Editorial Comment on: Thermal Response to High-Power Holmium Laser Lithotripsy by Aldoukhi et al.

The Holmium:YAG laser has become the lithotripter of choice for endoscopic treatment of urinary calculi. Two major strategies for Holmium:YAG laser lithotripsy are fragmentation with extraction (high pulse energy, low frequency, short pulse width) and dusting (low pulse energy, high frequency, long pulse width). ¹ The dusting technique is currently used for larger stones but some concerns exist regarding this technique: owing to the use of higher power settings up to 40 W (pulse energy 0.2–0.5 J, pulse frequency up to 80 Hz), there is greater energy deposition than with traditional (i.e., fragmentation) settings and an increased risk of thermal tissue injury. ²

Aldoukhi and coworkers from Michigan have set up a very nice protocol to investigate the caliceal fluid temperature changes during holmium laser activation. ² They used traditional and high powered energy settings from 5 to 40 W (0.5–1 J, 10–80 Hz) during a 60-second continuous Ho:YAG laser activation using different settings of irrigation flow (0–40 mL/minute). Most importantly temperature did never exceed 51°C during an activation time of 10 seconds. However, highest temperature (70.3°C) was measured using 1.0 J × 40 Hz without irrigation for 60 seconds of continuous laser firing. These results are of major importance for the endourologist using a laser in the upper urinary tract not only for stones but also for upper tract tumors. ³ Continuous lasering time and reduced irrigation are the limiting factors for temperature increase, which is important in dusting larger stones, especially in caliceal stones with reduced irrigant flow. Aldoukhi et al. support the findings from Butticè and coworkers. ⁴ They found a significant increase in the temperature with closure of the irrigation during Ho:YAG activation (0.5–4 J, 5–20 Hz, maximum 20 W). Their results would be even more pronounced using higher energy settings or the caliceal model of Aldoukhi and coworkers. Wollin et al. currently tested the impact of laser settings on the ureteral temperature during Ho:YAG lithotripsy. ⁵ They clearly demonstrated the impact of irrigation flow to balance the temperature during lithotripsy and found that even lower power settings (<20 W) might produce a clinically significant increase in the maximum temperature with decrease of irrigation.

In conclusion, the endourologist should be aware of the risk of temperature increase during continuous laser application and reduced irrigation in the upper urinary tract to avoid unnecessary damage of surrounding tissue.