SUMMARY Thallium-201 and microspheres were injected into the blood simultaneously during left circumflex (LC) occlusion in open chest dogs. The dogs were sacrificed 6-8 minutes later and regional myocardial 201T1 and microsphere concentrations determined. In dogs with permanent LC occlusion the myocardial 201T1 distribution approximated blood flow distribution as judged by the microsphere concentrations. Release of LC occlusion 45 seconds after 201T1 injection almost obliterated the myocardial 11'TI deficit in the area of the LC without changing the microsphere results, presumably a result of deposition of 11'T1 during reactive hyperemia. Either delaying the onset of reflow until 3 minutes or attenuating the magnitude of reactive hyperemia by LC stenosis markedly decreased the change in 20'Tl distribution due to reflow. We conclude that for a given degree of reversible regional myocardial ischemia at the time of 20'Tl injection, the perfusion deficit observed on the initial scintigram will be influenced by the subsequent duration of ischemia and by the magnitude of postischemic reactive hyperemia.THE CLINICAL USE of thallium-201 (201T1) scintigraphy for detecting reversible myocardial perfusion defects requires injecting the isotope during a state of temporary heterogeneity in coronary flow; for example, during exercise or coronary artery spasm. The heterogeneity may be so brief that it ends or even reverses its direction (due to postischemic reactive hyperemia) while isotope is still being actively removed from the blood by the myocardium. Thallium-201 uptake is greatest immediately after injection, when the blood level is highest, but continued uptake by myocardium can be detected by coronary arteriovenous sampling for at least 10 minutes after injection in animals. ' Though it is customary to relate the pattern of myocardial 201T1 deposition to the distribution of coronary blood flow that existed at the time of injection, this pattern might also be influenced by changes in flow distribution within several minutes after injection, when blood levels are still high. For any degree of initial ischemia, regional flow distribution during the first few minutes after injection is determined by at least two factors: duration of ischemia after injection (i.e., the interval between injection and cessation of exercise) and the degree of subsequent reactive hyperemia (i.e., degree of coronary stenosis or spasm). Either early restoration of flow to normal or marked reactive hyperemia might mask a profound but brief period of focal ischemia at the time of injection. In these experiments we examined the influence of these two factors on the formation of an initial 201T1 perfu-
Methods PreparationTwenty-eight fasting mongrel dogs that weighed 20-25 kg, pretreated with acetyl salicylic acid 300 mg to limit platelet aggregation, were anesthetized with halothane gas after premedication with acepromazine 10 mg and methohexital 100 mg induction. They were endotracheally intubated and connected to a Bird respirator using 1% halo...