Rubidium-82 kinetics after coronary occlusion: temporal relation of net myocardial accumulation and viability in open-chested dogs

Serial assessment of perfusion and viability during myocardial infarction has not been feasible, in part, because of the long half-lives of available tracers. Rubidium-82 (82Rb) is a generator-produced, positron-emitting potassium analog with a short half-life (75 sec) that permits repeated studies. To determine the temporal relation of net myocardial 82Rb accumulation to loss of viability during prolonged ischemia, a 2-3 mCi bolus of 82Rb was given to 46 open-chested dogs while regional myocardial time-activity curves were obtained with beta probes at baseline, and serially after coronary occlusion lasting 1-6 hr. Hearts were then stained with triphenyl tetrazolium chloride (TTC) to assess the viability of the epicardium under the probe to a depth corresponding to the range of positrons. Irreversible injury occurred in two out of 16 experiments at 1 hr and ten out of 15 experiments at 3 hr and also at 6 hr (p less than 0.05 vs. 1 hr). In viable myocardial samples, rubidium extraction increased with low flow as compared with nonischemic controls for all time periods but was unchanged (failed to increase) in nonviable tissue. Net 82Rb accumulation decreased during 1 to 6 hr of occlusion in irreversibly injured samples (0.28 +/- 0.19 to 0.16 +/- 0.07, p less than 0.05) but remained unchanged in myocardial tissue subsequently shown to be viable. For myocardial samples that were nonviable at 3 and 6 hr, changes in net accumulation of tracer became abnormal only after 6 hr of occlusion. The mechanisms primarily responsible for the decrease in net accumulation of 82Rb at 6 hr appeared to be leakage of tracer after first pass. Therefore, failure to increase extraction at low flows may be an early indicator of cell death, whereas membrane leakage occurs several hours after loss of viability.