Efficiency of Cardiac Contraction in Ischemic Heart Studied in Conscious Dogs

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We investigated how natural stresses affects efficiency of ventricular contraction in conscious dogs under the control condition, exercise and left ventricular dysfunction. We evaluated efficiency of ventricular contraction and its coupling with the arterial system in terms of optimal load and optimal heart. The optimal load is defined as that which extracts maximal work from a given ventricle. The optimal heart is defined as that which meets peripheral demand with minimum oxygen cost per unit time. Under the control condition at rest, the arterial system extract more than 95% of maximal external work from a given heart. At the same time, the hearts supplied flow to the peripheral system with a nearly minimum oxygen cost. With exercise, heart rate and cardiac output were nearly doubled. Both end-systolic elastance (Ees) and effective arterial elastance were increased while their ratio remained unchanged. Despite large changes in hemodynamics, external work of the heart was close to its maximal value. The oxygen consumption of the heart was also close to minimum in the presence of dramatically increased peripheral demand. Creation of cardiac dysfunction significantly increased the oxygen consumption of the heart. The external work of the heart, however, remained close to its maximal values. We conclude that the normal heart generates maximal external work with minimum oxygen cost during exercise as well as at rest. Deterioration of left ventricular function significantly lowers these optimalities.