Effects of load alteration and coronary perfusion pressure on regional end-systolic relations.

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Abstract
Left ventricular end-systolic pressure-wall thickness and pressure-segment length relations have been used to assess regional myocardial contractility. The purpose of these studies was to determine whether the method of afterload alteration, coronary perfusion pressure, and the definition of end systole affected regional end-systolic relations. In paced open-chest pig hearts (n = 10), left ventricular pressure, wall thickness, and segment length were measured during vena caval occlusions. First, regional end-systolic relations were obtained before and after an increase in afterload with intravenous phenylephrine. This comparison was performed with the coronary circulation intact (closed loop) and with coronary perfusion pressure controlled extracorporeally (open loop). Second, with the open-loop preparation, the independent effect of a range of coronary perfusion pressures (45-155 mm Hg) on end-systolic relations was assessed. In both procedures, end systole was defined by the maximal isochronal slope, termed regional Emax, and by the maximal ratio of left ventricular pressure to regional dimension, termed Ees. In the first procedure, the slope and x-axis intercept of regional Emax were unchanged with phenylephrine. In contrast, the slope of Ees decreased with phenylephrine, consistent with a decrease in contractility. The extrapolated x-axis intercept of Ees shifted in a direction consistent with an increase in contractility. In the second procedure, increases in coronary perfusion pressure did not affect regional Emax but increased the slope of Ees. A decrease in the coronary perfusion pressure, particularly with ischemia, shifted the x-axis intercepts of both Emax and Ees in a direction consistent with a decrease in contractility. Thus, regional end-systolic relations in intact hearts are dependent on the method of afterload alteration and the coronary perfusion pressure. Furthermore, Emax differs from Ees, so the assessment of regional relations depends on how end systole is defined.