Caliceal Fluid Temperature During High-Power Holmium Laser Lithotripsy in an In Vivo Porcine Model

Abstract

Introduction: With increasing use of high-power laser settings for lithotripsy, the potential exists to induce thermal tissue damage. In vitro studies have demonstrated that temperature elevation sufficient to cause thermal tissue damage can occur with certain laser and irrigation settings. The objective of this pilot study was to measure caliceal fluid temperature during high-power laser lithotripsy in an in vivo porcine model.

Methods: Four female pigs (30-35 kg) were placed under general anesthesia and positioned supine. Retrograde ureteroscopy with entry into upper or middle calices was performed. Thermocouples were placed into the calix by open exposure and puncture of the kidney or retrograde alongside the ureteroscope. A 242 μm laser fiber was positioned in the center of the calix and activated (0.5 J, 80 Hz, 40 W) for 60 seconds with high, medium, or no irrigation delivered in each trial. Finite element simulations of laser-induced heating in a renal calix were also performed.

Results: Peak temperatures of 84.8°C, 63.9°C, and 43.6°C were recorded for no, medium, and high irrigation, respectively. Mean time to reach threshold of thermal injury (t₄₃ of 120 minutes) was 12.7 and 17.8 seconds for no and medium irrigation. Thermal damage thresholds were not reached in high-irrigation trials. Numerical simulations revealed similar results with peak spatial average fluid temperatures of >100°C, 58.5°C, and 37.5°C during 60 seconds of laser activation for 0.1, 15, and 40 mL/minute irrigation, respectively.

Conclusions: High-power holmium laser settings (40 W) can induce potentially injurious temperatures in the porcine in vivo model, particularly with slower irrigation rates. Characterization of thermal dose across a broader range of laser parameter settings is underway to map out the thermal safety envelope.

Keywords: holmium laser; in vivo; lithotripsy; temperature; ureteroscopy.

FIG. 1.

Experimental setup showing (A) the pig positioned supine on the table with ureteroscope inserted to renal calix, (B) modified subcostal incision showing the exposed kidney and needle thermocouple, and (C) wire thermocouple secured to the ureteroscope.

FIG. 2.

Temperature recordings for all trials with no irrigation. Laser energy was applied starting at 10 seconds and ending at 70 seconds.

FIG. 3.

Temperature recording for all trials with medium irrigation. Laser energy was applied starting at 10 seconds and ending at 70 seconds.

FIG. 4.

Mean temperature with each irrigation rate. Laser energy was applied starting at 10 seconds and ending at 70 seconds.

FIG. 5.

Gross image of a bivalved kidney showing charred urothelium and thermal injury after three (60 seconds) trials at high, medium, and no irrigation.

FIG. 6.

Numerical simulation of temperature distribution after 2, 5, and 59 seconds of laser activation with 15 mL/minute irrigation. Room temperature irrigation fluid was introduced through a simulated ureteroscope (white tube) and started 20 seconds before laser activation. Simulated laser heating is generated by a small source 5 mm below the simulated ureteroscope, causing temperature elevation of the fluid and surrounding tissue. The scale bar is 1 cm.