Effect of Nitroglycerin

an 0.2 mg/min infusion ofTNG, and also after 1 mg/kg i.v. dipyridamole, ischemic flow was maintained in the face of a 20-30%o reduction in blood pressure. In this setting, nonischemic flow was unchanged during TNG and doubled after dipyridamole. With the addition of methoxamine in both dilator groups, blood pressure retumed to base line while flow to ischemic areas increased above base-line values (TNG, 0.16-0.20 ml/min per g, P < 0.01; dipyridamole, 0.180.31 ml/min per g, P < 0.05). Epicardial ST segment elevations increased during TNG infusion and were unchanged after dipyridamole, but with addition of methoxamine, ST segments became less elevated in both drug groups, concomitant with the observed increase in collateral blood flow. These data indicate thaA both types of coronary vasodilators, when used in conjunction with methoxamine to support blood pressure, reduce collateral resistance, increase collateral flow, and reduce epicardial ST-segment elevations. Received for publication 27 May 1976 and in revised form 25 August 1976. INTRODUCTION Coronary vasodilators increase myocardial blood flow in hearts with normal coronary arteries and may also increase flow when coronary artery occlusive disease is present (1). However, in the latter situation, increased flow may merely be going to myocardial regions fed by normal vessels rather than areas supplied by obstructed vessels. While some investigators have found that vasodilators increase flow to compromised myocardium (2-8), others have found a decrease or no change (9-14). Fam and McGregor (11) have pointed out that these discrepancies may be related in part to differences in action among the various coronary dilators. Nitrates (e.g. nitroglycerin), which effect mainly large epicardial conductive arteries, may selectively increase flow to ischemic areas, while nonnitrate dilators (e.g. dipyridamole), which affect primarily small precapillary resistance vessels, may decrease flow in areas of myocardial ischemia by dilating arterioles in nonischemic areas ("coronary steal"). The purpose of this paper was to study in more detail the effects of cornary vasodilators on left ventricular blood flow distribution during regional myocardial ischemia. Radioactive microspheres were used to separate subendocardial and subepicardial flows in ischemic and normal regions. Measurements were made during drug-induced hypotension and repeated after blood pressure had been returned to base line with the alpha-adrenergic agonist methoxamine.