MYOCARDIAL EFFICIENCY

Faithful adherence to the topic of heart efficiency would lead to a discussion of dynamic events, such as resistance, flow, and pressure. The following paragraphs, however, are restricted to a review of two factors that influence the calculation of myocardial efficiency: anaerobic metabolism (glycolysis) and oxygen consumption of the nonbeating heart. The final portions of the paper are devoted to a discussion of myocardial energy utilization. The efficiency of the heart can be determined by the ratio of the thermal equivalent of the work performed to the heat liberated. ’ Therefore, it can be indirectly calculated by comparing the work performed with the myocardial oxygen usage. The former is synonymous with energy utilization; the latter represents total energy production.2 Determinations of myocardial efficiency from the oxygen consumption of the heart cannot be made unless one is sure that anaerobic glycolysis in heart muscle is absent. An additional difficulty in the calculation of myocardial efficiency arises from t.he fact t.hat these calculations are valid only as long as the energy produced, aside from being degraded into heat, is utilized exclusively for “useful work. ” This represents only the mechanical work expended by the heart in overcoming resistance in the systemic and pulmonary circulation and in imparting momentum to the blood stream. Therefore, a knowledge of the oxygen consumption of the non-beating heart is essential in the determination of myocardial efficiency. In the presence of constant external work, the efficiency of the beating heart depends to a large extent on the oxygen consumption of the arrested organ. Catheterization of the coronary sinus has made it possible to study the metabolism of the heart in closed-chest animals and in man? These studies have indicated that fatty acids furnish the greatest contribution to the total energy production of the heart.2 In the fasting state, the relationship between the contribution of carbohydrates and non-carbohydrates to total energy production is about 1 to 3.2 The heart can use all substrates circulating in blood, and in general the amount of any individual substrate utilized depends on its arterial concentration.2 Storage of foodstuffs by the heart occurs; for example, carbohydrates are stored as glycogen and fatty acids as neutral fats. There is no evidence of glycolysis in the normally beating heart in silu.3 Under certain abnormal conditions, however, glycolysis of the heart muscle may play a role in energy production. According to expectation, this occurs when the oxygen supply to the heart muscle diminishes, as for example in myocardial infarction and following arrest of the coronary circulation.46 Under these conditions, lactate and pyruvate concentrations in coronary vein blood exceed those in arterial blood. Anaerobic glycolysis is reversible, provided * The work reported in this paper was supported by funds from the National Heart Institute, Grant No. H-2678 (c), Public Health Service, Bethesda, Md. , and the Life Insurance Medical Research Fund, the American Heart Association, Burroughs Wellcome & Co., Inc., and the Tobacco Industry Research Committee, New York, N. Y.