Nitroglycerin and heterogeneity of myocardial blood flow. Reduced subendocardial blood flow and ventricular contractile force.

The effects of both intracoronary and intravenous administration of nitroglycerin on transmural distribution of blood flow in the left ventricle after partial coronary artery occlusion was investigated using two independent methods. In 16 open chest, anesthetized dogs, tubing supplying the cannulated left coronary artery was partially occluded. Strain gauges sutured paralled to superficial and deep fibers of the myocardium separately recorded the contractile force of each layer. With occlusion set so that depression of the deep contractile force was imminent. 12 mug intracoronary nitroglycerin in seven dogs depressed only the deep contractile force without changing systemic hemodynamics. Intravenous administration of 180 mug nitroglycerin in nine dogs resulted in a decrease of deep contractile force and aortic pressure often associated with an increase in superficial contractile force. Distribution of myocardial blood flow during peak coronary flow after intracoronary administration of nitroglycerin or during a decrease in aortic pressure after intravenous nitroglycerin administration was determined by the tissue uptake of an intracoronary bolus of rubidium-(80). This was compared with the uptake of potassium-(42) injected before nitroglycerin. Intravenous or intracoronary administration of nitroglycerin caused a significant reduction in subendocardial blood flow with a decrease in the subendocardial/subepicardial ratio of isotope. These experiments suggest that under conditions of acute partial coronary occlusion, the autoregulatory response results in more fully dilated subendocardial vessels causing them to be less responsive to nitroglycerin. Nitroglycerin may then reduce the vascular resistance in the subepicardial more than the subendocardial vessels, resulting in a "steal" of blood flow from deep to superficial myocardium.