The effect of direct current (DC) countershock upon myocardial technetium-99m stannous pyrophosphate (PYP) uptake was studied in 22 dogs. All eight dogs imaged had positive abnormal PYP scintigrams that were usually indistinguishable from experimental infarction. In three animals, additional areas of radionuclide uptake were seen in overlying noncardiac tissue. Left and right ventricular myocardial PYP uptake averaged (+/- SEM) 23 +/- 5 times control and 24 +/- 6 times control, respectively. These activity ratios occurred without reduction in regional myocardial blood flow (RMBF), and were associated with histologic evidence of necrosis. The necrosis was usually epicardial, corresponding to the transmural site of greatest PYP uptake. The magnitude of PYP accumulation and the weight of damaged tissue increased with increasing applied energy. Thus, PYP uptake following DC countershock could result in false-positive interpretation of acute ischemic myocardial infarction. Since RMBF is normal in regions of PYP uptake, the major determinant of radionuclide accumulation is the extent of cellular damage.

SUMMARY The dual radionuclide myocardial distributions of imaging agents potassium43 (49K) and technetium-99m stannous pyrophosphate were studied in a 24-hour closed chest canine infarct preparation. In multiple myocardial biopsies in 20 dogs, tissue levels of both radionuclides were compared to either an index of tissue viability (myocardial creatine phosphokinase [CPK] depletion), or to estimates of regional myocardial blood flow as measured by the microsphere technique.Myocardial 4K uptake in the ischemic and infarcted zone correlated well with both CPK depletion (r = 0.73) and microsphere estimates of relative blood flow. The correlation with microspheres was excellent in the transmural sample (r = 0.93) as well as endocardial (r= 0.97) and epicardial (r = 0.86) portions.On the other hand, "tmTc-PYP myocardial uptake did not correlate with the extent of CPK depletion. Maximal uptake was frequently noted in border zones with only moderate CPK depletion, NONINVASIVE RADIONUCLIDE IMAGING techniques can visualize acute myocardial infarction in two general ways. On the one hand, a myocardial radionuclide distribution may be obtained with maximal incorporation in normal myocardium and minimal uptake in the region of the infarct. Imaging of such a distribution would demonstrate the zone of infarction and/or ischemia as a "cold spot," or region of relatively reduced myocardial radioactive tracer accumulation. Radioactive potassium and its analogs, rubidium, cesium, and thallium have been utilized for such studies.'-' Alternatively, certain radioactive tracers maximally concentrate in infarcted and/or ischemic myocardium, such that the region of infarction is visualized as a "hot spot," or "positive" zone of increased radionuclide accumulation. Prototype "positive" infarct imaging radionuclides include technetium-99m labelled stannous pyrophosphate, tetracycline, and glucoheptonate."-' Simultaneous assessment of an infarction with both types of externally detected radionuclide myocardial distributions might allow Received August 6, 1975; revision accepted for publication October 14, 1975. while lesser degrees of "9mTc-PYP uptake were noted in the central infarct zone where CPK activity was lowest. The relationship of ""mTc-PYP uptake to microsphere regional flow estimates demonstrated that '9mTc-PYP uptake was maximal at flows of 0.3 to 0.4 of normal. At lower flows, '9mTc-PYP uptake fell toward normal levels. A similar relationship was noted between the distributions of 99MTc-PYP and "K. In relatively high flow border segments (. 0.80 of normal), abnormal '9mTc-PYP uptake of five to six times normal persisted. The transmural distribution of "mTc-PYP demonstrated that in low flow regions 99mTc-PYP uptake was primarily epicardial, while in the higher flow ischemic periphery of the infarct endocardial uptake predominated. Thus, while there is a direct correlation between cationic "K myocardial uptake and regional myocardial viability and blood flow, no such direct relationship exists for 99mTc-PYP. This is in pa...

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The pathophysiological correlates of thallium-201 (201TI) myocardial uptake were studied in a 24-hour-old closed-chest canine infarct model. Reduction in regional 201Tl uptake correlated well with the magnitude of tissue creatine phosphokinase depletion and microsphere estimates of transmural blood flow. In low flow endocardial regions 201Tl occasionally under-estimated the magnitude of flow reduction. Even slight reductions of 201Tl uptake (less than 0.86 of normal) were associated with histopathological and histochemical evidence of necrosis.

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