THE EFFECT OF VAGAL-STIMULATION ON LEFT-VENTRICULAR SYSTOLIC AND DIASTOLIC PERFORMANCE

Vagal stimulation (VS) decreases heart rate and alters the loading con ditions of the left ventricle (LV). However, its effects on LV perform ance are not clearly defined. To evaluate the effects of VS on LV perf ormance, 20 anesthetized open-chest dogs were instrumented to measure LV pressure (LVP) and volume. VS resulted in a decrease in the slope o f the end-systolic pressure-volume relationship, without pacing and wi th pacing, after ansae subclaviae resection and after ansae subclaviae resection plus beta-adrenergic blockade. VS did not alter the time co nstant of isovolumic relaxation (tau) or the LV end-systolic pressure- tau relationship during vena caval occlusion. No change was noticed in the LV chamber stiffness constant without and with pacing. However, a significant increase in the LV filling fraction at one-third and one- half of diastole was observed. We examined the mechanism of this incre ase by examining the relationship of left atrial pressure (LAP) and LV P VS increased mean LAP 44% and increased the LAP-LVP gradient 42%. Th ese data suggest that VS exerts a significant negative inotropic effec t that is independent of its bradycardic effect and the level of sympa thetic tone. Despite the absence of lusitropic effects of VS, early fi lling of the LV is augmented most likely as a result of an increase in LAP.