## 4 Discussion
The highest temperature recorded, 70.3 \({}^{\circ}\)C, occurred with a laser setting of 40 Watts (1.0 J \(\times\) 40 Hz) and no irrigation after 60 s of continuous laser firing [19]. We conducted our study using 3D models printed from plasticized thermoplastic polyurethane, incorporating chemically synthesized stones. This allowed us to monitor real-time temperature changes during laser lithotripsy. Naturally, the obtained results do not allow for a direct translation to in vivo conditions, but they underscore exceptionally significant changes in the physical conditions within the pelivalyceal system. This translates into safety considerations for conducting the procedure on a patient. Based on the outcomes of our experiment, conducted exclusively under gravity irrigation, without the use of any additional pump irrigation, the maximum temperatures recorded during lithotripsy ranged from 40.3 \({}^{\circ}\)C to 50.9 \({}^{\circ}\)C for UAS 10/12 Fr. When applying a wider UAS (12/14 Fr), the range of maximum temperatures varied from 36.2 \({}^{\circ}\)C to 44.5 \({}^{\circ}\)C, contingent upon the specific laser settings. These results unequivocally indicated a significant correlation between the flow rate and the resultant maximum temperature. This phenomenon can be elucidated by the observation that the fluid flow facilitated by the 12/14 Fr UAS exhibited a magnitude three and a half times superior to that of the 10/12 Fr UAS. Moreover, this disparity in flow rates is contextualized by the comparatively modest twofold variance observed between the maximum and minimum laser power settings employed within the experimental framework. In the lower power spectrum, ranging from 12 W to 15 W, our data revealed no statistically significant temperature difference. However, escalating power settings from 12 W or 15 W to 18 W, 20 W, or 25 W led to substantial and statistically significant increases in temperature. Similar results were demonstrated in a study conducted on a live porcine model by [PERSON] et al. [20]. Using only gravity irrigation during flexible ureteroscopy without a UAS resulted in an elevation of IPT values to hazardous levels, even at the study's lowest laser power of 20 W. At the 20 W power setting, the incorporation of a UAS demonstrated an enhancement in temperature evolution, ensuring that safe IPT values were consistently maintained throughout the entire process. However, the positive impact of a UAS on IPT became nullified when the laser power was set to 40 W or higher, irrespective of the UAS size. Specifically, with the utilization of UAS 12/14, the maximum recorded IPT reached 63.4 \({}^{\circ}\)C at 40 W and 74.5 \({}^{\circ}\)C at 60 W [20]. The significant increase in temperature in the fluid during laser lithotripsy has also been confirmed in a single in vivo study on humans so far. During flexible ureteroscopy, the Ho:YAG laser lithotripsy in the group with the highest temperature increase after 60 s of laser activation was the one with settings at 1 J/20 Hz and an irrigation flow rate of 15 mL/min, while the lowest increase was observed in the group with settings at 0.5 J/20 Hz and an irrigation flow rate of 30 mL/min. When keeping the laser power setting constant, the temperature rise was significantly higher in groups with a lower irrigation flow rate. Similarly, with the same irrigation flow rate, the temperature rise was significantly higher in groups with a higher laser power setting [21]. In our study, when examining the temperature-escalation rate, it was noted that the rate of temperature increase during the initial minutes of laser activity was similar for both flow rates. A potential explanation for the minimal impact of flow rate on the rate of temperature increase at the initial stage of the procedures could be the uneven propagation of heat and the influx of the cooling fluid, which might flow through the shortest path from the end of the ureteroscope to the ureter, not significantly mixing with the heated fluid residing deeper within the system. _A_[PERSON] et al.