Energetics of the human heart ∗

Abstract

Observations were made in two groups of patients studied by coronary sinus catheterization. One group had clinical and hemodynamic evidence of left ventricular heart failure, while in the control group, no abnormality of the left ventricle was found. Myocardial oxygen consumption and its relationship to total body oxygen consumption and pressure development by the heart was determined in both groups at rest and during exercise. The major determinant of myocardial oxygen consumption appears to be muscle wall tension, which is governed (according to the Law of Laplace) by the intracavitary pressure and the mean radius of the heart chamber. The geometric shape assumed by a heart chamber defines the relationship between wall tension (or total oxygen consumption) and intracavitary pressure. The observation that the tension-time index per oxygen consumption per unit weight of left ventricle is essentially the same in normal and failing hearts at rest, suggests that hypertrophy in chronic congestive heart failure has kept abreast of the increased tension load imposed by cardiac enlargement. With exercise, the oxygen cost of pressure development by the normal left ventricle was found to fall, suggesting that the mean radius of that chamber had decreased. This change was not noted in the majority of congestive heart failure cases studied. The large rise in effective left ventricular filling pressure and the failure to increase stroke volume in the congestive heart failure group support the view that mean radius of the failing ventricle may actually increase during the stress of exercise. The manner in which volume changes affect the efficiency of a contracting heart chamber has been discussed. As determinants of myocardial oxygen consumption, the volume concept of Starling and the pressure index of Sarnoff (tension-time index) may be reconciled by the geometric Law of Laplace. Hypermetabolism in congestive heart failure may be due solely to an increase in myocardial oxygen consumption. While the normal left ventricle consumes approximately 5 per cent of the total body oxygen consumption, values as high as 27 per cent have been observed in patients with large, failing hearts.