An In Vitro Study of the Thermal Effect of Holmium Laser Lithotripsy of Ureteral Stones

Abstract Objective To observe the local thermal effect of holmium laser in ureteral models. Methods This study was conducted in July-August 2022 using a 3D printed kidney model (ureteral diameter: approximately 6 mm; length: approximately 20 cm). A clinically collected calcium oxalate monohydrate stone was polished into a 5 mm diameter sphere, placed under the ureteropelvic junction, and ligated with silk wire above the stone. The kidney model was placed in a water bath, and the temperature of the water bath was maintained at approximately 37°C to simulate the constant temperature of the human body. A fifth generation EMS pulse width tunable holmium laser was selected as the laser device. The laser fiber diameter was 200 μm. The thermometer was a multi-channel real-time thermometer with two temperature probes placed in the ureter. The distance between the probe and the fiber tip was maintained at 5 mm. Perfusion fluid: saline, temperature around 24°C; perfusion rate: 0, 10, 20, 30, 50 ml/min. We used a flexible ureteroscope with holmium laser fiber. The lithotripsy was performed by a physician. The holmium laser was continuously excited for 120 s. The temperature was measured and recorded once per second by an electronic thermometer. Each set of experiments was repeated three times. Results The operating graph of the temperature around the holmium laser fiber versus the time was recorded for different operating modes, and the change in temperature around the fiber was recorded at each time point in the absence of perfusion. The temperature of 43°C was reached around the fiber after an average of 9.2±3.0 s of laser excitation; the higher the power, the shorter the time to reach 43°C. The operating curve of the temperature around the laser fiber versus the time was recorded at the perfusion rate of 20 ml/min for each time period, and it was found that the temperature around the fiber was ≤43°C at ≤20 W of fragmentation power. At a perfusion flow rate of 50 ml/min, the holmium laser began to excite and reached a plateau between approximately 20 and 30 s. The plateau temperature powdered group > fragmentation group (p < 0.01). Conclusion During ureteroscopic holmium laser lithotripsy, holmium laser excitation will produce a local thermal effect. Keeping the lithotripsy power ≤ 20 W within the perfusion flow rate ≥ 20 ml/min can reduce the local high temperatures caused by the thermal effect of the holmium laser and reduce any thermal damage, while the local thermal effect of low-energy high-frequency powdering mode is the highest and the local thermal effect of high-energy low-frequency powdering mode is the lowest.