Left ventricular ejection activation in the in situ heart.

We tested, in the in situ heart, the hypothesis that the end-systolic pressure (ESP) of small ejecting contraction (EC) is greater than that of an isovolumic contraction (IC) with a similar end-systolic volume. We produced ECs with varying amounts of ejection by partial aortic occlusion while measuring left ventricular pressure, one or two left ventricular dimensions (anteroposterior and septal-lateral), and aortic flow. In 11 dogs, we plotted ventricular pressure against the time integral of aortic flow of ECs and IC with the same end-diastolic anteroposterior dimension. The end-systolic pressure-volume line was drawn from the peak isovolumic pressure-volume point tangential to the left upper corner of the pressure-volume loop of ECs. The slope of the tangential line of the middle EC, whose stroke volume was 51 +/- 8% of that of the control EC, was decreased by 54 +/- 16% compared with that of the control EC. In eight dogs with two pairs of crystals, left ventricular volume was controlled by partial vena caval occlusion, and ICs were produced by total aortic occlusion. The end-systolic pressure of small ejections exceeded (11.3 +/- 7.7 mmHg, P less than 0.01) those of isovolumic contractions with the same end-systolic cross-sectional area, whereas the end-systolic pressure of beats with large ejections was similar (-9.4 +/- 14.4 mmHg, P = NS) to an IC with the same end-systolic cross-sectional area. We conclude that the end-systolic pressure-volume point of beats with a small ejection is located above the isovolumic end-systolic pressure-volume relation in the in situ dog left ventricle.