The George E. Brown Memorial Lecture

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Abstract
The results presented are just the beginning of an attempt to study the reactions of the left coronary circulation in the unanesthetized dog to natural stresses of everyday life. They indicate in mild to moderate treadmill exercise, with little change in stroke cardiac output, that the flow and oxygen usage per heart beat does not greatly increase, so that the coronary circulation appears to be largely limited or monitored by the heart rate. During excitement and cardiac sympathetic nerve stimulation, the left coronary vascular bed undergoes massive dilatation, so that the left ventricle is able to increase its coronary circulation as the result of the combined effect of an increased number of heart beats and a greatly increased flow and oxygen usage per heart beat, the latter occurring despite a marked reduction in the duration of systole and of the cardiac cycle, and very little change in the pressure developed by the left ventricle. Although the left ventricular contraction imposes a large myocardial impediment to coronary flow, it permits a very sizable and variable contribution to systolic flow and to left coronary flow. In the resting dog, the left coronary inflow in systole generally approximates 15 to 60 per cent of that during diastole, and under different stress conditions, such as exercise, excitement, drug injections and irreversible hemorrhagic shock, both the systolic and diastolic flow per heart beat can increase by 300 to 400 per cent, the ratio between them remaining the same, or, at times, the volume of systolic flow can equal or exceed the diastolic flow. Evidence has been presented to show that much of this systolic flow represents intramural flow. These findings will, I believe, cause some recasting of our present views concerning the regulation of the coronary circulation.