The effect of ischemia on thallium-201 clearance from the myocardium.

To determine the effect of ischemia on myocardial clearance of thallium-201 (201Tl), we studied 12 dogs with ischemia produced after the injection of Tl. Tl was given I.V. 10 minutes before left anterior descending (LAD) coronary artery ligation. 85Sr-microspheres (MS) were administered 5 minutes later, and control biopsies were obtained from the myocardium. The LAD was tied and repeat biopsies obtained from the ischemic zone (IZ) and normal zone (NZ) 15 minures and 2 hours later. 46Sc-MS were given just before the final giopsy. Tl activity in the IZ was not significantly different from that in the NZ either before LAD occlusion or 15 minutes and 2 hours later. Tl clearance at the end of 2 hours was not significantly different (27 +/- 5% vs 28 +/- 5%, IZ vs NZ respectively) between the two zones. The half-time of Tl clearance from both the IZ and NZ was calculated at 4.5 hours (consistent with previously reported normal values). This occurred despite a decrease in regional myocardial blood flow to 24 +/- 6% of control (P less than 0.01) in the IZ and an increase to 47 +/- 14% of control (P less than 0.01) in the NZ during the study. We conclude that myocardial ischemia does not alter the normal rate of Tl clearance from the myocardium.