Loss of microspheres from ischemic canine cardiac tissue. An important technical limitation

Radionuclide-labeled tracer microspheres have been used frequently to map the time-course of dependent in coronary collateral flow. However, the stability of microspheres in ischemic tissue, over time, is unknown. To test how long microspheres can remain after infarction and yield a reliable measure of myocardial flow, tracer microspheres (15μm) were injected prior to left anterior descending coronary artery occlusion in 25 closed-chest, sedated dogs. The occlusion was released at 13 minutes in five dogs (control) or maintained in six dogs that were killed at 24 hours, six at 48 hours,and eight at 8 days. Control dogs were killed at 48 hours after release of occlusion. In control dogs, preocclusion central ischemic zone flow/normal zone flow (CZ/NZ) was not significantly different from unity: 0.97 ± 0.04 (mean ± SE) in the endocardium and 0.94 ± 0.02 in the epicardium. However, in dogs with sustained occlusion, preocclusion endocardial CZ/NZ was 0.66 ± 0.05 (P<0.01 from control) at 24 hours after occlusion. The magnitude of the apparent discrepancy in endocardial flow in the CZ was the same at 24 hours, 48 hours, and 8 days. Preocclusion epicardial CZ/NZ was significantly lower in dogs with sustained occlusion that were killed at 48 hours (0.66 ± 0.05; P<0.05 from control) and at 8 days (0.70 ± 0.05; P<0.05 from control), but not at 24 hours. Thus, there was significant loss of tracer microsphere radioactivity from the endocardium of ischemic tissue as early as 24 hours after occlusion, and from epicardium by 48 hours after occlusion. We conclude that microsphere estimates of collateral flow soon after sustained occlusion may be inaccurate when 24 or more hours elapse prior to tissue sampling.