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

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.