Physiology and pharmacology of the coronary circulation and myocardium, particularly in relation to coronary artery disease

The effect of a severely reduced tissue oxygen tension, whether caused by decreased oxygen delivery, increased oxygen utilization or both, is maximal coronary vasodilation and a hypodynamic ventricle.

Abstract

The heart is unusual in that its blood flow is relatively low in relation to its oxygen consumption. This provides the heart with a lower tissue fluid oxygen tension then any other organ in the body, a condition which may be optimal for blood flow regulation. A small increase in oxygen consumption, as in exercise, or a slight reduction in flow, as in coronary artery disease, is promptly followed by vasodilation, most likely due to accumulation of vasodilating metabolites. This provides the proper flow for the existing metabolic activity. Tissue fluid oxygen tension also influences myocardial function, a reduction in tension causing a decrease in myocardial contractile force. Necrosis follows if the reduction in tension is severe and prolonged for approximately eighteen minutes or more. Thus the effect of a severely reduced tissue oxygen tension, whether caused by decreased oxygen delivery, increased oxygen utilization or both, is maximal coronary vasodilation and a hypodynamic ventricle. Therapy is directed at increasing oxygen delivery and decreasing oxygen utilization. Delivery can be increased by increasing coronary blood flow, arterial oxygen content and oxyhemoglobin dissociation at the tissue level. Utilization can be decreased by reducing the fiber length at the onset of contraction, the strength of the contraction, the oxygen wastage during contraction, and the rate of contraction. The choice of therapy depends upon the circumstances surrounding a particular problem but at least as much attention should perhaps be given the matter of reducing oxygen utilization as the matter of increasing oxygen delivery.