Incidence of Carotid Stenosis in Coronary Artery Bypass Patients
In 1977, Mehigan and colleagues,24 used noninvasive testing in 874 patients before coronary grafting and found a 6% incidence of significant extracranial cerebrovascular disease. Ivey and colleagues25 reported that routine ultrasonic duplex scanning for a history of neurologic events or cervical bruits in 1035 patients having isolated CABG revealed significant carotid artery stenosis in 86 patients (8.3%). Faggioli evaluated 539 neurologically asymptomatic CABG patients and found 8.7% had a carotid stenosis greater than 75%.19 The rate rose from 3.8% for patients less than age 60 to 11.3% for patients older than age 60. Berens and colleagues, using routine carotid artery scanning in 1087 cardiac surgery candidates 65 years of age or older (91% with coronary disease), found 186 (17.0%) had a greater than 50% carotid stenosis and 65 (5.9%) had a greater than 80% carotid stenosis.20 Predictors of carotid artery disease were female gender, peripheral vascular disease, history of transient ischemic attacks (TIAs) or stroke, smoking history, and left main CAD. D'Agostino, using noninvasive testing in 1279 CABG candidates, found 262 (20.5%) had greater than 50% stenosis in at least one carotid artery and 23 (1.8%) had bilateral stenoses greater than 80%.26 Significant multivariable predictors of carotid disease were age, diabetes, female sex, left main CAD, prior stroke, peripheral vascular disease, and smoking.
Virtually all studies have emphasized that patient age is the single highest risk variable. One report demonstrated a dramatic, statistically significant increase in patients older than 60 years of age. Diabetes, smoking, and hypertension were important associated risk factors, such that, with all three risk factors in patients older than age 60, the incidence of significant carotid stenosis nearly doubled from about 8% to 14%.22
Diagnosis of Carotid Artery Disease
Essentials of Noninvasive Testing
Ultrasound-based Doppler interrogation for carotid stenosis exploits the Doppler effect; namely, the reflected or altered frequency of an ultrasound wave is shifted in proportion to the velocities of the sampled, flowing blood, which are increased in regions of significant arterial stenosis.
Doppler interrogation produces two data sets—Doppler-shifted flow velocities in regions of interest, and spectral analysis of turbulent flow, which lends a qualitative determination of stenosis severity. Doppler samples must be obtained in the tightest portion of the stenosis for accuracy.
Derived Doppler velocities include peak systolic velocity (PSV), end-diastolic velocity (EDV), and the ratio between the PSV in the internal carotid artery and in the proximal common carotid artery (ICA/CCA ratio). The derived ratio corrects for baseline variations in hemodynamics, such as cardiac output and increased overall flows that might be noted in contralateral internal carotid occlusion. PSV is the single most important criterion, followed by the ICA/CCA ratio. EDV is helpful in discriminating severe versus “very severe” lesions. An ICA/CCA ratio of more than 4.0 equates to a greater than 70% diameter stenosis, the general threshold for a flow-reducing lesion at basal conditions according to the physics of critical arterial stenosis.
Some general comments about the efficacy of ultrasound-based carotid noninvasive tests are in order because many vascular surgeons proceed to carotid endarterectomy based solely on these preoperative studies.15 Surgeons must be familiar with the specifics of noninvasive diagnostic criteria, and laboratories must have quality-control documentation of accuracy. Thorough knowledge of the translation of ultrasound-derived data to corresponding degrees of internal carotid artery stenosis is necessary.
Indications for Noninvasive Testing
Current indications for screening patients for carotid artery disease before surgical myocardial revascularization include:
An audible bruit in the neck
History of a prior stroke
History of transient ischemic attacks
Patients with severe peripheral vascular disease
Patients with a prior carotid endarterectomy
All of these indications are self-explanatory except the last. Because the incidence of carotid stenosis rises dramatically in patients older than age 65, there must be an age above which it is cost-effective to screen all patients for carotid disease. However, that age has not yet been determined. One would have to demonstrate the cost advantage of routine carotid endarterectomy versus that of strokes related to uncorrected carotid stenoses.
Role of Carotid Imaging Modalities
Catheter-Based Carotid Angiography
Although catheter-based carotid angiography yields excellent detailed images of the carotid and intracranial vessels, angiography is expensive, requires potentially nephrotoxic contrast material, and is not without risks, including arterial dissection and stroke. Cholesterol embolization owing to catheter manipulation in a diseased aorta can cause emboli to other vascular distributions, especially renal and/or other visceral arteries. (Indeed, in the Asymptomatic Carotid Atherosclerosis Study, one-half of the 2.3% stroke risk with carotid endarterectomy was referable to mandated angiography.27) For these reasons, conventional carotid angiography had all but vanished from the practice of many vascular surgeons by the year 2000.28 Ironically, current enthusiasm for carotid stenting has resurrected carotid angiography as a diagnostic and therapeutic tool.
Magnetic Resonance Angiography
Great enthusiasm in the 1990s arose for magnetic resonance angiography (MRA) to better define carotid lesions. Compared with catheter-based angiography, it was noninvasive, lacked nephrotoxicity, and when used with diffusion-weighted brain imaging, yielded an accurate map of the intracranial circulation. However, its limitations soon became obvious. Magnetic resonance vascular imaging relies on reflected magnetic pulses of flowing blood cells that vary as a function of flow turbulence. Because turbulent flow is characteristic of high-grade carotid stenoses, signal “dropout” in magnetic resonance imaging (MRI) is common. MRA suffers from low specificity with respect to detection of critical stenosis.29 Indeed, MRA alone can overestimate carotid stenosis severity. Thus, in a reversal of prior algorithms, we insist that stenosis severity information from MRA be verified by a duplex study.
Computed Tomographic Angiography
Computed tomographic angiography (CTA) is the noninvasive test of choice when supplemental information is required after duplex scanning. Although CTA requires iodinated contrast material, it can provide excellent arterial mapping from the aortic arch to the intracranial vasculature, which can be important to both vascular and cardiac surgeons. Accurate assessment of residual lumen diameter within a carotid artery lesion is obtained from both axial and three-dimensional (3D) reconstructed images. Current processing capabilities and the generation of 3D models have improved CTA accuracy in highly calcified lesions.