Predicting clinical efficacy of photoangiolytic and cutting/ablating lasers using the chick chorioallantoic membrane model: implications for endoscopic voice surgery.

Objectives/hypothesis: The optimal balance between a laser's clinical efficacy and collateral thermal damage is the major determinant for selection of a particular laser in endolaryngeal surgery. The chick chorioallantoic membrane (CAM) simulates the microvasculature of the human vocal fold and is, therefore, useful for testing effects of laser settings, mode of delivery, active cooling, and wavelength. Such information is essential for optimizing the effectiveness of lasers in treating laryngeal pathology while preserving vocal function.
Study Design and Methods: The thermal and coagulative effects of four lasers (585 nm PDL, 532 nm KTP, 2.01ìm Thulium, 10.6ìm CO2) were quantified at selected (and clinically relevant) energy settings before and after tissue cooling using the CAM model. Measures included imaging real-time vascular reactions in the CAM model (i.e., vessel coagulation and/or rupture), and post-procedure histologic analysis of CAM tissue. In each experiment, laser energy was applied to the CAM in a controlled manner. Cooling was done using a dermatological cold-air device, and temperatures were measured with a thermistor. Lasers tested included the photoangiolytic pulsed-dye (PDL) and KTP, as well as the ablative/cutting CO2 and thulium lasers. The vessel rupture/coagulation and thermal effects of various energy-delivery parameters on the CAM, with and without cooling, were assessed. After removal of the CAM, specimens were stained as whole-mounts, photographed at 4X magnification, and evaluated by two independent, blinded surgeon reviewers. The efficacy of increased pulse-width (KTP laser) on treating larger vessels (>0.5 mm) and the effects of extravasated blood on photoangiolysis were also evaluated.
Results: Photoangiolytic lasers: Vessel coagulation/rupture rates showed that the PDL caused more frequent vessel rupture than the KTP laser. For both lasers, cooling the CAM by approximately 20 degrees C resulted in 30% - 60% reduction in the thermal-damage zone (P < .05). Cooling reduced the efficacy of coagulation with the PDL but not with the KTP laser. The clinically observed phenomenon that laser heating of extravasated blood increases thermal damage and decreases efficacy of coagulation was clearly evident in the CAM model. Ablative lasers: The thermal-damage zone of the CO2 laser (0.3 mm spot size) was not significantly different with or without cooling (0.32 mm2 and 0.34 mm2, respectively) (P = .30). However, when the spot size was defocused to 1 mm, the thermal-damage zone was over 2x greater when the tissue was not cooled (0.74 mm2 vs 0.35 mm2) (P < .002). The thermal-damage zone of the Thulium laser was reduced by an average of 58% for the three power settings tested when the CAM was air-cooled (P < .05).
Conclusions: The CAM was an excellent model for studying the effects of photoangiolytic lasers, for which optimal pulse-widths exist for vessel coagulation. Smaller vessels coagulated reliably at pulse widths >15 msec, and larger vessels required pulse widths >35 msec for optimal coagulation. Cooling the target tissue decreased the thermal-damage zone created by photoangiolytic lasers. While cooling had no effect on the efficacy of coagulation with longer pulse widths (KTP), tissue cooling decreased the coagulation rate at shorter pulse widths (PDL). The thermal effects of cutting/ablating lasers can be reduced with cooling, but the CAM was not a good model with which to study coagulation/rupture rates in cutting/ablating lasers.