Thallium Clearance from the Myocardium

Serial myocardial imaging after a single dose of thallium is often used in evaluating patients with coronary artery disease. Because the appearance of the image is related to myocardial thallium content, it is important to know if transient ischemia occurring after tracer administration will influence the rate of myocardial thallium clearance. To answer this question 13 mongrel dogs (eight study dogs and five controls) were studied as follows. The left anterior descending (LAD) coronary artery and the anterior interventricular vein (AIV) were isolated. A catheter was inserted in the AIV to collect venous blood draining from the myocardium in the LAD distribution. Next, a snare was positioned around the LAD proximal to the tip of the AIV catheter. Thallium was given intravenously and paired samples of arterial and AIV blood were obtained every 10 minutes for 1 hour to determine thallium activity and potassium and hydrogen ion concentration. The LAD was then occluded for 10 minutes and paired blood samples were obtained. Next, the snare was released and paired blood samples were obtained beginning 10 seconds, 4, 10 and 20 minutes after reperfusion. Regional myocardial blood flow at the time of thallium administration and during the coronary occlusion was determined by the microsphere technique. After 20 minutes of reperfusion, the dog was sacrificed. The hearts were sectioned and counted for thallium and microsphere activity. During the coronary occlusion, the rate of thallium clearance (,uCi/min X 10-2) from myocardium drained by the AIV (-1.1 ± 0.3, mean ± SEM) was similar to the rate just before occlusion (-0.68 ± 0.21, p = NS), as well as to the rate during the first minute of reperfusion (-0.77 ± 0.55, p = NS). In contrast, compared with the rate (mEq/min X 10-3) just before occlusion (-0.55 ± 0.61) a significant (p < 0.001) increase in output of potassium (-7.15 ± 1.04) by myocardium in the AIV distribution was observed during the coronary occlusion. Likewise, hydrogen ion output (mEq/ min X 10-11) also increased significantly (p < 0.05) during the occlusion (12.26 + 0.94) compared with the rate just before occlusion (-6.98 ± 0.78). At postmortem examination the ratio of thallium activity in the ischemic zone to that of the normal zone (1.01 ± 0.01) did not differ significantly from the ratio of thallium activity of the LAD to that of the circumflex region (1.04 ± 0.05) in control dogs in which no occlusion was performed. We conclude that transient ischemia occurring after thallium administration does not influence the normal rate of thallium clearance from the myocardium and thus may not be detected on serial scans.