A 65-year-old man with hypertension, familial hypercholesterolemia, type 2 diabetes mellitus, and angina pectoris presented for resection of a sigmoid colon tumor. Stress imaging demonstrated an anteroseptal region of ischemia. Coronary angiography showed a critical lesion of the left anterior descending coronary artery and a 50% stenosis of the proximal circumflex coronary artery. Percutaneous transluminal coronary angioplasty with drug-eluting stent (DES) implantation was performed successfully on the left anterior descending lesion 6 weeks before surgery. The patient was maintained on metoprolol, aspirin, and clopidogrel therapy. Clopidogrel was discontinued 7 days before surgery.
General anesthesia was induced with etomidate, midazolam, and fentanyl. Maintenance anesthesia consisted of oxygen, sevoflurane, and fentanyl. Muscle relaxation was provided with vecuronium. During tumor mobilization, the heart rate increased from 70 to 120 beats per minute. Blood pressure remained stable at 130/70 mm Hg. On the V5 electrocardiogram (ECG) lead, 2 mm of horizontal ST-segment depression was noted, but no abnormality was seen in lead II. An additional dose of fentanyl was associated with slowing of the heart rate to 95 beats per minute but no change in the ST-segment depression in V5.
What are the determinants of myocardial oxygen supply?
A major concern in the anesthetic management of patients with coronary artery disease (CAD) is maintaining a favorable balance between myocardial oxygen supply and demand ( Figure 1-1 ). Myocardial oxygen supply is tenuous in patients with CAD because blockages in coronary arteries by atherosclerotic plaques, thrombi, and emboli disrupt the flow of oxygen-rich blood to heart muscle distal to the obstruction. Coronary perfusion is preserved by maintaining both coronary perfusion pressure and length of the diastolic interval. The left coronary artery is perfused during diastole. The right coronary artery is perfused during both diastole and systole. It is important to prevent shortening of the diastolic interval by preventing increases in heart rate. Coronary perfusion pressure is maintained by ensuring normal to high diastolic arterial pressure and normal to low left ventricular end-diastolic pressure (LVEDP).
Explain the determinants of myocardial oxygen consumption (demand).
Heart rate, myocardial contractility, and myocardial wall tension are the three major determinants of myocardial oxygen consumption. Heart rate is probably the most important parameter regulating myocardial oxygen supply-demand balance. Decreasing heart rate increases oxygen supply by prolonging diastole (allowing for more subendocardial perfusion) and decreases oxygen demand. The association between tachycardia and myocardial ichemia is well documented. Severe bradycardia should be avoided because it causes decreased diastolic arterial pressure and increased LVEDP. β-Adrenergic blocking drugs are commonly used to maintain mild bradycardia in patients with CAD.
Myocardial contractility refers to the ability of the heart to generate force at a given preload. Myocardial contractility is very difficult to measure and is poorly described by cardiac output or even left ventricular ejection fraction. Determination of loading conditions and measurement of velocity, force, and extent of muscle shortening facilitate description of myocardial contractility. Decreased myocardial contractility is associated with decreased myocardial oxygen demand, and decreasing myocardial contractility may be beneficial in patients with CAD. Specifically, agents that depress myocardial contractility but are not potent vasodilators may be beneficial as long as coronary perfusion pressure is maintained. Examples of such agents include midazolam and etomidate.
All volatile anesthetic agents decrease systemic blood pressure by decreasing vascular resistance in a dose-dependent fashion. Although isoflurane is the most widely studied volatile anesthetic for its effect on coronary artery dilation and coronary artery steal, sevoflurane and desflurane show similar mild vaodilating effects. Intravenous anesthetic agents such as propofol, midazolam, etomidate, and ketamine have shown slight negative inotropic effects, but thiopental may have strong negative inotropic effects, which could explain the hypotension associated with its use. Dexmedetomidine is associated with decreases in heart rate and cardiac output in a dose-dependent manner but usually offers excellent hemodynamic stability. “Myocardial depressants” could be useful for patients with CAD as long as coronary perfusion pressure is maintained because they theoretically decrease myocardial oxygen demand.
Myocardial oxygen supply and demand are kept in balance by properly managing left ventricular preload, afterload, heart rate, and contractility. Major increases in preload (left ventricular end-diastolic volume) add to the volume work of the heart (increased demand) and decrease coronary perfusion pressure because of the associated increase in LVEDP (decreased supply). Nitroglycerin assists in maintaining a normal to low preload (see later). Excessive increases in afterload result in increased pressure work of the heart (wall tension) during systole (increased demand) despite the increase in coronary perfusion pressure. At the other end of the spectrum, extreme vasodilation (decreased afterload) decreases the diastolic arterial pressure and decreases myocardial oxygen supply ( Table 1-1 ).