Effect of Coronary Vasodilator Drugs on Retrograde Flow in Areas of Chronic Myocardial Ischemia

The effects of two coronary vasodilator drugs, nitroglycerin and dipyridamole, on the coronary collateral flow were studied in twelve dogs with chronic myocardial ischemia and in five normal dogs, by means of the retrograde flow technique. In the ischemic dogs with well developed collateral circulation, both drugs caused a reduction in systemic blood pressure. Dipyridamole caused a significant reduction in both the retrograde flow and peripheral coronary pressure as well. Nitroglycerin, by contrast, did not cause a significant reduction in either retrograde flow or peripheral coronary pressure in spite of a greater fall in mean arterial pressure. When arterial pressure was held constant artificially, nitroglycerin caused a large increase of retrograde flow in chronic "ischemic" dogs while dipyridamole did not. In normal dogs these differences between the two drugs were not observed. Both drugs caused a reduction of retrograde flow and peripheral coronary pressure which paralleled the reduction in systemic pressure. In both ischemic and normal dogs, dipyridamole caused a reduction in the coronary arteriovenous oxygen difference. Nitroglycerin did not change this value. Reported differences between these two drugs, together with these observations, suggest that nitroglycerin may have an action which is to be seen only in the presence of well developed collateral circulation; this is to increase flow through collateral channels without significantly altering total coronary artery flow. Dipyridamole does not seem to alter collateral flow but does increase total coronary flow. The site of action of these two drugs in the coronary vascular tree must thus be different.