Effects of nitroglycerin and propranolol on the distribution of transmural myocardial blood flow during ischemia in the absence of hemodynamic changes in the unanesthetized dog

Abstract

Chronically instrumented awake dogs were used to study the effects of nitroglycerin and propranolol on the transmural distribution of myocardial blood flow during transient ischemia. Studies were carried out 7-14 d after implantation of an electro-magnetic flowmeter probe and balloon occluder on the left circumflex coronary artery, placement of epicardial minor axis sonar crystals, and implantation of left atrial, left ventricular, and aortic catheters. The occluder was inflated to completely interrupt flow for 10 s followed by partial release to reestablish flow at 60% of the preocclusion level. During this partial release, which served as the control for the study, regional myocardial blood flow was measured with 7- to 10- μm radioactive microspheres. After control measurements, seven dogs were given nitroglycerin (0.4 mg i.v.) and eight dogs propranolol (0.2 mg/kg i.v.). 5 min later the occlusion and partial release sequence was repeated, and regional myocardial blood flow was measured when heart rate, aortic and left ventricular end-diastolic pressure, and minor axis diameter were unchanged from control values. The data values were selected so that total flow to the ischemic region during partial release after nitroglycerin or propranolol administration was not significantly different from flow during the control partial release. After nitroglycerin administration endocardial flow (endo) in the ischemic region increased from 0.46 ± 0.07 to 0.59 ± 0.06 ml/min per g (P<0.006); epicardial flow (epi) decreased from 078 ± 0.09 to 0.70 ± 0.08 ml/min per g (P<0.04). The endo:epi ratio increased from 0.65 ± 0.07 to 0.92 ± 0.10 (P<0.05). In contrast, administration of propranolol produced no significant change in transmural fow (endo, 0.42 ± 0.02 and 0.46 ± 0.03 ml/min per g; epi, 0.71 ± 0.06 and 0.70 ± 0.07 ml/min per g) or in the endo:epi ratio (0.60 ± 0.03, 0.66 ± 0.06) in the ischemic region. Nitroglycerin and propranolol produce different effects on the transmural distribution of blood flow to ischemic myocardium. Nitroglycerin can increase blood flow to the underperfused endocardium in the absence of alterations in heart size, hemodynamic parameters, and total transmural flow to the ischemic region. Under similar conditions, propranolol has no significant effect on the transmural distribution of blood flow to an ischemic region.