Effects of dipyridamole-induced vasodilation on myocardial uptake and clearance kinetics of thallium-201

Abstract

Myocardial thallium-201 (201Tl) uptake and clearance after intravenous administration of dipyridamole (150 micrograms/kg) were determined in 12 open-chest anesthetized dogs with a partial coronary artery stenosis. 201Tl (1.5 mCi) was injected intravenously and myocardial biopsy specimens were obtained 10 min, 60 min, and 2 hr after injection. Serial changes in 201Tl activity in the normal zone and in the zone of partial stenosis were correlated with microsphere-determined regional blood flow and distal coronary pressure. Another nine dogs with equivalent stenosis not given dipyridamole before 201Tl served as controls. In the 12 dogs given dipyridamole, 201Tl activity at 10 min in the zone of stenosis was reduced to 42 +/- 5% of initial normal zone activity (p less than .001) and remained at 44 +/- 3% of initial normal zone activity at 2 hr. There was a good correlation (.81) between the percent reduction in myocardial 201Tl activity and the percent reduction of peak hyperemic flow as determined by measuring the percentage difference in peak coronary flow after a transient 10 sec occlusion under control and stenotic conditions. In contrast, 201Tl clearance was rapid in the normal zone, with 201Tl activity decreasing to 55 +/- 3% of initial normal zone activity by 2 hr. A redistribution pattern was produced because of the disparate clearance rates from hyperperfused and relatively hypoperfused myocardial regions. The relative 201Tl defect decreased from 58% to 11% from 10 min to 2 hr. In the normal zone dipyridamole increased epicardial flow from 1.03 +/- 0.09 (SEM) to 3.52 +/- 0.36 ml/min/g (p less than .0001) and endocardial flow from 1.19 +/- 0.09 to 2.96 +/- 0.20 ml/min/g (p = .0001). In the zone of partial stenosis the increase in epicardial flow after dipyridamole was less marked (1.01 +/- 0.10 to 1.55 +/- 0.15 ml/min/g; p = .009) and endocardial flow decreased (0.84 +/- 0.11 to 0.64 +/- 0.15 ml/min/g; p = .04). Coronary perfusion pressure distal to the stenotic zone fell from 65 +/- 3 to 50 +/- 3 mm Hg after dipyridamole. In the nine control dogs with equivalent stenosis, 201Tl uptake and washout were not significantly different in the stenotic zone compared with the normal zone. These data indicate that dipyridamole-induced vasodilation in the presence of a partial stenosis results in diminished uptake and delayed clearance compared with increased uptake and more rapid clearance in normally perfused myocardium producing an initial 201Tl defect with delayed redistribution.(ABSTRACT TRUNCATED AT 400 WORDS)