Atrial Contribution to Stroke Volume in Dogs with Chronic Heart Block

In dogs studied without thoracotomy, under morphine-pentobarbital anesthesia and with an average heart rate of 71 beats/minute, atrial systole contributes to ventricular function to a small but significant degree. Stroke volume increased by 7 to 8% when atrial systole was timed to occur 0.1 second prior to closure of the atrioventricular valves, and decreased a similar magnitude when the contribution of atrial systole to ventricular filling was rendered ineffective by the use of an A-V delay of zero so that atrial emptying into the ventricles was prevented by the closed A-V valves. Increases or decreases in right atrial pressure of several centimeters of water induced by infusion of 6% dextran and by hemorrhage, respectively, were not associated with demonstrably significant alterations in these effects.