Current concepts of pathophysiology, circadian patterns, and vasoreactive factors associated with myocardial ischemia detected by ambulatory electrocardiography.

Transient ST-segment changes during continuous ECG monitoring occur not only in many clinical ischemic syndromes, but also in a proportion of the normal population. The pathophysiology of episodes of ST-segment change that represent transient periods of myocardial ischemia varies according to the underlying disease process, which may include stable coronary artery disease, unstable angina, variant angina, and syndrome X. Patients with stable coronary artery disease have episodes of ischemia as a result of an imbalance between increases in myocardial oxygen demands and changes in coronary blood flow due to physiologic changes in coronary vasomotor tone. Both these factors are subject to a circadian rhythm that results in a preponderance of ischemia in the morning hours. Vasospasm, often beyond the physiologic range, in localized segments of epicardial coronary arteries causes ischemia and ST-segment changes in variant angina, whereas luminal thrombosis with superimposed vasoconstriction is an important cause of continued ischemia in unstable angina.