Time course of technetium-99m sestamibi myocardial distribution in dogs with a permanent or transient coronary occlusion

The influence of duration of coronary occlusion and reperfusion on technetium-99m hexakis-2-methoxyisobutylisonitrile (99mTc-sestamibi) myocardial redistribution between necrotic, salvaged and non-ischaemic myocardium was investigated in dogs submitted either to a 90-min or a 24-h permanent left descending coronary artery occlusion (groups 1 and 2) or to a 90-min occlusion followed by 30 min, 6 h or 22.5 h of reperfusion (groups 3, 4 and 5). In all groups, 99mTc-sesta-mibi and radiolabelled microspheres were injected at 45 min of occlusion. After delimiting the area at risk and the infarct by Evans blue perfusion and triphenyltetrazolium chloride staining, radioactivity of heart slices from normal, viable-ischaemic and necrotic myocardium was measured in a gamma counter. A significant (P<0.001) linear relationship between 99mTc-sestamibi distribution and myocardial blood flow was observed in the area at risk of groups 1 (r=0.92), 2 (r=0.84), 3 (r=0.90), 4 (r=0.93) and 5 (r=0.58). In all groups, the mean percentage of 99mTc-sestamibi uptake in the ischaemic over normal zone overestimated significantly (P<0.05) the mean percentage of the ratio in myocardial blood flow measured with microspheres (group 1: 13.3±1.4 vs. 7.7±1.2; group 2: 15.9±2.0 vs 5.6±1.2; group 3: 14.9±1.6 vs 6.2±1.0; group 4:20.9±1.7 vs 10.9±1.8; group 5: 51.0±2.7 vs 14.0±2.0). This overestimation of blood flow by 99mTc-sestamibi approximated a ratio of 2 after brief (90 min) or sustained (24 h) occlusion and after 30 min or 6 h of reperfusion, but increased to almost 4 in group 5 with prolonged (22.5 h) reperfusion, indicating a significant redistribution of the tracer. In this group, redistribution was more pronounced in the viable-ischaemic zone as indicated by a 99mTc-sestamibi uptake vs the normal zone of 66.5%±5.5% (P<0.05) as compared to 31.5%±3.3% in the necrotic zone, whereas the uptake in the necrotic zone of the 24 h permanently occluded group averaged 22.1%±3.4%. We conclude that upon 6 h of reperfusion, 99mTc-sestamibi myocardial distribution is still flow dependent, with a net overestimation of the ischaemic myocardial blood flow. Prolonged reperfusion redistributes the tracer to the viable and necrotic myocardium in a 2:1 ratio. In these conditions, reperfusion may be overestimated and infarct size underestimated.