The effect of collateral flow and myocardial viability on the distributionof technetium-99m sestamibi in a closed-chest model of coronary occlusion and reperfusion

Myocardial uptake of technetium-99m sestamibi at low coronary flow rates ov erestimates blood flow, but the relative impact of flow and viability on Tc -99m-sestamibi kinetics is unclear. The objective of this study was to dete rmine the effect of myocardial viability and the degree of collateral blood flow on the uptake and retention of Tc-99m-sestamibi by examining three an imal models of coronary occlusion and reperfusion, each reflecting a differ ent state of viability and collateral blood flow, Three closed-chest animal models were studied: canine (high collateral flow, preserved viability), p orcine (low collateral flow, absent viability) and porcine with slowly occl usive coronary stents producing infarction and enhanced collateral blood fl ow (high collateral flow, absent viability). There were seven dogs, seven p igs and six pigs, respectively, in each animal model. Animals from all thre e models were subjected to a 40-min total left anterior descending artery ( LAD) occlusion followed by 2 h of reperfusion, Tc-99m-sestamibi and radiola belled microspheres were injected during LAD occlusion 10 min prior to repe rfusion, Animals were sacrificed after 2 h of reperfusion now, Ex situ hear t slice imaging to determine risk area was followed by viability staining t o determine infarct size. Slices were subsequently sectioned into equally s ized radial segments and placed in a gamma well counter. Risk area as deter mined by ex situ Tc-99m-sestamibi imaging was not significantly different b y model, Pathological infarct size differed significantly by model [canine = 1% +/- 1% of the left ventricle (LV); porcine = 13% +/- 8% LV; porcine wi th stent = 14% +/- 7% LV; P=(0.002)]. Collateral blood flow by microspheres during occlusion tended to differ among models (overall P=0.08), with the canine and porcine with stent models having relatively high flow rates comp ared with the acute porcine model. Tc-99m-sestamibi activity correlated wit h microsphere blood now in all three models, with r values for individual a nimals (n=20) ranging from 0.86 to 0.96 (all P<0.0001), There was a signifi cant difference in the regression line intercepts (P<0.0001) and slopes (P< 0.01) among the three models comparing 99mTc-sestamibi uptake with myocardi al blood flow. Tc-99m-sestamibi uptake overestimated blood flow to a greate r extent in the canine model (high flow with viability) than in the porcine model (low flow, absent viability). Despite enhanced collateral flow, ther e was significantly less overestimation of flow in the porcine stent model (high flow, absent viability). In conclusion, at low now rates Tc-99m-sesta mibi activity overestimates myocardial blood now. This effect is most prono unced in myocardium with significant collateral flow and preserved viabilit y, consistent with over-extraction or redistribution of the tracer. The eff ect is markedly decreased in non-viable myocardium regardless of blood now.