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Myocardial Contractile Reserve and Perfusion Defect Severity with Rest and Stress Dobutamine ^99mTc-Sestamibi SPECT in Canine Stunning and Subendocardial Infarction

¹ Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Medical Institutions, Baltimore, Maryland
² Centro di Studio per la Medicina Nucleare, CNR, Naples, Italy

Myocardial contractile reserve and resting perfusion scintigraphy provide independent information to assess myocardial viability. The purpose of this study was to simultaneously evaluate both with ^99mTc-sestamibi SPECT and low-dose dobutamine in canine stunning and subendocardial infarction (SEMI). Methods: Eighteen dogs were included in the study: 7 normal, 7 stunned, and 4 with SEMI. Closed-chest stunning and SEMI were produced by angioplasty balloon occlusion of the left anterior descending artery (20 and 90 min, respectively). Subsequent radiolabeled mircospheres confirmed reflow, and ^99mTc-sestamibi was then administered at rest. Gated SPECT and MRI tagging were performed at rest and during low-dose dobutamine infusion (5 µg/kg/min). SPECT systolic wall thickening index (SWI) and MRI radial strain quantified myocardial contraction. Postmortem 2,3,5-triphenyltetrazolium chloride staining quantified SEMI. Results: Defect severity by SPECT in the anterior wall was mild and was not statistically different for the stunned versus SEMI groups (P = not significant). At rest, anterior wall SPECT SWI was significantly higher in the normal versus stunned groups (21.1 ± 3.1 vs. 10.1 ± 9.0; P = 0.0002) and the normal versus SEMI groups (21.1 ± 3.1 vs. 2.6 ± 6.0; P = 0.000002). With low-dose dobutamine, SWI increased significantly compared with rest for the stunned group (29.1 ± 10.4 vs. 10.1 ± 9.0; P = 0.000007) but did not increase significantly for the SEMI group (11.0 ± 11.3 vs. 2.6 ± 6.0; P = 0.09); SWI during low-dose dobutamine infusion for the stunned group was comparable to that for the normal group (29.1 ± 10.4 vs. 28.2 ± 7.0; P = 0.80). SWI also showed correlation with MRI radial strain (r = 0.42; P = 0.00015). Conclusion: Defect severity for stunned myocardium and SEMI was mild and was not significantly different. Contractile reserve was significantly different in stunned myocardium and SEMI. ^99mTc-Sestamibi SPECT at rest and with low-dose dobutamine is a promising new technique to simultaneously assess myocardial perfusion and contractile reserve.

Key Words: myocardial infarction • myocardial stunning • ^99mTc-sestamibi • contractile reserve • myocardial viability

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