IMPACT OF REGIONAL VENTRICULAR-FUNCTION, GEOMETRY, AND DOBUTAMINE STRESS ON QUANTITATIVE TC-99M-SESTAMIBI DEFECT SIZE

Background. Serial myocardial perfusion imaging with Tc-99m-sestamibi (MIBI) was used to evaluate infarct risk area and salvage after thrombolysis. The purpose of this investigation was to determine whether changes in MIBI defect size observed after reperfusion may result in part from distortion of regional and global left ventricular geometry. Methods and Results. Twenty-five open-chest dogs were subjected to either 15 minutes (groups 1A and 1B) or 3 hours (group 2) of left anterior descending coronary artery occlusion followed by 3 hours of reperfusion. MIBI was injected before occlusion (group IA) or during occlusion (groups 1B and 2), and serial ECG-gated planar imaging was performed. Dobutamine was infused after 3 hours of reperfusion (groups 1B and 2) to transiently alter left ventricular size and function. Perfusion defect magnitude (DM) and extent (DE) were serially quantified with circumferential profile analysis of end-systolic (ES), end-diastolic (ED), and summed images. Flow was assessed with radiolabeled microspheres and correlated with myocardial MIBI activity. Myocardial thickening was assessed in the risk area with sonomicrometers. In group IA dogs, ischemic dyskinesis produced large artifactual quantitative MIBI defects on ES images (DM, 9.3+/-1.3; DE, 27.8+/-6.0) that were significantly smaller on ED images (DM, 4.5+/-0.9, P<.05; DE, 4.4+/-2.3, P<.05). In addition, DM and DE correlated inversely with myocardial thickening on ES images (DM, r=-.84; DE, r=-.78) and summed images (DM, r=-.72; DE, r=-.61) but not ED images (DM, r=-.12; DE, r=-.15). An index of defect reduction derived from summed images correlated well with thickening fraction in stunned dogs (group 1B, r=.89) but poorly in infarcted dogs (group 2, r=.41) subjected to dobutamine stress. Conclusions. Tc-99m-MIBI defect size may be affected by alteration of left ventricular geometry. Changes in regional function may confound analysis of risk area and myocardial salvage with serial Tc-99m-MIBI imaging and may also affect defect size during pharmacological stress with dobutamine. Dobutamine Tc-99m-MIBI imaging may be useful for distinguishing viable and nonviable myocardium.