Recognition of reversible and irreversible myocardial injury by technetium pyrophosphate extraction kinetics

The need for a more accurate method of detecting episodes of myocardial ischemia during cardiac operations, particularly during the ischemic arrest interval, prompted us to investigate the usefulness of measuring the active extraction of technetium pyrophosphate in identifying and quantitating ischemic injury. Twenty-four adult mongrel dogs were subjected to cardiopulmonary bypass, and normothermic global ischemia was induced by cross-clamping the proximal aorta. Technetium pyrophosphate (1 mCi) was injected through a standard cardioplegia line with normal saline, simulating administration of cardioplegic solution, upon placement of the aortic cross-clamp (time 0), at 15, 30, 45, and 60 minutes of global ischemia, and with the onset and completion of ischemic contracture. Radioactive counts were recorded over the heart at 1 second intervals, and the extraction fraction and half-time of clearance were calculated. The extraction fraction increased from 0.22 at time 0 to 0.58 at 15 minutes, 0.82 at 30 minutes, 0.85 at 45 minutes, and 0.91 at 60 minutes. The halftime increased from a baseline of 114 seconds (time 0) to a maximum of 321 seconds at 60 minutes of ischemia. The onset and completion of ischemic contracture showed a return toward baseline of both the extraction fraction and halftime of clearance, with an extraction fraction of 0.44 and 0.46 and a halftime of 135 and 133 seconds, respectively. These data clearly show that reversible myocardial injury increased the extraction and reduced the clearance of technetium pyrophosphate and that the magnitude of change related to the extent of injury. The progression to irreversible myocardial injury decreased the active extraction of technetium pyrophosphate. This simple procedure for real-time documentation of myocardial injury promises to provide easily obtainable endpoints of injury for use during cardiac operations in humans.