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 openchested 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.’ T HE INFLUENCE of arterial blood pressure and cardiac output on coronary flow and cardiac gaseous metabolism, and the interrelation between the latter two, have been studied recently in this department (1-3). The present study deals with the cha nges in coronary flow and cardiac oxygen consumption induced by changes in heart rate. No clear relationship of coronary flow to heart rate has emerged from the literature. This may be a result of the differences in physiological preparations employed to measure coronary flow (4-6) and th e different methods used to obtain cardiac rate variations (7-1 I). In the heartlung preparation oxygen consumption (I 2) Received for publication December 5, 1955. increases as heart rate is increased, and its increment tremely is less at extremely low rates (13, 14). high In a than more at exintact preparation, Wegria (IS) has recently shown that electrically induced auricular tachycardia results in an increased coronary flow and cardiac oxygen consumption, while the cardiac work and efficiency is decreased. The present study also employed a relatively intact preparation and the experiment was designed to determine the effects of heart rate over a wide range of work loads. Adjustments in cardiac output and blood pressure were predetermined and carefully controlled to keep them constant while the heart rate changes were produced. This was done in order 355 by 10.0.33.6 on S etem er 8, 2017 http://ajple.physiology.org/ D ow nladed fom 356 LAURENT, BOLENE-WILLIAMS, WILLIAMS AND KATZ