Myocardial thallium-201 kinetics during coronary occlusion and reperfusion: Influence of method of reflow and timing of thallium-201 administration

Thallium-201 (²⁰¹Tl) uptake and redistribution kinetics were examined in an open-chest canine preparation of occlusion and reperfusion. Seven dogs (group I) underwent 3 hr of sustained occlusion and received 1.5 mCi of ²⁰¹Tl after 40 min of occlusion of the left anterior descending coronary artery (LAD). Group II (n = 18) underwent 60 min of LAD occlusion followed by sudden and total release of the ligature. Group IIa (n = 8) received intravenous ²⁰¹Tl during occlusion of the LAD, whereas group IIb (n = 10) received intravenous ²⁰¹Tl at the time of peak reflow. Group III dogs (n = 26) also underwent 60 min of LAD occlusion that was followed by gradual reflow through a residual critical stenosis. Animals in this group also received ²⁰¹Tl either before (IIIa; n = 16) or after reflow was established (IIIb; n = 10). In group I, the relative ²⁰¹Tl gradient (nonischemic minus ischemic activity) decreased from 88 ± 8% (mean ± SEM) to 59 ± 6% during 3 hr of coronary occlusion (p = .034). After rapid and total reperfusion (group IIa), this gradient decreased from 71 ± 6% during occlusion to 26 ± 5% after reflow (p<.001). After slow reperfusion through a residual stenosis (group IIIa), the gradient decreased from 81 ± 5% to 31 ± 5% (p<.001) (p = .56 compared with group IIa). In rapidly reperfused dogs receiving intravenous thallium during peak reflow (IIb), initial ²⁰¹Tl activity in the ischemic zone was 155 ± 20% of initial normal activity and fell to 93 ± 13% of normal after 2 hr of reperfusion. Similarly, in dogs reperfused slowly through a critical stenosis (IIIb), which received ²⁰¹Tl during reflow, ²⁰¹Tl activity soon after reflow was 94 ± 4% of initial normal and decreased to 80 ± 6% at 2 hr of reperfusion (p = .10). Histochemical evidence of necrosis was present in the biopsy region in 80% of the 20 dogs subjected to triphenyl tetrazolium chloride (TTC) staining. Microsphere-determined transmural blood flow was similar in all groups during LAD occlusion and final flows after 2 hr were comparable in all subgroups undergoing reflow. Ischemic zone flow (% normal) was significantly higher at the time of ²⁰¹Tl administration in groups IIb (192 ± 25%) and IIIb (110 ± 5%), which received ²⁰¹Tl during reflow, than in groups IIa (31 ± 9%) and IIIa (22 ± 5%), which received ²⁰¹Tl during occlusion. These differences in flow at the time of administration of ²⁰¹Tl explain the different thallium uptake patterns observed. These data suggest that after 1 hr of LAD occlusion there is no difference between rapid reperfusion through a totally patent vessel and slow reperfusion through a critical stenosis with regard to ultimate degree of flow restoration or magnitude of ²⁰¹Tl redistribution in instances in which ²⁰¹Tl is given before reflow. With both methods of reperfusion a residual ²⁰¹Tl gradient is seen. Administration of ²⁰¹Tl during reflow, however, probably overestimates the degree of myocardial salvage as reflected by final ²⁰¹Tl uptake values. In dogs rapidly reperfused, a relative 'hot spot' of ²⁰¹Tl activity was observed in the ischemic zone when ²⁰¹Tl was administered at peak reflow, despite histochemical evidence of necrosis. These results have clinical implications with respect to the timing of ²⁰¹Tl administration and interpretation of serial ²⁰¹Tl scintigrams in patients with acute myocardial infarction undergoing thrombolysis.