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 minutes, 6-hour intravenous infusions were begun of saline (controls, n = 18), NG in doses to reduce mean arterial pressure by 10% but not below 29 mm Hg (n = 15), or NG followed by methoxamine (MX) to correct the NG-induced fall in blood pressure (n = 10). After sacrifice 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, both 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 three groups. Coronary blood flow (CBF), measured by 7-10-μm radioactive microspheres, increased by more than 50% throughout the occluded bed (p < 0.005) after NG, and was more than the spontaneous increase seen in controls (p < 0.05), but MX had no additional effect on CBF over NG alone. Six-hour infusions of NG therefore decreased infarct size and improved CBF, and addition of MX to reverse the systemic effects of NG did not lessen the benefit. The results suggest that under the conditions of this study, myocardial protection by NG did not depend on a decrease in myocardial oxygen demands, but rather on an increase in collateral flow resulting from a direct vasodilating action of NG on the coronary bed.