Thermal Response to High-Power Holmium Laser Lithotripsy

Objectives: The aim of this study was to investigate caliceal fluid temperature changes during holmium laser activation/lithotripsy using settings up to 40 W power output with different irrigation flow rates. Materials and Methods: The experimental system consisted of a glass test tube (diameter 10mm/length 75mm) filled with deionized water, to mimic a calix. Real-time temperature was recorded using a thermocouple (Physitemp, NJ) positioned 5mm from the bottom of the tube. A 200m laser fiber (Flexiva; Boston Scientific, MA) was introduced through the working channel of a disposable ureteroscope (LithoVue; Boston Scientific) and the laser fiber tip was positioned 15mm above the bottom of the test tube. Deionized water irrigation (room temperature) through the working channel of the ureteroscope was delivered at flow rates of 0, 7-8, 14-15, and 38-40mL/minute. A 120-W holmium laser (pulse 120; Lumenis, CA) was used. The following settings were explored: 0.5Jx10Hz, 1.0Jx10Hz, 0.5Jx20Hz, 1.0Jx20Hz, 0.5Jx40Hz, 1.0Jx40Hz, and 0.5Jx80Hz. During each experiment, the laser was activated continuously for 60 seconds. Results: Temperature increased with increasing laser power output and decreasing irrigation flow rate. The highest temperature, 70.3 degrees C (standard deviation 2.7), occurred with laser setting of 1.0Jx40Hz and no irrigation after 60 seconds of continuous laser firing. None of the tested laser settings and irrigation parameters produced temperature exceeding 51 degrees C when activated for only 10 seconds of continuous laser firing. Conclusion: High-power holmium settings fired in long bursts with low irrigation flow rates can generate high fluid temperatures in a laboratory caliceal model. Awareness of this risk allows urologist to implement a variety of techniques (higher irrigation flow rates, intermittent laser activation, and potentially cooled irrigation fluid) to control and mitigate thermal effects during holmium laser lithotripsy.