Effects of calcium entry blockade on myocardial blood flow.

The calcium entry-blocking drugs produce effects on the coronary vasculature that might be expected to exert anti-ischemia activity. Although these agents cause little vasodilation of the epicardial coronary arteries during basal conditions, they block vasoconstriction that can increase stenosis severity during isometric exercise and interrupt coronary artery spasm in patients with variant angina. Administration of the calcium blockers causes transient vasodilation of the coronary resistance vessels, followed by decreased responsiveness to a brief ischemic stimulus. This results in decreased coronary reactive hyperemia after transient coronary occlusion. By preventing excessive ischemic vasodilation of the resistance vessels, these agents can enhance perfusion of the subendocardium distal to a flow-limiting coronary stenosis. The calcium entry blockers have relatively little effect on the immature coronary collateral vessels that exist at the time of acute coronary occlusion. Diltiazem, however, has been demonstrated to increase collateral blood flow in animals in which chronic coronary occlusion has resulted in growth of moderately well-developed collateral vessels.