Instantaneous Pressure-Volume Relationships and Their Ratio in the Excised, Supported Canine Left Ventricle

Abstract

We have previously shown in the normally ejecting canine left ventricle that E(t), the time-varying ratio of instantaneous pressure, P(t), to instantaneous volume, V(t), is little affected by end-diastolic volume or aortic pressure. The present study on an excised, supported canine heart preparation indicates that the thesis on E(t) is also valid for either totally isovolumic or auxobaric beats. Intraventricular volume was measured more accurately than it was in the previous study by a new volumetric system. Regression analysis of the data showed that the instantaneous pressure-volume relationship could be approximated by the equation P(t) = E(t).[V(t) - V_d], where V_d is an empirical constant, over a wide range of intraventricular volume. Similar E(t) curves were obtained from both isovolumic and auxobaric beats for a given contractile state. When the contractile state of the preparation was enhanced by a constant-rate infusion (0.2 µg/min) of norepinephrine or isoproterenol into the coronary artery, the peak magnitude of E(t) increased 63% from 3.6 mm Hg/ml and the time to peak E(t) shortened 10% from 175 msec. We conclude that the present investigation substantiates our earlier study which established a link between E(t) and the contractile state of the heart.