Coronary autoregulation and optimal myocardial oxygen utilization.

The complex relationship among myocardial contractility, preload, afterload, and coronary autoregulation was studied using both analytical and numerical methods. To study autoregulation and coronary reserve changes in response to changes in cardiac oxygen consumption and in arterial pressure generation, a new variable was introduced: myocardial resistance to oxygen flow (RO2). This variable was defined as the ratio of the coronary driving pressure to left-ventricular oxygen uptake. High values for this variable indicate small consumption relative to the generated aortic pressure. Conditions which produce the highest obtainable value for RO2 are considered as optimal. An expression relating RO2 to ventricular hemodynamic variables was developed and studied using a mathematical model of the cardiovascular system. The model included a mechanism of local autoregulation based on the assumption that, in steady state, the amount of oxygen consumed equals the amount extracted from coronary blood. Heart rate, peripheral resistance, end-diastolic volume, and myocardial contractility were varied while the coronary circulation was adjusted to meet ventricular oxygen consumption at each state. The model predicts that, for each state of the circulation, there is an optimal level of cardiac contractility for which the coronary reserve is maximized.