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Arbutamine Stress Perfusion Imaging in Dogs with Critical Coronary Artery Stenoses: ^99mTc-Sestamibi Versus ²⁰¹Tl

Experimental Cardiology Laboratory, Cardiovascular Division, Department of Medicine, University of Virginia Health System, Charlottesville, Virginia

Having previously shown that dobutamine reduces ^99mTc-methoxyisobutylisonitrile (sestamibi [MIBI]) uptake in normal myocardium by elevating intracellular calcium, we hypothesized that arbutamine, which has less inotropic effect than dobutamine, might cause less reduction in MIBI uptake, thereby improving defect contrast. In this study using a canine model, we compared the effects of arbutamine stress on myocardial blood flow, myocardial MIBI uptake, and systolic thickening in the presence of a coronary artery stenosis. Methods: Arbutamine was infused (0.5–250 ng/kg/min) in 8 open-chest dogs with critical coronary stenoses that abolished flow reserve. At the time of peak arbutamine effect, MIBI (296 MBq), ²⁰¹Tl (27.75 MBq), and microspheres were coinjected. The dogs were killed 5 min later, and myocardial tracer activities and flow were quantified by well counting. Ex vivo imaging of heart slices was also performed. Results: Arbutamine increased mean heart rate, peak positive left ventricular pressure and its first time-derivative, and normal-zone myocardial thickening. Stenotic zone flow and thickening did not increase during arbutamine infusion. MIBI uptake versus flow was significantly lower than ²⁰¹Tl uptake at the same flow values. By imaging, defect magnitude (stenotic/normal) was greater for ²⁰¹Tl than MIBI (0.56 vs. 0.57; P < 0.001). Conclusion: In the presence of coronary stenoses that abolished regional flow reserve, myocardial uptake of MIBI, compared with ²⁰¹Tl, significantly underestimated the arbutamine-induced flow heterogeneity. The attenuation of MIBI uptake and diminished defect contrast during arbutamine stress were comparable with those previously reported for dobutamine stress.

Key Words: arbutamine • ^99mTc-sestamibi • coronary disease