Myocardial kinetics of thallium-201 after dipyridamole infusion in normal canine myocardium and in myocardium distal to a stenosis.

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The purpose of the present study was to define myocardial and blood thallium-201 (Tl-201) kinetics after infusion of dipyridamole in normal canine myocardium and in myocardium distal to a coronary artery stenosis. Miniature radiation detector probes were implanted in the left ventricle in 39 open-chest dogs. A balloon constrictor was placed around the proximal left circumflex coronary artery. Electromagnetic flow probes were positioned proximally around both the left circumflex and left anterior descending coronary arteries. In five control dogs (group 1) the balloon occluder was not inflated; in 12 dogs (group 2) a mild stenosis was created such that resting flow was not reduced, yet the hyperemic response after 10 s of total occlusion was partially attenuated; in nine dogs (group 3) a moderate stenosis was created such that resting flow was not reduced, yet the hyperemic response was completely eliminated; and in 13 dogs (group 4) a severe stenosis was created such that resting flow was reduced. After intravenous dipyridamole (0.08 mg/kg . min-1 x 4 min), 1.5 mCi Tl-201 was injected intravenously and probe counts were collected continuously for 4 h. The mean 4-h fractional myocardial Tl-201 clearance for nonstenotic zones was 0.35, 0.27 for group 2 stenotic zones, 0.19 for group 3 stenotic zones, and 0.05 for group 4 stenotic zones (P less than 0.0001). After reaching peak activity, myocardial Tl-201 activity cleared biexponentially with a final decay constant lambda 2 = 0.0017 +/- 0.0001 min-1 (SE) for nonstenotic zones, 0.0011 +/- 0.0001 min-1 for group 2 stenotic zones, and 0.0006 +/- 0.0001 min-1 for group 3 stenotic zones (P less than 0.01). Group 4 stenotic zone Tl-201 clearances were negligible (decay constant essentially zero). Blood Tl-201 activity decayed triexponentially with a final blood lambda 3 = 0.0018 +/- 0.0001 min-1, which was almost identical to the final myocardial lambda 2 decay constant. Thus, the rate of myocardial Tl-201 clearance can distinguish between coronary stenoses of graded hemodynamic severity. These results may be applicable to quantitative techniques for determining myocardial Tl-201 clearance rates on serial clinical images after dipyridamole administration.

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