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

TLDR

The study used a new volumetric system to measure intraventricular volume more accurately in an excised, supported canine left ventricle preparation. The results confirm that the instantaneous pressure‑volume ratio E(t) remains stable across varying end‑diastolic volumes and aortic pressures, is reproducible in both isovolumic and auxobaric beats, can be modeled as P(t)=E(t)[V(t)-Vd], and rises by 63 % with a 10 % faster peak when contractility is increased by catecholamines, supporting its role as a marker of contractile state.

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.