SUMMARY The initial myocardial uptake of thallium-201 depends on myocardial blood flow distribution. The phenomenon of delayed thallium redistribution after transiently or chronically altered myocardial perfusion has been described. The net myocardial accumulation of thallium-201 after injection depends upon the net balance between continuing myocardial extraction from low levels of recirculating thallium in the blood compartment and the net rate of efflux of thallium from the myocardium into the extracardiac blood pool. These experiments were designed to measure separately the myocardial extraction and intrinsic myocardial efflux of thallium-201 at normal and at reduced rates of myocardial blood flow. The average myocardial extraction fraction at normal blood flow in 10 anesthetized dogs was 82 ± 6% (±SD) at normal coronary arterial perfusion pressures and increased insignificantly, to 85 ± 7%, at coronary perfusion pressures of 10-35 mm Hg. At normal coronary arterial perfusion pressures in 12 additional dogs, the intrinsic thallium washout in the absence of systemic recirculation had a half-time (T½h) of 54 ± 7 minutes. The intrinsic cellular washout rate began to increase as distal perfusion pressures fell below 60 mm Hg and increased markedly to a T½ of 300 minutes at perfusion pressures of 25-30 mm Hg. A second, more rapid component of intrinsic thallium washout (T' 2.5 minutes) representing approximately 7% of the total initially extracted myocardial thallium was observed. The faster washout component is presumed to be due to washout of interstitial thallium unextracted by myocardial cells, whereas the slower component is presumed due to intracellular washout. The net clearance time of thallium measured after 'Lv. injection is much longer than the intrinsic myocardial cellular washout rate because of continuous replacement of myocardial thallium from systemic recirculation. Myocardial redistribution of thallium-201 in states of chronically reduced perfusion cannot be the result of increased myocardial extraction efficiency, but rather, is the result of the slower intrinsic cellular washout rate at reduced perfusion levels.DESPITE the widespread use of thallium-201 perfusion scintigraphy in clinical situations, there remain significant gaps in our knowledge concerning the kinetics of the radionuclide in normal and ischemic myocardium. The initial distribution of thallium in the myocardium immediately after i.v. injection is the result of both blood flow delivery of the tracer to the heart and the extraction of the tracer by the myocardium. ' 2 The delayed or equilibrium distribution, particularly under conditions of altered perfusion, is less well understood. The determination of relative myocardial thallium concentration as a function of time in sequential imaging studies is being used increasingly to detect and evaluate coronary artery disease.1' 3-9 The clinical use of delayed thallium redistribution imaging has brought about an acute need for an improved understanding of the mechanism of thallium kinet...

These data suggest a possible role for intracoronary urokinase during angioplasty for unstable angina. The lack of effect after infarction may represent a greater thrombus burden or degree of plaque disruption. A trial utilizing higher doses of urokinase in a larger patient group is in progress.

STUDY OBJECTIVE - The aim of the study was to assess the value of triphenyltetrazolium chloride (TTC) staining as an indicator of non-viable myocardium after early reperfusion of ischaemic myocardium. DESIGN - Left anterior descending artery occlusion was performed in pigs for various lengths of time and at two different sites (proximal and distal). After 120 min reperfusion, TTC was injected distal to the occlusion while the remainder of the myocardium was stained with Evans Blue. Myocardial enzymes were measured in non-ischaemic zone, regions of risk and in necrotic zones and related to staining characteristics. SUBJECTS - 31 male Hampshire pigs, weight 34-39 kg, were studied. Twelve were excluded because of resistant ventricular fibrillation or poorly defined areas of infarction. In the remaining 19 pigs, proximal occlusion was carried out in 15 and distal in four. Occlusion lasted for 15 min in six animals, for 30 min in eight (four of which were the animals with distal occlusions), and for 45 min in five. MEASUREMENTS and RESULTS - Biopsies from non-ischaemic zones, regions of risk and necrotic zones were analysed for creatine kinase and lactate dehydrogenase. In the 15 min group, myocardial creatine kinase in the region of risk (red stained) was similar to the non-ischaemic (blue) zone, but in the 30 min distal occlusion group it was reduced. After 30 and 45 min of proximal occlusion, creatine kinase activity in the necrotic (white) zone was reduced compared to the red zone in the same group, and in the red zone of both groups it was reduced compared to the non-ischaemic area. CONCLUSIONS - The red zone, as defined by TTC staining, may be associated with significant creatine kinase depletion after relatively brief periods of occlusion and subsequent reperfusion. This suggests that the red region may be a heterogeneous area of dead and viable cells.

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