Effect of nitroglycerin and aortic occlusion on myocardial blood flow

To better characterize the cardiac and peripheral effects of nitroglycerin during aortic occlusion, we measured myocardial blood flow in 43 normal and pentobarbital (PB)-depressed dogs (groups I to VII). PB was continuously infused in groups IV to VII to maintain reduced cardiac output and contractility. In groups VI and VII nitroglycerin was administered at 2 micrograms/kg/min. Sequential injections of radioactive microspheres (10 micrometers) and hemodynamic measurements were performed during 2-hour occlusions of the infrarenal aorta. The hearts were divided into endocardium, midmyocardium, and epicardium; total and regional blood flows and the ratio of endocardial to epicardial blood flow (endo/epi) were calculated. The results (mean +/- SEM) were subjected to analysis of variance. Normal dogs that underwent aortic occlusion had predictable increases in peripheral vascular resistance. Coronary vascular resistance fell (P less than 0.05) and endo/epi ratios were maintained above 1. Following PB administration, the myocardial blood flow uniformly fell (1.08 +/- 0.34 to 0.55 +/-0.09 ml/min/gm, P less than 0.001), and the animals not treated with nitroglycerin demonstrated decreased endo/epi ratios (1.11 +/- 0-.07 to 0.83 +/- 0.08, P less than 0.001). Although nitroglycerin did not prevent decreases in total myocardial blood flow, endo/epi ratios were maintained above 1 in treated animals (group VI, 1.04 +/- 0.08; group VII, 1.18 +/- 0.17). Furthermore, the increases in left ventricular end diastolic pressure in the untreated animals were significantly greater than those in animals receiving nitroglycerin (P less than 0.01). Despite severe cardiac depression, nitroglycerin maintained normal transmural distribution favoring the endocardium. Since coronary and peripheral vascular resistances were not altered, this benefit most probably reflects decreased ventricular wall tension secondary to preload reduction.