Myocardial kinetics of thallium-201 after stress in normal and perfusion-reduced canine myocardium

Abstract

Despite the emerging use of quantitative computer programs for assessing myocardial thallium uptake and clearance after exercise, little is known about the kinetics of thallium after exercise stress. Accordingly, 11 mongrel dogs with experimental left anterior descending coronary stenoses were given thallium during norepinephrine infusion to simulate exercise. The infusion was discontinued and thallium activity was monitored regionally using miniature radiation detectors for 3 hours. Heart rate, arterial pressure and double product all increased significantly during norepinephrine infusion. The mean fractional myocardial thallium clearance was lower (0.47 ± 0.03 [± standard error of the mean]) for the stenosis zone than for the no-stenosis zone (0.57 ± 0.03) (p < 0.0001). The stress blood flow ratio (stenosis/no-stenosis zone = 0.27 ± 0.06) was significantly lower than the final thallium activity ratio (0.68 ± 0.07) (p < 0.001), consistent with thallium redistribution occurring over the 3-hour period. Myocardial thallium activity in the stenosis zone peaked in a mean of 2.2 minutes, then washed out biexponentially with a final decay constant of 0.0035 ± 0.0005 min −1. Myocardial thallium activity in the no-stenosis zone peaked within 1 minute in all dogs, then washed out biexponentially, with a final decay constant of 0.0043 ± 0.0003 (p < 0.001 compared with stenosis zone). In conclusion, fractional clearance of thallium can differentiate myocardium distal to a coronary artery stenosis from that supplied by a normal coronary vessel.