EPIDEMIOLOGY AND RISK ASSESSMENT

In 2001, coronary artery disease (CAD) was estimated to occur in 13.2 million individuals in the United States (6.4%), resulting in approximately 500,000 deaths, 2 million hospital discharges, and a societal cost of $133 billion. CABG surgery is clearly the established cornerstone of treatment of advanced degrees of CAD. Although its absolute frequency has recently declined, there is no doubt that it will remain a common procedure and that its complexity will continue to increase for many decades to come. An understanding of the basic epidemiology of CABG surgery and of risk assessment for patients undergoing it is important for the anesthesiologist for a variety of reasons, including interactions with surgeons and cardiologists; enhancing clinical management of patients by recognizing high-risk characteristics and situations where preoperative management may not be adequate (such that delay of a planned elective procedure or additional perioperative interventions are required); developing a better sense of long-term trends in surgical practice that may impact on future practice volume (e.g., growth or decline of CABG techniques); and changes in complexity of such procedures that may influence reimbursement or additional training requirements.

Preoperative risk assessment for patients undergoing CABG has evolved dramatically over the past 2 decades. The Department of Veterans Affairs in the 1970s established the first large-scale, multicenter surgical outcomes database applying rigorous statistical methodology for comparing outcomes between centers. This group and others have pioneered methodology for adjusting for different severity of illness between patients (i.e., risk adjustment) using multiple preoperative and perioperative variables thought to be of intrinsic value (usually by expert consensus) that could be easily captured and have high consistency of definition.

The Society of Thoracic Surgeons (STS) instituted a voluntary clinical database system with this approach in the early 1990s that has continued to grow rapidly as cardiac surgical groups are increasingly interested in benchmarking their practices against others.⁴ Many tertiary centers (e.g., Cleveland Clinic) and regional consortiums of hospitals (e.g., Northern New England Cardiovascular Disease Study Group) maintain databases, and some publish statistical models. Many states have established and maintain risk-adjusted mandatory reporting systems for hospital and individual surgeon performance (with New York State being an early and influential pioneer). A new scoring system (EuroSCORE) based on outcomes in 128 centers in eight European countries has received increasing attention. It appears to compare favorably with the STS model in North American patients.⁵ It is freely accessible by means of an interactive web-based calculator (www.euroscore.org) and is decidedly simpler and faster to use than the STS’s scoring system, which is now also freely accessible to the public at http://www.sts.org/sections/stsnationaldatabase/riskcalculator/index.html.

PATHOPHYSIOLOGY OF CORONARY ARTERY DISEASE

Anatomy

The anesthesiologist should be familiar with coronary anatomy if only to interpret the significance of angiographic findings. The coronary circulation and common sites for placement of distal anastomoses during CABG are shown in Figures 13-1 to 13-3.

Figure 13-1 Thirty-degree left anterior oblique angiographic view of the heart, which best shows the right coronary artery. Arrows indicate common sites of distal vein graft anastomoses.

(From Stiles QR, Tucker BL, Lindesmith GG, et al: Myocardial Revascularization: A Surgical Atlas. Boston, Little, Brown, 1976.)

Figure 13-2 Ten-degree right anterior oblique angiographic view of the heart, which best shows the left main coronary artery dividing into the circumflex and left anterior descending arteries. Arrows indicate common sites of distal vein graft anastomoses.

(Adapted from Stiles QR, Tucker BL, Lindesmith GG, et al: Myocardial Revascularization: A Surgical Atlas. Boston, Little, Brown, 1976.)

Figure 13-3 Seventy-five-degree left anterior oblique angiographic view of the heart, which best shows branches of the left anterior descending and circumflex coronary arteries.

(Adapted from Stiles QR, Tucker BL, Lindesmith GG, et al: Myocardial Revascularization: A Surgical Atlas. Boston, Little, Brown, 1976.)

The right coronary artery (RCA) arises from the right sinus of Valsalva and is best seen in the left anterior oblique view on coronary cineangiography. It passes anteriorly for the first few millimeters; it then follows the right atrioventricular (AV) groove and curves posteriorly within the groove to reach the crux of the heart, the area where the interventricular septum (IVS) meets the AV groove. In 84% of cases, it terminates as the posterior descending artery (PDA), which is its most important branch, being the sole supply to the posterior-superior IVS. Other important branches are those to the sinus node in 60% of patients and the AV node in approximately 85% of patients. Anatomists consider the RCA to be dominant when it crosses the crux of the heart and continues in the AV groove regardless of the origin of the PDA. Angiographers, however, ascribe dominance to the artery, right coronary or left coronary (circumflex), that gives rise to the PDA.

The vertical and superior orientation of the RCA ostium allows easy passage of air bubbles during aortic cannulation, CPB, or open valve surgery. In sufficient concentration (e.g., coronary air embolus), myocardial ischemia involving the inferior LV wall segments and the right ventricle may occur (Fig. 13-4). In contrast, the nearperpendicular orientation of the left main coronary ostium makes air embolization much less common.

Figure 13-4 The vertical and superior orientation of the right coronary artery arising from the aortic root is identified by transesophageal echocardiography (TEE). The TEE transducer in the esophagus is on the top of the screen, and the patient’s chest wall is on the bottom. Retained air preferentially enters the right coronary artery (RCA), which may cause inferior ischemia, depending on the amount of air and the coronary perfusion pressure. Elevation of perfusion pressure using phenylephrine is often used to treat coronary air embolus. The left main artery (not visible) arises at approximately 3 o’clock on this image.

(Image courtesy of Martin J. London, MD, University of California, San Francisco, CA [www.ucsf.edu/teeecho].)

The left coronary artery arises from the left sinus of Valsalva as the left main coronary artery. This is best seen in a shallow right anterior oblique projection. The left main coronary artery courses anteriorly and to the left, where it divides in a space between the aorta and pulmonary artery (PA). Its branches are the left anterior descending (LAD) and circumflex arteries. The LAD passes along the anterior intraventricular groove. It may reach only two thirds of the distance to the apex or extend around the apex to the diaphragmatic portion of the left ventricle. Major branches of the LAD are the diagonal branches, which supply the free wall of the left ventricle, and septal branches, which course posteriorly to supply the major portion of the IVS. Although there may be many diagonal and septal branches, the first diagonal and first septal branches serve as important landmarks in the descriptions of lesions of the LAD.

The circumflex arises at a sharp angle from the left main coronary artery and courses toward the crux of the heart in the AV groove. When the circumflex gives rise to the PDA, the circulation is said to be left dominant and the left coronary circulation supplies the entire IVS and the AV node. In approximately 40% of patients, the circumflex supplies the branch to the SA node. Up to four obtuse marginal (OM) arteries arise from the circumflex and supply the lateral wall of the left ventricle. All of the previously described epicardial branches give rise to small vessels that supply the outer third of the myocardium and penetrating vessels that anastomose with the subendocardial plexus. This capillary plexus is unique in that it functions as an end-arterial system. Each epicardial arteriole supplies a capillary plexus that forms an end loop rather than anastomosing with an adjacent capillary from another epicardial artery. Significant collateral circulation does not exist at the microcirculatory level. This capillary anatomy explains the very distinct areas of myocardial ischemia or infarction that can be related to disease in a discrete epicardial artery.

CAD most commonly affects the epicardial muscular arteries with rare intramyocardial lesions. However, severe disorders of the microcirculation and primary impairment of coronary vascular reserve in normal coronary arteries have been described, especially in diabetics, female patients, and those with variant angina. Atherosclerosis in all organs is most common at the outer edges of vessel bifurcations, because in these regions blood flow is slower and changes direction during the cardiac cycle resulting in less net shear stress than in other regions with more steady blood flow and higher shear stress. Low shear stress has been shown to stimulate an atherogenic phenotype in the endothelium. Epicardial lesions can be single but are more often multiple. A combined lesion of the RCA and both branches of the left coronary artery is referred to as triple-vessel disease. The left coronary artery supplies the thickest portions of the left ventricle, at least the exterior two thirds of the IVS, and the greater part of the atria. Most bypass grafts are done on the left coronary system.

Venous drainage of the myocardium is primarily to the coronary sinus, which drains 96% of the LV free wall and septum, and the remainder of the venous return goes directly into the right atrium.⁶ A small fraction may enter other cardiac chambers directly through the anterior-sinusoidal, anterior-luminal, and thebesian veins.

Myocardial Ischemia and Infarction

In patients with CAD, myocardial ischemia usually results from increases in myocardial oxygen demand (Fig. 13-5) that exceed the capacity of the stenosed coronary arteries to increase their oxygen supply. However, the determinants of myocardial oxygen balance are complex, and alterations may have several effects. For example, an increase in blood pressure (i.e., increased afterload) increases wall tension and oxygen demand while also increasing coronary blood flow (CBF). It is now appreciated that myocardial ischemia may occur without changes in systemic hemodynamics and in awake patients may occur in the absence of chest pain (i.e., silent ischemia), particularly in diabetic patients.

Figure 13-5 Factors determining myocardial oxygen supply and demand.

In atherosclerotic heart disease, the fundamental lesion is an intimal lipid plaque that causes chronic stenosis and episodic thrombosis, occurring most often in an epicardial coronary artery, thereby reducing myocardial blood supply. Characteristics of the vulnerable plaque include high lipid content, a thin fibrous cap, a reduced number of smooth muscle cells, and increased macrophage activity. The lipid core is the most thrombogenic component of the plaque. Fuster described five phases in the progression of CAD by plaque morphology. Phase 1 is a small plaque present in many people younger than 30 years and usually progresses very slowly depending on the presence of risk factors associated with CAD (i.e., elevated low-density lipoprotein cholesterol). Phase 2 is a plaque with a high lipid content that has the potential to rupture. If it ruptures, it will lead to thrombosis and increased stenosis (phase 5), possibly producing unstable angina or an acute coronary syndrome. The phase 2 plaque usually does not rupture; it instead progresses onto phases 3 and 4, with enlargement and fibrous tissue organization, which may ultimately produce an occlusive plaque at phase 5.⁷

ANESTHESIA FOR CORONARY ARTERY BYPASS GRAFTING

Conventional CABG with CPB is still the most commonly performed cardiac surgical procedure (Box 13-1). Fast-track management with early extubation (4 to 8 hours postoperatively) has become the standard of care in nearly all centers. OPCAB is increasing steadily, although its use tends to be very frequent in some centers or infrequent in others as various surgeons have become “early adopters” or are waiting for firm evidence-based recommendations from future randomized, controlled trials. However, anesthetic management of the sickest patients undergoing multivessel operations combined with valve repair or replacement, repeat operations, and other complex procedures (e.g., ventricular septal defect repairs along with CABG after acute myocardial infarction) has changed relatively little over the past decade as the long duration of surgery usually mandates greater cumulative doses of fixed anesthetic agents with overnight or even prolonged postoperative mechanical ventilation. However, many clinicians have adopted use of infusions of shorter-acting agents (e.g., sufentanil, propofol, remifentanil), avoided large cumulative dosing of fixed agents with potentially long half-lives (e.g., midazolam), and now rely on a volatile anesthesia “base,” taking a “wait and see” attitude toward early extubation if surgery is “smooth” and physiologic parameters remain within acceptable limits (e.g., good urine output, normothermic, adequate hematocrit).