Effects of heart rate on the dynamics of force development in the intact human ventricle.

SINCE Bowditch's classic description of the effect of stimulation frequency on the force of contraction of the frog ventricle,' the relations between contractile force and heart rate have been studied in many animal species.2 7 Indeed, Kruta8 has suggested that "the basic mechanism of inotropic effects of all changes of frequency (or even of one single interval) exists in all vertebrate myocardial preparations, and is one of the most striking and important features of the mechanical response of the heart muscle." It has been observed, however, that the detailed nature of the inotropic response to changing frequency varies between species,4' and while the influence of changing heart rate in man has been studied in relation to hemodynamics,9 12 no direct measurements of the force-frequency relationships in the intact human heart have been made. In the present study, the effects of alterations in heart rate on the dynamics of force development were determined in the human right ventricle in the course of corrective intracardiac operations. Coronary blood flow was not interrupted, and this obviated the limitations of myocardial oxygen supply encountered with in vitro preparations. It was observed that velocity of myocardial contraction and the duration of contraction varied inversely with heart rate as in other species. In the human heart, however, force of contraction was found to change little with large alterations in rate.