Characterization of Fluid Dynamics and Temperature Profiles During Ureteroscopy with Laser Activation in a Model Ureter.

Ureteral thermal injury has been reported in patients following ureteroscopy with laser lithotripsy due to overheating of fluid within the ureter. Proper understanding of this risk necessitates knowing the volume of fluid available to absorb laser energy. This can be approximated as the volume of fluid that mixes during laser activation, since energy transfer through fluid is dominated by convection. Objectives of this study were to determine the volume of fluid that mixes during laser activation at different irrigation rates and to characterize the temporal/spatial temperature distribution in a model ureter. The model ureter consisted of a plastic tube-160 mm length and 5.3 mm inner diameter. Irrigation was first applied with clear, then dyed, deionized water at rates from 8 to 40 mL/min. The laser was activated at 20 W (0.5 J/40 Hz). The distances the dyed fluid propagated were measured and volumes calculated. Temperatures were recorded from six thermocouples-five embedded within the tube and one affixed to the ureteroscope. Thermal dose was calculated using the Dewey and Sapareto methodology. The volume of total fluid mixing in the model ureter was ≤1.26 ± 0.10 cm, consistent with a sharp temperature increase after laser activation from -5 to 25 mm from the ureteroscope tip. With irrigation rates ≤12 mL/min, calculated thermal dose within the model ureter exceeded the threshold of tissue injury and extended greater distances along the ureter with lower irrigation rates. The volume of total fluid mixing within the model ureter was found to be small thus conferring a greater risk of ureteral thermal injury. A thermocouple positioned near the tip of the ureteroscope reasonably approximates temperature in front of the ureteroscope. Until temperature sensors are incorporated into ureteroscopic systems, laser power settings should be carefully selected to minimize risk of ureteral thermal injury.