Many patients requiring AVR have coexistent coronary artery disease (CAD). In North America, more than one-third of AVR procedures are accompanied with coronary bypass graft surgery. This proportion will indeed increase as the surgical population continues to age. Risk assessment for ischemic heart disease is complicated in patients with aortic valve disease because angina may be related to true ischemia from hemodynamically significant coronary lesions, or other causes such as left ventricular wall stress with subendocardial ischemia or chamber enlargement in the setting of reduced coronary flow reserve. Because traditional coronary risk stratification is unreliable in aortic valve patients, at Sunnybrook Health Sciences Center, it is our practice to routinely perform diagnostic coronary angiography on all patients greater than age 35.
Myocardial Protection and Cardiopulmonary Bypass
Isolated AVR is performed using a single, two-stage right atrial venous cannula and an arterial cannula into the ascending aorta for systemic perfusion of oxygenated blood. A retrograde cardioplegia cannula may be placed into the coronary sinus via the right atrium. A left ventricular vent cannula is placed in the right superior pulmonary vein and advanced into the left ventricle to ensure a bloodless field and prevent ventricular distention with aortic insufficiency. Once cardiopulmonary bypass (CPB) is initiated, aorta and pulmonary artery are dissected to expose the anterior aortic root to the left coronary artery. Careful dissection of the pulmonary artery from the aorta ensures that the cross-clamp will be fully occlusive on the aorta and prevents inadvertent opening of the pulmonary artery with the aortotomy incision. Care is needed to present pulmonary artery injury, as this tissue is substantially more friable than the aorta.
After the cross-clamp is applied, myocardial protection is initially delivered as a single dose of high potassium blood through the ascending aorta. This will trigger prompt diastolic arrest unless there is moderate to severe AR. Myocardial protection is maintained by continuous infusion of cold or tepid oxygenated blood cardioplegia delivered via direct cannulation of both coronary ostia after the aorta has been opened. In the case of the short left main coronary artery, antegrade cannulation may preferentially perfuse either the LAD or circumflex system. In the presence of severe coronary artery disease, antegrade cardioplegia may not perfuse myocardial segments distal to significant coronary obstruction. Furthermore, direct cannulation of left main coronary artery risks endothelial injury and potential dissection or promotion of atherosclerosis development.
An alternative method of cardioprotection in aortic valve cases is retrograde cardioplegia, in either intermittent or continuous forms, and used in isolation or in combination with antegrade cardioplegia. This is helpful in patients with significant AR or severe concomitant coronary disease. However, there are some questions regarding the quality of RV perfusion using retrograde alone. If retrograde cannula cannot be guided into the coronary sinus, conversion to bicaval cannulation will allow opening the right atrium and direct placement of the cannula into the coronary sinus. Do not place the retrograde cannula beyond the origin of the right coronary vein ostium in the coronary sinus to ensure adequate right ventricular myocardial protection.
Aortotomy, Valve Excision, and Debridement
Once the cross-clamp has been applied and cardioplegic arrest has been achieved, the aorta is opened either with a transverse or oblique aortotomy. The low transverse aortotomy is a common approach to the aortic valve when using stented bioprostheses or mechanical valves. The aortotomy is started approximately 10 to 15 mm above the origin of the right coronary artery (RCA) and extended anteriorly and posteriorly. The initial transverse incision over the RCA may also be extended obliquely in the posterior direction into the noncoronary sinus or the commissure between the left and noncoronary cusps (Fig. 33-3). The oblique incision is often used in patients with small aortic roots, in whom root enlargement procedures may be required (see the following) and may also be used to tailor a larger ascending aorta.
Exposure and aortotomy incision. A two-stage venous cannula is in place in the right atrial appendage. The aortotomy (dashed line) may be made in the transverse or oblique direction.
Morphology of the valve is then inspected (Fig. 33-4). The valve cusps are incised with scissors at the right cusp between the right coronary ostium and the commissure between the right and noncoronary cusps (Fig. 33-5). Mayo scissors or special right-angled valve scissors are usually used at this stage and the calcific deposits are removed from the aortic wall. One to two millimeters of tissue are left behind to support a sewing surface. Right cusp excision is carried first toward the left coronary cusp and then toward the noncoronary cusp and the cusp is removed as a single piece if possible. Excision is then carried toward the left and noncoronary commissure along the noncoronary cusp and then the left coronary cusp. A moistened radiopaque sponge is placed into the outflow area to catch calcific debris, which must be removed before placing the valve sutures. Thorough decalcification is then performed with a scalpel or rongeur. Debridement of all calcium deposits back to soft tissue improves seating of the prosthesis and decreases the incidence of paravalvular leak and dehiscence.
The exposed aortic valve.
The aortic valve after leaflet excision.
Care must be taken to prevent aortic perforation while calcific deposits are debrided from the aortic wall, particularly at the commissure between the left and noncoronary cusps, where perforation is most likely. Several anatomic relationships must be respected during valve excision (Fig. 33-6). The bundle of His (conduction system) is located below the junction of the right and noncoronary cusps in the membranous septum. Deep debridement in this area can result in permanent heart block. The anterior leaflet of the mitral valve is in direct continuity with the left aortic valve cusp. If it is damaged during decalcification, an autologous pericardial patch is used to repair the defect.
Anatomical relationships of the aortic valve.
Once debridement is completed, the aortic root is copiously flushed with saline while the left ventricular vent is stopped. To prevent pushing debris into the left ventricle, saline in a bulb syringe is flushed through the left ventricular vent and out the aortic valve in an antegrade manner instead of retrograde through the valve. The irrigation solution is suctioned with the external wall suction and not into the cardiotomy suction.
After the native valve has been excised, the annulus is sized with a valve-sizer designed for the selected prosthetic device. The valve is secured to the annulus using 12 to 16 double-needled interrupted 2-0 synthetic braided pledgeted sutures that are alternating in color. The pledgets can be left on the inflow/ventricular side or the outflow/aortic side of the aortic annulus (Figs. 33-7 and 33-8). Placing the pledgets on the inside of the annulus allows supra-annular placement of the valve and generally will allow implantation of a slightly larger prosthesis. In cases in which the coronary ostia are close to the annulus, supra-annular placement may only be possible along the noncoronary cusp. Mattress sutures are first placed in the three commissures and retracted to assist visualization. Some surgeons will place the commissural suture between the right and noncoronary cusps from the outside of the aorta (ie, the pledget is left on the outside of the aorta) to prevent injury to the conduction system. Pledgeted mattress sutures are then placed in a clockwise fashion typically starting in the noncoronary cusp. Sutures may be placed into the sewing ring of the prosthetic valve with each annular suture or after all annular sutures are placed. The sutures for each of the three cusps are held separately with three hemostats and retracted while the prosthesis is slid into the annulus. Sutures are then tied down in a balanced fashion alternating among the three cusps.
Placement of sutures with pledgets below the annulus.
Placement of sutures with pledgets above the annulus.
Aortic Closure and De-Airing
The aorta is closed with a double row of synthetic 4-0 polypropylene sutures. The first suture line is started on the right side at the posterior end of the aortotomy, and the double-needled suture is secured slightly beyond the incision to ensure there is no leak in this region. One end of the suture is run as a horizontal mattress anteriorly to the midpoint of the aortotomy, and then the second end of the suture is run anteriorly, slightly superficial to the horizontal mattress suture, in an over-and-over manner. On the left side, a similar technique is performed, the aorta is de-aired (described in the following), and the two sutures are tied to themselves and to each other at the aortotomy midpoint.
During AVR, air is entrained into the left atrium and ventricle, and aorta. This must be removed to prevent catastrophic air embolization. Immediately before tying the suture of the aortotomy, the heart is allowed to fill, the vent in the superior pulmonary vein is stopped, the lungs are inflated, and the cross-clamp is briefly partially opened. The influx of blood should expel most air from these cavities out of the partially open aortotomy. Closure of the aortotomy is then completed and the cross-clamp is fully removed. The cardioplegia cannula in the ascending aorta and the left ventricular vent are then placed on suction to remove any residual air as the heart begins electrical activity. A small needle (21-gauge) is used to aspirate the apex of the left ventricle and the dome of the left atrium. To prevent air entrainment, the left ventricular vent must be removed while the pericardium is filled with saline irrigation. De-airing maneuvers are verified with visualization using transesophageal echocardiography to verify that all air has been removed from the left side of the heart. Vigorous shaking and careful manual compression of the heart while suctioning through the aortic vent (ie, cardioplegia tack) is helpful to remove air trapped within trabeculations. Once de-airing is complete, the aortic vent is removed. The patient is then weaned from CPB and decannulated in the standard fashion. If patients are pacemaker-dependent when weaned from CPB in the operating room, it is recommended to insert atrial pacing wires to allow for synchronous atrioventricular pacing.
Concomitant Coronary Artery Bypass Grafting
Operative technique is modified when there is concomitant CAD to optimize myocardial protection. Distal anastomoses are performed before AVR so that antegrade cardioplegia may be administered through these grafts during the operation. The left internal mammary artery should be used for revascularization of the left anterior descending artery, because this may improve long-term survival in aortic valve patients.29 This anastomosis is performed after the aortotomy is closed to ensure that the coronary circulation is not exposed to systemic circulation during cardioplegic arrest and to prevent trauma to the anastomosis during manipulation of the heart.
Concomitant Ascending Aortic Replacement
In general, the ascending aorta is replaced electively when maximal diameter exceeds 5.5 to 6.0 cm; however, with Marfan's syndrome or equivalent connective tissue disorder the cutoff is 4.5 to 5.0 cm. In the setting of concomitant AVR, aortic replacement is advised if the ascending aortic diameter is greater than 5.0 cm. Patients with bicuspid aortic valves have an underlying aortopathy that leads to significant risk of late ascending aortic complications, and these patients should have replacement of their ascending aorta if its diameter exceeds 4.5 cm at the time of AVR30 (Fig. 33-9).
Freedom from ascending aortic complications for patients with a bicuspid aortic valve with an ascending aortic diameter of less than 4 cm, 4.0 to 4.5 cm, and 4.5 to 4.9 cm at the time of aortic valve replacement. (Reproduced with permission from Borger MA, et al: Should the ascending aorta be replaced more frequently in patients with bicuspid aortic valve disease? J Thorac Cardiovasc Surg 2004; 128:677.)
Aortic Root Enlargement Procedures
Detailed descriptions of aortic root enlargement procedures are presented in a later chapter. Briefly, either an anterior or posterior annular enlargement procedure may be performed in a patient with a small aortic root to allow for implantation of a larger valve. The posterior approach is the most commonly used aortic root enlargement procedure in adults and can increase the annular diameter by 2 to 4 mm. Nicks and colleagues in 1970 described a technique of root enlargement in which the aortotomy is extended downward through the noncoronary cusp, through the aortic annulus to the anterior mitral leaftlet.31 In 1979, Manouguian and Seybold-Epting described a procedure extending the aortotomy incision in a downward direction through the commissure between the left and noncoronary cusps into the interleaflet triangle and into the anterior leaflet of the mitral valve.32 The anterior approach is generally used in the pediatric population. Described by Konno and colleagues in 1975, this technique, which is also known as aortoventriculoplasty, is used when more than 4 mm of annular enlargement is required.33 Instead of a transverse incision, a longitudinal incision is made in the anterior aorta and extended to the right coronary sinus of Valsalva and then through the anterior wall of the right ventricle to open the right ventricular outflow tract. The ventricular septum is incised, allowing significant expansion of the aortic annulus and left ventricular outflow tract.
Reoperative Aortic Valve Surgery
Repeat sternotomy after AVR may be performed for valve-related complications, progressive ascending aortic disease, or CAD. Valve-related causes include structural valve deterioration, prosthetic endocarditis, prosthesis thrombosis, or paravalvular leak. Chest re-entry is the most hazardous portion of any repeat cardiac procedure. At Sunnybrook Health Sciences Center, our routine practice is to obtain an adequate lateral chest x-ray and computed tomography scan to determine the proximity of cardiac structures to the posterior sternum. Cardiopulmonary bypass is instituted through the femoral vessels for any concerns about chest re-entry. An oscillating saw is used to open the sternum and the dissection is kept as limited as possible. Extreme caution must be employed during dissection when there are patent bypass grafts.
Once cardioplegic arrest is established, the old prosthesis is excised with sharp dissection. Care must be taken to remove all sutures and pledgets from the annulus. Annular injuries caused while excising the prosthesis are repaired with pledgeted interrupted sutures. Removal of stentless prostheses may be particularly difficult in this regard. In the setting of endocarditis, aggressive debridement of infected tissue must be performed with appropriate annular reconstruction with pericardium when root abscesses are present.34 All foreign graft material, including Dacron aortic grafts, must be excised in the presence of active endocarditis.
In the presence of a Dacron prosthesis in the ascending aorta, chest re-entry may be extremely hazardous because exsanguination will occur if the graft is inadvertently opened during dissection. To limit the systemic consequences of exsanguination at normothermia, the patient should be placed on femoro-femoral CPB and cooled to 20°C before chest reentry. If the Dacron graft is accidentally opened, local control of the bleeding is established and CPB is stopped. Under circulatory arrest, atrial venous cannulation is instituted and the graft is controlled distal to the tear. Cardiopulmonary bypass may then be restarted. In all repeat aortic procedures, rigorous myocardial protection must be applied because these procedures often have very long ischemic times. Antegrade cold blood cardioplegia is usually employed in a continuous fashion throughout the case by selective cannulation of the coronary ostia. Retrograde cardioplegia may have benefit in the setting of patent old saphenous vein grafts.35