LOSS OF RADIOACTIVE MICROSPHERES FROM CANINE NECROTIC MYOCARDIUM

To verify whether there is a loss of radioactive microspheres (RM) from regions undergoing myocardial infarction, we injected 7-10 μm RM into the left atrium in 75 dogs before left circumflex (LC) coronary artery occlusion and measured the myocardial RM content when the dogs were killed 6-96 hours later. The preocclusion RM content per gram in the occluded LC bed was not significantly different from that in the nonoccluded left anterior descending (LAD) coronary bed in dogs killed within 6 hours and in those without necrosis and killed later. In contrast, in dogs with necrosis, the relative RM content in the LC bed was significantly reduced, indicating an apparent loss of RM averaging 19% (LC/LAD at 12-24 hours = 0.84; 48 hours = 0.80; 96 hours = 0.80). The percent of apparent RM loss from transmural samples correlated closely with the percent necrosis in the samples (r=0.93, P<0.001). Desiccation of samples revealed that edema in the infarct accounted for about 40% of this apparent loss. Most of the remaining 60% appeared to represent true physical loss as evidenced microscopically by decreased RM counts in infarcted tissue, especially in zones of coagulative necrosis. Furthermore, following systemic RM injections and, more importantly, selective intracoronary RM injections into the occluded coronary bed, more RM were found in the lungs and neck lymph nodes in dogs with necrosis than in dogs without necrosis. These results indicate that apparent RM loss from necrotic myocardium is related both to increased tissue water and true physical RM loss and suggest that measurements of flow in infarct regions by RM may be reduced falsely.