Thermally Controlled Laser Irradiation of the Myocardium with Intraoperative Ultrasound Monitoring

We used intraoperative ultrasonography (IOUS) to study the feasibility and safety of Nd:YAG laser irradiation of the myocardium in 26 canine left ventricular segments. During the laser irradiation process, the myocardial temperature was monitored and surface cooling was used. Afterward, intraoperative ultrasonographic scans, which enabled the evaluation of the irradiated lesions, were compared with cross sections made through the tissues. The total dose of laser energy ranged from 200 to 3,600 joules, and the estimated volume of irradiated lesions ranged from 76.8 to 2590 mm3. There were significant correlations between the laser discharge output (in joules [J]) and the irradiated lesion volume (P less than 0.001), and between the laser energy density (in J/mm2) and the depth of the lesions (P less than 0.01). Macroscopic examination of the cross sections of irradiated myocardium revealed that the lesions were well-demarcated, but not charred or perforated. Thus, we could obtain a satisfactorily large zone of laser photocoagulation without inducing tissue damage, if surface cooling was used and the myocardial temperature was monitored. IOUS was successful in visualizing and locating the irradiated lesions that were seen as hypoechoic, clearly outlined nodules. We submit that this study has established the technical feasibility and dose-response relationship of thermally controlled laser irradiation, and has demonstrated the usefulness of IOUS for the precise localization and monitoring of such laser treatment.