Effect of altered resistive load on left ventricular systolic mechanics in dogs

The authors tested the possibility that end-systolic volume is reduced at a given aortic and left ventricular (LV) pressure when systemic vascular resistance is reduced. In seven anesthetized dogs pretreated with propranolol to block reflex changes in contractility, the authors measured aortic pressure and ventricular volumes under baseline conditions and after reducing resistive load by opening two 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 less than 0.01) and left ventricular end-systolic volume decreased (P less than 0.005) despite constant aortic pressure. In seven additional dogs, pretreated with propranolol, the authors measured left ventricular pressure, instantaneous and peak left ventricular flow before and after resistive load was reduced by opening one shunt. Despite constant left ventricular pressures, instantaneous, peak and total flow increased when resistance was reduced. It was concluded that the left ventricle shortens farther and faster during ejection against the same aortic and LV pressure when resistive afterload is reduced.