Effect of Ventricular Rate on the Cardiac Output in the Dog with Chronic Heart Block

The effect of heart rate on cardiac output and associated rate induced hemodynamic changes in dogs with chronic heart block is presented and discussed. At heart rates below 60/min., the stroke volume was maximum and relatively constant, and the cardiac output was largely rate-dependent. These relationships did not exist at ventricular rates above 60/min. Cinefluorographic evidence of decreased diastolic ventricular filling with increasing ventricular rates is presented. At very slow and very fast ventricular rates, the cardiac output decreased despite an increase in right atrial and right ventricular end-diastolic pressure. At slow rates the decreased output was due to rate alone, while at rapid rates it was attributed to the decreased diastolic filling period and resistance to ventricular distention, resulting in decreased diastolic filling.