Roles of baroreflex in controlling energy efficiency of ventricular arterial coupling

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The heart contracts incessantly over the life time. The average number of contraction for men exceeds 2.5 billion times. Since each ejection requires large amount of energy, the total amount of energy expenditure by the heart becomes enormous. Thus it is conceivable that, in response to changes in demands for the heart, the regulatory system adjusts ventricular contraction so that energy efficiency of the heart becomes maximal.To test this hypothesis, in chronically instrumented dogs, we measured efficiency of ventricular contraction during exercise. The results indicated that the oxygen consumption of the left ventricle was nearly minimum at rest for a given amount of cardiac work. At the same time, external work of the left heart was maximal for a given preload. These optimal conditions were hardly affected by exercise.To further investigate what regulatory mechanisms are responsible for this optimization, we evaluated the roles of baroreflex. In acute open-chest dogs, we isolated th … More e carotid sinuses and altered intrasinus pressure. We measured left ventricular end-systolic elastance and effective arterial elastance beat-by-beat. The effect of the baroreflex on these elastances were quantitated using transfer functions in the frequency domain. The transfer functions from intrasinus pressure to end-systolic elastance and to effective arterial elastance resembled each other. They showed characteristics of the second order delay system. The gains in the low frequency range were similar. The corner frequencies were around 0.03 Hz and were not significantly different. Since efficiency of ventricular ejection was a function of the ratio of end-systolic elastance to effective arterial elastance, these findings suggested the ratio may well be maintained during the entire time course of baroreflex. This is to say that the optimal condition was maintained when the baroreflex system stabilizes arterial pressure. Whether this optimization is truly nature's goal when ventricular arterial coupling is regulated remains to be investigated. Less