Patterns of Laser Activation During Ureteroscopic Lithotripsy: Effects on Caliceal Fluid Temperature and Thermal Dose

Characterizing patterns of laser activation is important for assessing thermal dose during laser lithotripsy. The objective of this study was twofold: first, to quantify the range of operator duty cycle (ODC) and pedal activation time during clinical laser lithotripsy procedures, and second, to determine thermal dose in an in vitro caliceal model when 1200 J of energy was applied with different patterns of 50% ODC for 60 seconds. Methods: Data from laser logs of ureteroscopy cases performed over a 3-month period were used to calculate ODC (lasing time/lithotripsy time). Temporal and rolling 1-minute average power tracings were generated for each case. In vitro experiments were conducted using a 21 mm diameter glass bulb in a 37°C water bath, simulating a renal calix. A LithoVue ureteroscope with attached thermocouple was inserted and 8 mL/min irrigation was delivered with a 242 μm laser fiber within the working channel. In total, 1200 J of laser energy was applied in five different patterns at 20 W average power for 60 seconds. Thermal dose was calculated using the Sapareto and Dewey t 43 method. Results: A total of 63 clinical cases were included in the analysis. Mean ODC was 32% overall and 63% during the 1-minute of greatest energy delivery. Mean time of pedal activation was 3.6 seconds. In vitro studies revealed longer pedal activation times produced higher peak temperature and thermal dose. Thermal injury threshold was reached in 9 seconds when 40 W was applied at 50% ODC with laser activation patterns of 30 seconds on/off and 15 seconds on/off. Conclusion: ODC was quantified from clinical laser lithotripsy cases: 32% overall and 63% during 1-minute of peak power. Time of pedal activation is an important factor contributing to fluid heating and thermal dose. Awareness of these concepts is necessary to reduce risk of thermal injury during laser lithotripsy procedures.