Myocardial technetium 99m sestamibi kinetics after reperfusion in a canine model

Clinical sestamibi imaging protocols after reperfusion therapy are based on the premise that redistribution does not occur. However, animal studies that use punch biopsies or imaging have variably reported either some or no redistribution. This study was designed to (1) utilize implantable radiation detectors to determine whether sestamibi is redistributed after reperfusion, (2) accurately determine the time course, extent, and kinetics of the redistribution, and (3) determine whether sestamibi kinetics can be used to document reperfusion and salvage after a single dose of sestamibi. Twenty-five dogs were injected with 5.0 mCi of technetium 99m sestamibi and microspheres during circumflex occlusion, and reperfusion occurred within 5 minutes in group 1 (15-minute occlusion) and group 2 (1-hour occlusion). Group 3 was not reperfused. Sestamibi activities in the normal and occluded zones were monitored with radiation detectors for 2 hours, and serial gamma camera imaging and arterial blood sampling was begun. No dogs in group 1 and all dogs in groups 2 and 3 had infarcts as shown by triphenyltetrazolium chloride stain. The final occluded/normal zone technetium 99m activity ratio was significantly higher than the flow ratio at the time of sestamibi injection only in the group 1 dogs (0.51 +/- 0.05 vs 0.38 +/- 0.06, p = < 0.0001). In addition, the mean 2-hour fractional sestamibi clearance from the occluded/reperfused zone (0.03 +/- 0.02) was significantly slower in the group 1 dogs compared with normal zone clearance (0.09 +/- 0.01, p = 0.03). Gamma camera images demonstrated large posterior wall perfusion defects initially, which persisted 2 hours later with no visual evidence of redistribution in any of the reperfused dogs in groups 1 and 2. Thus in an experimental animal model under ideal conditions, sestambi is redistributed into reperfused viable myocardium; however, the amount of this redistribution is small and could not be perceived by visual image analysis. Sestamibi is not redistributed into reperfused nonviable myocardium or into nonreperfused myocardium. Therefore sestamibi kinetics after a single dose of tracer in an experimental animal model can be used to document reperfusion of viable myocardium but cannot differentiate reperfusion of the infarcted territory from nonreperfused infarcted myocardium.