Dynamic behaviour of coronary stenosis in response to increasing heart rate in the dog

In order to assess the responses of coronary stenosis to an elevation of myocardial metabolic demand, the effects of pacing induced tachycardia on systemic and coronary haemodynamics were examined during four grades of compliant coronary stenosis which preserved stenosis vasomotility in eight open chest dogs. Grades of the coronary stenosis were defined as trifling, mild, moderate, and severe by pressure gradients of 12(0.5), 19(1.0), 28(1.4) and 37(1.6) mm Hg (mean [SEM]), respectively. Stepwise increases in heart rate (+30, +60 and +90 beats.min-1) decreased the resistance caused by trifling and mild coronary stenosis by 47(11.9) and 26(8.8)%, respectively, with an increase in coronary blood flow and a decrease in pressure gradient across the stenosis. By contrast, the resistance caused by moderate and severe stenosis increased by 96(24.2) and 148(63.2)%, respectively, with a decrease in coronary blood flow and an increase in pressure gradient. During lower grades of coronary stenosis, flow-dependent dilatation of large epicardial coronary arteries reduced the stenosis severity, whereas during higher grades of stenosis, passive narrowing of the stenosed arterial segment due to arteriolar vasodilatation outstripped the flow-dependent stenosis dilator effects and resulted in an intensification of the stenosis.