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Coronary Artery Bypass Surgery
Cardiovascular disease is the leading cause of death of both sexes in the United States and all industrialized nations, and is increasingly becoming an important cause of death in developing countries. According to the Centers for Disease Control and Prevention, approximately 600,000 people die annually in the United States as a result of cardiac disease; 150,000 of them are aged younger than 65 years. In 2017, approximately 790,000 Americans presented with a myocardial infarction (MI). Of these, approximately 525,000 were new MIs, and approximately 210,000 were recurrent MIs. Acute and chronic coronary syndromes result in inadequate delivery of oxygen to the myocardium and subsequent disturbances in oxidative metabolism. Insufficient coronary flow of nutrients to myocardial cells results in angina. If prolonged, myocardial ischemia leads to myocardial cell death. The most straightforward solution to this interruption of blood flow through coronary arteries is to bring new or additional blood flow through alternative pathways, thus bypassing the obstructed coronary arteries. The development of coronary artery bypass graft (CABG) surgery was fostered by this understanding.
Etiology and Pathogenesis
The presence of risk factors for atherosclerosis—advanced age, genetic predisposition, male sex, hypertension, diabetes mellitus, renal disease, hyperlipidemia, and cigarette smoking—all result in a propensity for the normally thin intima of coronary arteries to increase in both thickness and smooth muscle cell content. This earliest stage of atherosclerosis is caused by the proliferation of smooth muscle cells; the formation of a tissue matrix of collagen, elastin, and proteoglycan; and the accumulation of intracellular and extracellular lipids. Thus, the first phase of atherosclerotic lesion formation is focal thickening of the intima with an increased presence of smooth muscle cells and the extracellular matrix. Intracellular lipid deposits also accumulate. Next, lesions called fatty streaks form. A fatty streak is an accumulation of intracellular and extracellular lipids that are visible in diseased segments of affected arteries. As the lesion evolves, a fibrous plaque can form from continued accumulation of fibroblasts that cover proliferating smooth muscle cells that are laden with lipids and cellular debris. Plaques progress in complexity as ongoing cellular degeneration leads to ingress of blood constituents and calcification. The necrotic core of the plaque may enlarge and become calcified. Hemorrhage into the plaque may disrupt the smooth fibrous surface, causing thrombogenic ulcerations. Clot organization on the plaque surface often occludes, or nearly occludes, the arterial lumen, further decreasing blood flow (see also Chapter 14 ).
Just as the rapidity of atherosclerotic lesion formation varies from individual to individual, the presentation of ischemic heart disease also varies. Objective evidence of myocardial ischemia is identified with concurrent coronary angiographic evidence of flow-limiting atherosclerotic lesions. The need for surgical treatment usually arises from presentation of an individual with an acute coronary syndrome and multivessel coronary artery disease (CAD) or with stable but debilitating angina. Examples of indications for urgent CABG include postinfarction angina, ventricular septal defect, acute mitral regurgitation, free wall rupture, and/or cardiogenic shock in patients admitted to the hospital with acute MI. Each of these acute conditions warrants surgical intervention and revascularization.
Differential Diagnosis
The differential diagnosis of myocardial ischemia includes atherosclerotic and nonatherosclerotic causes of epicardial coronary artery obstruction. Nonatherosclerotic causes include congenital anomalies, myocardial bridges, vascularities, aortic dissection, aortic valve stenosis, granulomas, tumors, and scarring from trauma, as well as vasospasm and embolism. Many of these entities may also be indications for CABG.
Other diseases that mimic angina include esophagitis due to gastrointestinal reflux, peptic ulcer disease, biliary colic, visceral artery ischemia, pericarditis, pleurisy, thoracic aortic dissection, and musculoskeletal disorders.
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