Mechanisms and time course for the disappearance of thallium-201 defects at rest in dogs. Relation of time to peak activity to myocardial blood flow

The kinetics of thallium (Tl)-201 in ischemic myocardium was studied with a balloon constrictor to create a fixed reduction in distal left anterior descending coronary arterial pressure in 19 dogs. After 30 to 60 minutes of partial occlusion, Tl-201 was administered intravenously. Tracer activity was monitored continuously for 4 hours in both the normal and reduced flow zones using implanted miniature cadmium telluride radiation detectors. Microsphere-determined regional myocardial blood flow, heart rate, mean arterial pressure, mean left atrial pressure, mean distal left anterior descending coronary arterial pressure and sonomicrometer-determined myocardial wall thickness did not change significantly during each study. Thallium-201 time-activity curves for the normal zones (flow 1.00 ml/min per g or more) demonstrated an initial rapid increase to 80 to 90 percent of peak, an early peak (mean 20 minutes) and then a monoexponential decrease in activity (decay constant λ = 0.0013 min^-1). Thallium-201 time-activity curves for mildly and moderately ischemic zones (flow = 0.40 to 0.99 ml/min per g) demonstrated a progressive delay in the time to peak activity with progressive reductions in flow (r = 0.84). Thallium-201 time-activity curves for severely ischemic zones (flow less than 0.40 ml/min per g) demonstrated a rapid initial increase, and then no further increase in activity, probably because of cell death with a resultant decreased ability to accumulate Tl-201. When the Tl-201 activity ratio (reduced/normal flow zone) was calculated as a function of time for each dog, there was a progressive increase in this ratio over time for all dogs, although the rate of increase was slower for dogs with increasing degrees of flow reduction. This increasing ratio over time would correspond to disappearance of an initial Tl-201 resting scintiscan defect over time. The mechanism for the increasing ratio in dogs with mild or moderate flow reduction was both clearance of Tl-201 from normal zones and accumulation of Tl-201 by ischemic zones. However, the mechanism for the increasing ratio in dogs with severe flow reductions was a faster rate of Tl-201 loss from the normal compared with the reduced flow zone.