Effect of intravenous nitroglycerin on collateral blood flow and infarct size in the conscious dog.

Abstract

This study was performed to determine whether nitroglycerin (NG) can increase collateral flow to ischemic myocardium and reduce ultimate infarct size. Permanent occlusion of the mid-circumflex coronary artery was produced in 43 previously instrumented conscious dogs and within 3 min, 6 h i.v. infusions were begun of saline (18 controls); NG in doses to reduce mean arterial pressure by 10% but not < 90 mm Hg (15); or NG followed by methoxamine (MX) to correct the NG-induced fall in blood pressure (10). After sacifice 2 days later, the occluded coronary bed was defined by postmortem coronary arteriography and masses of infarct and occluded bed were measured by planimetry of weighed rings of the left ventricle (LV). Infarct size was significantly less with NG than saline as a percent of LV (12.1 vs. 6.4%, P < 0.05) and as a percent of occluded bed (32.0 vs. 15.9%, P < 0.005). NG plus MX did not reduce infarct size more than NG alone: 6.6 vs. 6.4% of LV and 16.0 vs. 15.9% of occluded bed. Masses of LV and occluded bed did not differ significantly among the 3 groups. Coronary blood flow (CBF) measured by 7-10 .mu.m radioactive microspheres increased by > 50% throughout the occluded bed (P < 0.005) after NG and was more than the spontaneous increase seen in controls (P < 0.05); MX had no additional effect on CBF over NG alone. Six hour infusions of NG decreased infarct size and improved CBF; addition of MX to reverse the systemic effects of NG did not lessen the benefit. Myocardial protection by NG evidently did not depend on a decrease in myocardial O2 demands but on an increase in collateral flow resulting from a direct vasodilating action of NG on the coronary bed.