Nearly two-thirds of postoperative myocardial infarctions occur during the first 3 days postoperatively (189). Although the pathophysiologic factors are complex, the causes of postoperative myocardial ischemia and infarction are related to decreased myocardial oxygen supply coupled with increased myocardial oxygen requirements. In postoperative patients, conditions that decrease oxygen supply to the myocardium include tachycardia, increased preload, hypotension, anemia, and hypoxia (190). Conditions that increase myocardial oxygen consumption are tachycardia, increased preload, increased afterload, and increased contractility. Tachycardia and increased preload are the most important causes of ischemia, because both conditions decrease oxygen supply to the myocardium while simultaneously increasing myocardial oxygen demand. Tachycardia decreases the diastolic time, which, when the coronary arteries are perfused, decreases the volume of oxygen available to the myocardium. Increased preload increases the pressure exerted by the myocardial wall on the arterioles within it, thus decreasing myocardial blood ﬂow.
Other factors associated with perioperative myocardial ischemia include physiologic responses to the stress of intubation, intravenous or arterial line placement, emergence from anesthesia, pain, and anxiety. These stresses result in catecholamine stimulation of the cardiovascular system, resulting in increased heart rate, blood pressure, and contractility, which may induce or worsen myocardial ischemia. Loss of intravascular volume because of third spacing of ﬂuids or postoperative hemorrhage can induce ischemia.
Postoperative myocardial infarction is often difficult to diagnose. Chest pain, which is present in 90% of nonsurgical patients with myocardial infarction, may be present in only 50% of patients with postoperative infarction because myocardial pain may be masked by coexisting surgical pain and the use of analgesics (175). It is important to maintain a high level of suspicion for postoperative infarction in patients with coronary artery disease. The presence of arrhythmia, congestive heart failure, hypotension, dyspnea, or elevations of pulmonary artery pressure may indicate infarction and should prompt a thorough cardiac investigation and electrocardiographic monitoring. Measurement of creatinine phosphokinase myocardial band (CPK-MB) isoenzyme and troponin T levels are the most sensitive and specific indicators of myocardial infarction, and assessments should be obtained for all patients suspected of myocardial infarction (189).
Despite the high incidence of silent myocardial infarction, routine use of postoperative electrocardiography (ECG) for all patients with cardiovascular disease is controversial.
Many patients will exhibit P-wave changes that spontaneously resolve and do not represent ischemia or infarction. Conversely, patients with proven myocardial infarctions may show few, if any, ECG abnormalities. The American College of Cardiology and American Heart Association advise consideration of postoperative surveillance via ST-segment monitoring for myocardial infarction in patients with known or suspected coronary artery disease (191). In a review of over 2,400 patients, the sensitivity of predicting postoperative cardiac events was 55% to 100%, specificity 37% to 85%, positive predictive value 7% to 57%, and negative predictive value 89% to 100% (192). If routine screening of asymptomatic patients is desired, ECG should be performed 24 hours following surgery because significant ECG changes that occur immediately postoperatively will persist for 24 hours. It is prudent to continue serial ECG assessments for at least 3 days postoperatively.
Postoperative management of patients with coronary artery disease is based on maximizing delivery of oxygen to the myocardium and decreasing myocardial oxygen utilization.