First Intraluminal Temperature Measurement during Ho:YAG-Laser Exposure at an In-Vitro URS

Introduction: The laser is a high energy instrument which can melt metals like nitinol. So it is very important to know under which conditions it is dangerous to perform an endourologic lithotripsy. We measure the temperature increase during laser exposure in an underwater in-vitro ureter model. For comparison, temperatures with and without irrigation and with different distances from the laser fiber to the thermometer are measured. Materials and Methods: We used the Ho:YAG-laser (Vera PulseTM, Coherent) with a 365 μm laser fiber. The settings of the laser were 0.6 J with a frequency of 5 Hz which is the minimum setting for that type of laser. The experimental setup was closely aligned with the clinical situation. A metal container was filled with 0.9% sodium chloride (NaCl) solution (Temp. 36.8°) and a catheter with an inner diameter of 4 mm was attached to the rim of the container. The tip of the thermometer was attached inside the catheter through a waterproof hole. The laser fiber was guided by means of a rigid URS video device (11.5 F). We had four different settings during the measurement: 1) Distance of 0.5 cm between the laser and the thermometer; without irrigation, 2) Distance of 0.5 cm between the laser and the thermometer; with irrigation, 3) Distance of 1 cm between the laser and the thermometer; without irrigation, 4) Distance of 1 cm between the laser and the thermometer; with irrigation. Results: The maximum overall temperature was recorded in the 1) and 3) setting, both featuring no irrigation. The maximum temperature was ~50°C in both settings, with the 1) setting reaching the maximum temperature after 50 seconds and hence approximately twice as fast as the 3) setting. During measurements with a NaCl solution flow we couldn’t detect any noticeable increase in temperature, neither at short nor at long distance between the laser fiber and the thermometer. Conclusion: There is a relevant heating in the ureter beside an endourologic lithotripsy. In our model we could reproduce a maximum heating until ~50°C without irrigation and no heating with irrigation. Without irrigation there is a relevant bubble formation which should be an indicator for the surgeon to stop lithotripsy due to a temperature increase which could harm surrounding tissue.