Effect of Altered Resistive Load on Left Ventricular Systolic Mechanics in Dogs

Abstract
The possibility that end-systolic volume is reduced at a given aortic and left ventricular (LV) pressure when systemic vascular resistance is reduced was studied. In 7 anesthetized dogs pretreated with propranolol to block reflex changes in contractility, aortic pressure and ventricular volumes were measured under baseline conditions and after reducing resistive load by opening 2 peripheral arterio-venous shunts. When resistive load was reduced, blood pressure was maintained with plasma volume expansion. Real size biplane ventricular end-diastolic areas were obtained using scintigraphic techniques and end-diastolic volumes were calculated using the area-length equation. Thermodilution stroke volume was subtracted from end-diastolic volume to obtain end-systolic volume. When resistive load decreased, mean ejection rate doubled (P < 0.01) and LV end-systolic volume decreased (P < 0.005), despite constant aortic pressure. In 7 additional dogs, pretreated with propranolol, LV pressure, instantaneous and peak LV flow were measured before and after resistive load was reduced by opening 1 shunt. Despite constant LV pressures, instantaneous, peak and total flow increased when resistance was reduced. Evidently, the LV shortens farther and faster during ejection against the same aortic and LV pressure when resistive afterload is reduced.