Autonomic nervous stimulation affects left ventricular relaxation more than left ventricular contraction

We studied the effects of stimulation of the vagal and also the sympathetic efferent cardiac nerves on left ventricular (LV) relaxation and contraction in 11 anesthetized, open-chest dogs. In each dog, we paced the ventricles at a fixed rate of 120 beats.min-1 and kept the systemic arterial pressure constant. The maximum rate of LV pressure decline, (dP/dt)min, and the time constant of LV isovolumic pressure decline, tau, were used as our indexes of LV relaxation. The maximum rate of LV pressure rise, (dP/dt)max, was used as our measure of LV contractility. Vagal stimulation decreased (dP/dt)min more than (dP/dt)max (P less than 0.01) when examined at frequencies that ranged from 2 to 12 Hz. Vagal stimulation at 12 Hz reduced (dP/dt)min by 26% (P less than 0.001) and increased tau by 57% (P less than 0.0001) but decreased (dP/dt)max by only 20%. Sympathetic stimulation, at frequencies that ranged from 2 to 12 Hz, increased (dP/dt)min more than (dP/dt)max (P less than 0.001). Sympathetic stimulation at 12 Hz increased (dP/dt)min by 130% (P less than 0.0001) whereas it increased (dP/dt)max by 60% (P less than 0.0001). Sympathetic stimulation at 12 Hz decreased tau by 74% (P less than 0.0001). Our studies suggest that cardiac autonomic nerve stimulation affects left ventricular relaxation more than left ventricular contraction.