Effect of coronary occlusion and myocardial viability on myocardial activity of technetium-99m-sestamibi The timing effect of sestamibi administration with respect to the onset of myocardial ischemia and reperfusion was studied in swine. In different groups of animals sestamibi was administered prior to coronary artery occlusion, during occlusion, or 1/2 hour following reperfusion. Sestamibi administered prior to coronary occlusion resulted in an insignificant decrease in 99mTc activity in the ischemic zone. However, infarct zone activity was reduced to 62 +/- 14% of the nonischemic zone. In contrast, administration during coronary occlusion resulted in similar significant reductions of both ischemic and infarct zone activity. Administration of sestamibi during reperfusion resulted in normal ischemic zone activity and markedly reduced activity in the infarct zone. Significantly reduced activity in the infarct zone was found to be independent of the timing of sestamibi administration with respect to the onset of myocardial ischemia and/or reperfusion. Thus, cell viability appears required for uptake and retention of isotope activity. 