Comparison of continuous versus pulsed CO2 and Nd:YAG laser-induced pulmonary parenchymal lung injury in a rabbit model

Background and Objective Laser techniques have recently been described for treatment of patients with emphysema and bullous lung disease. Laser exposure of the pulmonary parenchyma during these procedures is complicated by laser‐induced lung injury with substantial associated morbidity. Animal investigations are needed to develop methods that reduce lung injury. We hypothesized that the depth of injury could be reduced by pulsing laser exposures, with the goal of limiting thermal effects to more superficial tissue levels. In this study, we compared acute and chronic histologic injury resulting from pulsed‐ versus continuous‐mode CO2 and Nd:YAG laser pulmonary parenchymal exposures in rabbits. Study Design/Materials and Methods A total of 40 New Zealand White (NZW) rabbits underwent thoracotomy followed by exposure with CO2 laser (n = 10 continuous vs. n = 10 pulsed at 250 Hz with duty cycle 0.15 ms) or ND:YAG laser (n = 10 continuous vs. n = 10 pulsed at 10 Hz with duty cycle 0.10 sec) to the visceral pleural surface using 1 minute of laser exposure (5 watts, defocused to 70 W/cm2 power density) to the exposed lung surface. Rabbits were sacrificed at 4 and 21 days post‐injury, and lungs were examined histologically. Results CO2 and Nd:YAG laser treatment resulted in substantial pulmonary parenchymal injury. While CO2 laser‐induced damage was distinct from Nd:YAG histologically, pulsed‐mode laser exposures did not reduce lung injury for either laser. Acute edema occurred to depths of 1180 ± 338 μm for continuous‐mode CO2 laser exposures compared to 1,340 ± 430 μm in pulsed mode (p = .77). For Nd:YAG laser exposure, acute edema depth was 750 ± 748 μm continuous versus 1120 ± 367 μm pulsed mode (p = .65). Chronic lung fibrosis depth was 450 ± 164 μm for CO2 continuous Injury From Pulsed‐ Vs. Continuous‐Mode Laser mode compared to 575 ± 170 μm in pulsed mode (p = .61). Lung fibrosis depth for Nd:YAG was 550 ± 96 μm, continuous versus 484 ± 180 μm pulsed mode (p = .76). Conclusion The similarity in injury between pulsed‐ and continuous‐mode exposures suggests that thermal relaxation times are long relative to the selected pulse frequencies in intact living rabbit lungs. Alternatively, brief high‐energy pulsations may increase focal temperatures with a tendency to increase injury depth relative to the penetration of the laser light. Thus, pulsed laser modes in these settings appear to be ineffective in reducing laser‐induced lung injury in clinical settings. © 1