Effect of heart rate on coronary circulation

The direct and secondary effects of tachycardia on the coronary circulation were analyzed in the dog's heart with atrioventricular block, pacing the heart at rates of 60, 120, 180 and 240 per minute from ventricular automatism (heart rate 45 on an average). The effect of bradycardia was also studied by turning off the pacemaker from various heart rates. 1. In the initial phase of marked tachycardia, a decrease in coronary blood flow and an increase in coronary vascular resistance were observed, presumably due to direct mechanical effect of tachycardia on the coronary vasculature. Increases in coronary vascular resistance were 4.9%, 8.5%, 26.7% and 38.8% at ventricular rates of 60, 120, 180 and 240, respectively. 2. The coronary blood flow increased in the late phase of marked ventricular tachycardia. This suggested the effect of metabolic and humoral factors. The coronary vascular resistance decreased by 1.0%, 12.5%, 30.5% and 32.2% at the pacemaker rates of 60, 120, 180 and 240, respectively. 3. Changes in heart rate from ventricular tachycardia to the ventricular automatism were considered to reveal the effect of bradycardia. The coronary vascular resistance remained unchanged in bradycardia starting from ventricular rates of 60 and 120, but it decreased in bradycardia from ventricular rates of 180 and 240, and averaged -15.3% and -24.6%, respectively.