Load Independence of the Instantaneous Pressure-Volume Ratio of the Canine Left Ventricle and Effects of Epinephrine and Heart Rate on the Ratio

TLDR

The study measured the instantaneous pressure‑volume ratio E(t) = P(t)/[V(t)–Vd] in canine left ventricles using plethysmographic volume measurement, right‑heart bypass, cardiac nerve sectioning, and epinephrine infusion to manipulate contractility. E(t) curves consistently peaked near the end of ejection and were load‑independent, but epinephrine infusion doubled Emax and shortened Tmax, while increased heart rate proportionally shortened Tmax without affecting Emax, indicating that Emax and Tmax directly reflect ventricular contractility.

Abstract

As a means of assessing ventricular performance, we analyzed the time-varying ratio of instantaneous pressure, P(t), to instantaneous volume, V(t), in the canine left ventricle. Intraventricular volume was measured by plethysmography, while the right heart was totally bypassed. The cardiac nerves were sectioned, and an epinephrine infusion was used to alter the contractile state. The instantaneous pressure-volume ratio was defined as E(t) = P(t)/[V(t) - V d ], where V d is an experimentally determined correction factor. We found that (1) all the E(t) curves thus defined were similar in their basic shape and attained their peak near the end of the ejection phase regardless of the mechanical load, the contractile state, or the heart rate, (2) under a constant heart rate and contractile state extensive changes in preload, afterload, or both did not alter the peak value of E(t), Emax, or the time to Emax from the onset of systole, Tmax, and (3) these parameters of E(t) markedly changed with epinephrine infusion or increases in heart rate. At an epinephrine infusion rate of 2 µg/kg min -1 , Emax increased to 12.2 ± 4.5 ( SD ) mm Hg/ml (N = 9) from its control value of 6.6 ± 1.2 mm Hg/ml before the infusion. Simultaneously, Tmax shortened from 191 ± 29 msec to 157 ± 26 msec. Increases in the paced heart rate proportionally shortened Tmax (45% per 100-beats/min change in heart rate) without any effect on Emax. We concluded that E(t), represented by Emax and Tmax, explicitly reflects the ventricular contractility.