The ventricular pressure-volume diagram revisited

Abstract

This is a review of ventricular pressure-volume relation and force length relation of heart muscle. Twitching heart muscle yields complex instantaneous curves which are dependent on the history of the contractile event. In comparison, the canine ventricle seems to yield a considerably simpler pressure-volume-time relation curve. At end-diastole, the relation curve is identical to the nonlinear passive pressure-volume curve. As contraction begins and proceeds, the relation curve increases its slope E(t) and shifts its volume axis intercept V(d)(t) to the left. At the same time it becomes more and more rectilinear. The magnitude of and the time to the maximum (end-systolic) slope of the relation curve (E(max) and t(max) are determined almost entirely by the contractile state of the ventricle; how the ventricle is preloaded and afterloaded (history of contraction) causes minimal difference unless the preload or afterload is excessively unphysiological. What is described is a simplified picture of the contraction of the ventricular chamber. Admittedly E(t), or E(max), is a phenomenological parameter of ventricular contraction rather than something directly associated with the fundamental mechanism in the contractile machine of muscle. Effort to find the correspondingly simple F-L relation in heart muscle has yielded unsatisfactory results. On the other hand, E(max) should not be misconstrued as an entirely empirical index with no physiological basis. Having a dimension of volume elastance, it carries a clear notion of active stiffening of ventricular wall. Further, it can be used to predict end-systolic volume (and stroke volume) if end-systolic pressure (and end-diastolic volume) is specified. Because of this feature modelers of the entire circulatory system have used E(t) for many years without experimental data, which are now available at least for the canine left ventricle.