Cardiac work and contractility.

It has been customary to regard the heart as a pump maintaining the flow of blood, and to assess the work done by the heart from the pressure and volume of blood leaving the ventricles. While correct in physical terms, measurement of external cardiac work in this way is a poor index of the myocardial oxygen consumption which is related to the work done by the ventricular muscle. Systolic pressure seems to be a more important determinant of ventricular work than stroke volume, and under some conditions myocardial oxygen consumption can be predicted from the systolic pressure, duration of systole, and heart rate (Sarnoff et al., 1958). This relationship does not hold in other situations, as ventricular work depends on the force of the contraction in the ventricular wall rather than on the systolic pressure. The force developed for a given pressure increases as the cavity becomes largert, so ventricular work is affected by the size of the chamber as well as by the pressure and volume of blood ejected. A better prediction of myocardial oxygen consumption follows if the calculation is based on ventricular wall force instead of pressure, as alterations in ventricular volume are then taken into consideration (Britman and Levine, 1964). Some work is wasted at the early stages of contraction in stretching non-contractile tissue in the ventricular wall, but this factor becomes relatively unimportant once ejection has begun. Accurate measurement of ventricular wall force requires a detailed knowledge of the changes in ventricular volume during systole. The ventricles can be regarded as spherical in shape without producing serious error in these calculations. The dilatation of the ventricle in heart failure might be expected to lead to an increase in ventricular work, but the effect of greater force in the ventricular wall is offset by the mechanical advantage of a larger chamber. Less shortening of the muscle