LEFT VENTRICULAR TIME-VARYING ELASTANCE

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The evaluation of left ventricular (LV) performance remains a difficult problem in clinical practice and physiologic investigation. Data from isolated canine hearts suggest that the LV behaves as an elastic structure that stiffens in a predictable manner during systole, thus, it can be described as a time-varying elastance. The goals of this project are to test the ability of this hypothesis to describe the relation between LV systolic pressure and volume in the in situ heart; and to investigate two new methods of evaluating LV performance that we have derived from the time varying elastance hypothesis. The first method involves the relation between the peak rate of LV pressure rise (dP/dt-max) and the LV end-diastolic volume. We will test the hypothesis, supported by our preliminary data, that this relation is linear, insensitive to alterations in loading conditions, and very sensitive to inotropic state. Secondly, we will evaluate a new method of calculating E-max, the slope of the LV end-systolic pressure-volume relation, using a relation derived from the time-varying elastance hypothesis and supported by preliminary date. This relation describes E-max as a function of dP/dt-max, the LV end-systolic and end-diastolic volumes, and the time to end-systole. Thus E-max can be determined from measurements under a single set of conditions, avoiding the need to alter loading conditions to produce a range of LV end-systolic pressure-volume points. These studies will be carried out in dogs chronically instrumented with a micromanometer to measure LV pressure, and 3 pairs of ultrasonic crystals to determine LV volume. Aortic flow will be measured with an electromagnetic flow probe. LV pressure and volume will be varied by caval occlusions. Inotropic state will be altered by dobutamine infusion, while loading conditions will be varied by infusion of methoxamine and dextran and by transient occlusions of the descending aorta. Isovolumic beats will be produced by transient complete occlusion of the ascending aorta. These studies will provide new information on the descriptors of LV systolic function in the in situ heart and two new methods of evaluating LV performance that may be independent of loading conditions.