Effects of changes in preload, afterload and inotropic state on ejection and isovolumic phase measures of contractility in the conscious dog

Abstract

Despite much investigation, the usefulness of various indexes employed clinically for detecting alterations in ventricular contractility in the intact circulation remains controversial. The effects of acute preload, afterload and contractility changes on both ejection and isovolumic phase measures of left ventricular function were analyzed in normal, trained conscious dogs instrumented with micromanometers and endocardial ultrasonic diameter gauges. Rapid volume overload increased the excursion of the left ventricular diameter (delta LVD) by 7 percent above the control level, but mean velocity of circumferential shortening (VCF) did not change significantly; peak rate of left ventricular pressure rise (dP/dt) increased by 11 percent and (dP/dt)/DP40 (DP = developed pressure) was augmented by 10 percent, but maximal [(dP/dt)/LVP], or "Vpm," decreased by 20 percent. Pressure overload by phenylephrine infusion decreased delta LVD by 15 percent and mean VCF fell by 26 percent; peak dP/dt and (dP/dt)/DP40 remained unaltered, but VPM was reduced by 37 percent. Isoproterenol augmented peak dP/dt by 55 percent, and (dP/dt)/DP40, Vpm and mean VCF were increased comparably. Propranolol decreased these measures equally by about 16 percent. Therefore, in the conscious animal in the steady state, isovolumic phase indexes were mildly influenced by acute volume loading, wheras ejection phase indexes were not. Acute increases in aortic pressure markedly reduced ejection phase measures, whereas the isovolumic indexes were unaffected. All of the indexes studied were comparably sensitive to acute alterations in contractility, but we conclude that no single measure can always be used for defining an acute contractility change in the intact circulation.