinely use a lumbar drain for aneurysms extending below T9.50 Reimplanting intercostals and lumbar arteries between T9 and L1 can be an important adjunct.84 The aortic cross-clamp is sequentially moved distally to perfuse branches as they are reimplanted. The combination of distal perfusion, cerebrospinal fluid drainage, and reimplanting large intercostal and lumbar arteries has significantly reduced the incidence of paraplegia at our institutions. Additional techniques used for spinal cord protection include measurement of sensory and motor evoked potentials, regional epidural cooling, and the use of a variety of pharmacologic agents for cellular protection.

The techniques used for replacement of the descending thoracic aorta are identical to those described for treatment of acute type B dissection. The extent of resection, however, for chronic type B dissection is usually greater with the goal to remove all dissected aorta at risk for rupture or symptoms. Usually these operations can be performed through the left chest, but more extensive aneurysms or cases of visceral malperfusion require a thoracoabdominal incision or a staged repair. The proximal anastomosis is ideally made to undissected normal aorta but infrequently the distal arch is involved, which requires alteration in surgical strategy.

As mentioned, we prefer the combination of partial left heart bypass and cerebrospinal fluid drainage for spinal cord protection. Sites for cannulation are the left inferior pulmonary vein and the left femoral artery or descending thoracic aorta. Depending on location and extent of aneurysm, the distal arch is mobilized first. The area between the left common carotid and left subclavian artery is circumferentially dissected, and the left subclavian artery is independently controlled. Partial left heart bypass is then initiated. Ideally, clamps are placed between the left subclavian and left common carotid arteries and on the aorta distal to the involved segment. If the entire descending thoracic aorta is diseased, the clamp is placed on the mid-thoracic aorta to perform the proximal anastomosis first. The aorta is opened and small intercostal arteries are oversewn. The proximal anastomosis is made to normal aorta whenever possible with running 3-0 Prolene; 4-0 Prolene is used if the tissue is fragile. The clamp is moved distally onto the graft to inspect the proximal anastomosis and achieve hemostasis. Several centimeters of the dissection flap is then resected from the lumen of the distal aorta and the distal anastomosis created to the adventitia of the chronic dissection. In more extensive thoracoabdominal disease, the clamp is progressively moved distal as intercostal arteries below T7 to L2 and visceral vessels are reimplanted (Fig. 50-25). Bypass is terminated and the operation completed.

Full cardiopulmonary bypass with deep hypothermic circulatory arrest may be necessary in cases in which the proximal anastomosis cannot be safely or adequately performed with a clamp in the usual position.

Results

The operative mortality for chronic type A dissections is between 4 and 17%.84,85 The stroke rate after repair is 4%, with early neurologic complications occurring in 9%.27 Regular follow-up of the aortic valve is necessary when the native valve is preserved at the initial operation. This is best performed using TTE on a yearly basis. Early reports indicated that nearly 20% of patients require reoperation secondary to progressive aortic regurgitation. The most recent data from David et al., however, reveal a 90 ± 4% 5-year freedom from severe or moderate aortic insufficiency in patients with aortic root aneurysm and 98 ± 2% in patients with ascending aortic aneurysm following valve-sparing operation.78

The perioperative mortality after surgical treatment of chronic type B dissections has been reported to be as low as 10% with a rate of permanent paraplegia of 9%.27 Long-term survival for type A and B chronic dissections is similar with rates at 1, 5, 10, and 15 years after surgery of 78, 60, 45, and 27%, respectively.86 Approximately one third of deaths are cardiac-related, and at least 15% of deaths are related to complications or extension of the aortic dissection.

Recently, endovascular therapies have been used for management of chronic type B aortic dissections. The outcomes cited on Table 50-6 are similar to the outcomes after endovascular treatment of acute dissections. Although the results of these studies look promising, this topic needs to be studied further before recommendations or indications of endovascular repair in chronic type B dissections are elucidated.

Considerable improvement in the treatment of patients with acute and chronic aortic dissection has occurred over the last 50 years. The management of aortic dissections will continue to evolve as improved medical, endovascular, and surgical techniques are refined. At the present time, surgical therapy remains the standard of care for acute type A aortic dissections. Uncomplicated type B dissections should continue to be treated with optimal medical management and close surveillance until further studies define whether "prophylactic" endovascular repair will play a role in these patients. The treatment of acute complicated type B dissections has significantly evolved over the last few years, with endovascular therapies now providing an alternative to surgical therapy. The long-term outcomes of endovascular treatment are still unclear but appear to be promising. Patients undergoing operative intervention for aortic dissection will undoubtedly benefit from the novel basic and clinical research taking place in the areas of spinal cord and cerebral protection, strategies for cardiopulmonary bypass, improved vascular graft and endograft technology, and procedures for preservation of the aortic valve. Such progress may even permit advancement in our greatest remaining clinical challenge, those patients who are hemodynamically unstable following aortic dissection.