An Appreciation of the Coronary Circulation

Coronary sinus catheter techniques for evaluation of coronary flow and myocardial metabolism have the drawback that a global sampling method is used to evaluate a regional disease (coronary artery disease). Studies on the coronary circulation are further limited by the fact that interventions acting on the coronary bed may simultaneously modify several of the principal determinants of coronary blood flow. Results are also influenced by differences among species, and whether the coronary vascular bed is normal or pathologically narrowed. Because coronary flow is intimately coupled to myocardial oxygen demand, interpretation of values as abnormal require simultaneous evaluation of some index of myocardial oxygen consumption. Under normal conditions, myocardial flow is predominantly in diastole, and is subject to compromise by factors that abbreviate diastole (e.g., tachyarrhythmias). Autoregulation maintains constant coronary blood flow over a range of perfusion pressures (60-130 mm Hg), and increased flow demands are normally met by coronary vasodilation (coronary flow reserve). In proximal coronary stenosis, this capacity for additional vasodilation may be significantly reduced, and flow to potentially ischemic beds beyond the stenosis may be maintained by collaterals. Pharmacologic coronary vasodilation in this situation can result in coronary steal. When perfusion pressure decreases below the autoregulatory range, or when coronary flow reserve is exhausted early, as in coronary stenosis, flow becomes dependent on mechanical factors including duration of diastole and the perfusion pressure. In these situations, monitoring heart rate and diastolic pressure would allow reasonable assessment of adequacy of coronary flow and myocardial perfusion.