Reduction of the duration of isovolumic relaxation in the ejecting left ventricle of the dog: residual volume clamping

Abstract

The individual effects of stroke volume and speed of ejection on the duration of the isovolumic relaxation phase were analyzed in the canine left ventricle with a constant end-systolic residual volume. A new technique was employed to maintain the ventricular end-systolic residual volume at a desired constant value regardless of wide changes in stroke volume and speed of ejection in a given inotropic background. The duration of isovolumic relaxation, which was defined to be the time taken for ventricular pressure to fall from its end-systolic level to its 75, 50 and 25% levels, markedly decreased with increases in stroke volume. The reduction amounted to as much as 30-50% when stroke volume was increased from zero (isovolumic) to 20-25 ml. The degree of shortening of the duration of isovolumic relaxation was largely independent of changes in speed of ejection which ranged from about 100-800 ml/s at a constant stroke volume of 15 ml. Stroke volume itself could be a major determinant of the duration of isovolumic relaxation. The mechanism of the observed phenomenon might be a manifestation of the uncoupling effect of muscle shortening on contractile state.