INFLUENCE OF SYSTOLIC PRESSURE PROFILE ON RATE OF LEFT-VENTRICULAR PRESSURE FALL

We examined the influence of the systolic left ventricular pressure (LVP) waveform on the rate of isovolumetric LVP fall, as assessed by the time constant-tau. Seven open-chest dogs were instrumented with a micromanometer in the left ventricle, with segment length gauges in the anterior and posterior midwall of the left ventricle, and with a balloon-tipped catheter in the proximal aorta. The intra-aortic balloon was inflated before the onset of ejection (early) or during midejection (late) to produce timed and graded increases in peak LVP of 2-20 mmHg. The rate of LVP fall slowed significantly more with late than with early increases in LVP (tau increased 1.5 +/- 0.5 vs. 0.5 +/- 0.3% mmHg increase in peak LVP, respectively, P < 0.001). For a similar increase in peak LVP, there was a progressively greater increase in tau when the timing of balloon inflation was progressively delayed from early to late ejection in 10-ms increments). The differential effect of early vs. late pressure increases on tau was not related to regional differences in segment length behavior nor to an increase in regional nonuniformity between anterior and posterior sites. We conclude that under the experimental conditions of an intact, ejecting left ventricle, the systolic pressure profile is an important determinant of the rate of pressure fall. The rate of LVP fall slows in direct proportion to the magnitude of increase in systolic pressure. The sensitivity to systolic load increases progressively throughout the ejection period, so that the rate of LVP fall slows significantly more with late than with early pressure increases. These results may be related to a progressive delay in cross-bridge inactivation with greater and later increases in systolic load. The delayed isometric relaxation with late systolic loads illustrates load dependence and the separation in time between isometric and isotonic relaxation.