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Experimental Myocardial Cryoinjury: Local Electromechanical Changes, Arrhythmogenicity, and Methods for Determining Depth of Injury

To characterize the electromechanical effects of acute and 2‐week‐old cryoinjury, programmed stimuiation and epicardial M‐mode echo mapping (7.5 mHz) were performed prior to, at 15 minutes and 35 ± 2 days after cryoinjury in 10 dogs. Epicardial and intramural bipolar and unipoiar electrograms were recorded in five of the dogs. Cryoinjury was produced with a 5 mm in diameter flat cryoprobe at ‐60°C applied to the left ventricular epicardium for 10 minutes at each of six contiguous sites. Cryoinjury, acutely and at 2 weeks, was characterized by a loss of normal intramural systolic thickening and a decrease in echo density. The maximum depth of cryoinjury determined by echo ranged from .55 to .85 cm acutely, and .50 to .80 cm chronically and it correlated acutely and chronically (r = .80; r = .85) with pathologically documented depth of cryoinjury at 2 weeks. Acutely, the presence of either an abnormal intramural unipolar or bipolar electrogram also defined the depth of cryoinjury within .16 cm. In contrast, epicardiai electrograms were not useful for judging depth of cryoinjury. A QS complex on the epicardial unipolar electrogram was recorded over the cryoinjury acutely and at 2 weeks in four of five dogs despite preserved subendocardial systolic wall thickening and normal histology extending 40 to 65% of the wall thickness. Epicardiai bipolar eiectrograms uniformly showed a marked decrease in amplitude and siope of the intrinsic deflection, a small hut insignificant increase in width and no multicomponent activity. Ventricular fibrillation was reproducibly initiated with three ventricular extrastimuli in five of 10 dogs at 2 weeks. No ventricular tachycardia was initiated. Thus, cryoinjury produces characteristic echogram and electrogram changes that are immediate and persistent. The distinct echo signature as well as the presence of abnormal intramural electrograms can be used to judge the depth of cryoinjury. Cryoinjury does not potentiate the inducibility of ventricular tachycardia. The significance of the inducibility of ventricular fibrillation cannot be determined. Our cryoinjury model can be used to characterize electrograms which represent recordings of myocardial depolarization through an interface of cellular necrosis and early fibrosis.