Effects of Heart Rate on Coronary Flow and Cardiac Oxygen Consumption

The effects of electrically-induced heart rate (up to 300 beats/min.) on coronary flow and cardiac oxygen consumption has been studied in an open-chested intact preparation. A significant correlation between heart rate, coronary flow and cardiac oxygen consumption has been found at each level of cardiac work. Coronary flow and cardiac oxygen consumption increase with a rise in heart rate and seem to approach a limit at extremely rapid heart rates. Nomograms relating heart rate, work and oxygen consumption of the heart have been constructed. From these it is seen that heart rate is an important factor in determining the myocardial oxygen consumption; this is true at each level of cardiac work. The significance of these findings have been discussed relative to the unanesthetized animal. Under conditions of excessive heart rate or cardiac load which presumably lead to unusually high energy requirements, a radical departure from the expected ‘normal’ values was found in coronary flow, coronary A-V oxygen difference and cardiac oxygen consumption. This confirms the presence of ‘spontaneous’ change previously described by us. Its meaning and significance is discussed. Under these conditions of ‘spontaneous’ change coronary flow was increased, oxygen consumption decreased and coronary venous oxygen was raised as the coronary A-V oxygen difference declined. This ‘spontaneous’ change shifted the relationship of coronary flow to oxygen consumption. Furthermore, it led to a rise in coronary flow despite an elevation in coronary venous oxygen. It would seem that at this time the energy metabolism of the heart must change in that less oxygen is extracted from the blood despite an increase in its availability. The heart may therefore operate anaerobically in part or may make use of other hydrogen acceptors in the blood. The fact that this kind of metabolism may last as long as an hour or more, excludes the ordinary type of ‘oxygen debt.’