ABSTRACTTo determine safety and efficacy of neodymium:YAG laser irradiation of the endocardium, temperatures at both the epicardium and the endocardium were recorded for thermal damage evaluation. A total of 48 coagulation lesions were created at power settings of 20 and 30 W in 20 open chest dogs by transcatheter endocardial laser irradiation. Tissue temperatures were monitored by epicardial thermography (Tepi), and by endocardial thermocouples at the catheter tip (Tprox) and 4 mm below the endocardial surface (Tdist). In group I the optical fiber extended 1 mm from the catheter and irradiation times ranged from 3 to 60 sec.Tepireached ≥57° after a weighted average of 5 sec of laser irradiation (n= 44). In group II the fiber was retracted 1 mm from the catheter tip, and irradiation times were 100 to 150 sec.Tepireached ≥57°C after a weighted average of 30 sec (n= 4). Blood vessels were recognized as heat sinks until coagulation occurred. Lesion volume showed a proportional increase with total delivered energy. From the observed timeframes in epicardial temperature rise it is suggested that total direct light absorption at the epicardium was the main contribution toTepi, and the Nd:YAG laser can efficiently create transmural lesions. The epicardial temperatures remained below 80°C in combination with the constant movement of the epicardial wall suggested safety from thermal damage to the ambient
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