TIME-COURSE OF TC-99M SESTAMIBI MYOCARDIAL DISTRIBUTION IN DOGS WITH A PERMANENT OR TRANSIENT CORONARY-OCCLUSION

The influence of duration of coronary occlusion and reperfusion on tec hnetium-99m hexakis-2-methoxyisobutylisonitrile (Tc-99m-sestamibi) myo cardial redistribution between necrotic, salvaged and non-ischaemic my ocardium 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 reperf usion (groups 3, 4 and 5). In all groups, Tc-99m-sestamibi and radiola belled microspheres were injected at 45 min of occlusion. After delimi ting the area at risk and the infarct by Evans blue perfusion and trip henyltetrazolium chloride staining, radioactivity of heart slices from normal, viable-ischaemic and necrotic myocardium was measured in a ga mma counter. A significant (P<0.001) linear relationship between Tc-99 m-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 Tc-99m-sestamib i uptake in the ischaemic over normal zone overestimated significantly (P<0.05) the mean percentage of the ratio in myocardial blood flow me asured 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 overe stimation of blood flow by Tc-99m-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 prolong ed (22.5 h) reperfusion, indicating a significant redistribution of th e tracer. In this group, redistribution was more pronounced in the via ble-ischaemic zone as indicated by a Tc-99m-sestamibi uptake vs the no rmal 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 per manently occluded group averaged 22.1%+/-3.4%. We conclude that upon 6 h of reperfusion, Tc-99m-sestamibi myocardial distribution is still f low dependent, with a net overestimation of the ischaemic myocardial b lood flow. Prolonged reperfusion redistributes the tracer to the viabl e and necrotic myocardium in a 2:1 ratio. In these conditions, reperfu sion may be overestimated and infarct size underestimated.