A holmium laser is a high-energy pulsed laser with a wavelength of 2100 nm. Its lithotripsy mechanism includes photothermal and photomechanical effects.[1] The photothermal effect is the absorption of laser energy by water to form a vapor bubble cavity. Then, the laser energy reaches the stone surface through the vapor bubble cavity channel, and the resulting micro-vapor bubbles break the stone during expansion or fragmentation.[5] The photothermal mechanism is that the temperature of the stone rises rapidly after the laser energy is absorbed, and the high temperature causes the stone to decompose; while the laser energy is absorbed by the water and the resulting vacuole expands to form a micro-vapor bubble, which in turn can cause the stone to break down.[6] As the laser energy is absorbed by water in the liquid environment, a thermal effect phenomenon is generated. Whether this phenomenon causes local high tissue temperatures and burns local tissue is an important indicator for evaluating its efficacy.[7] Research shows that body temperature > 43 ℃ will cause substantial damage to cells; local tissue temperature > 45 ℃ will cause tissue damage, and at temperature > 60 ℃, tissue protein thermal coagulation denaturation results and irreversible damage occurs.[8, 9]
The holmium laser has a penetration depth of approximately 0.3 mm and has good precision cutting ability, which is ideal for endoscopic treatment. Theoretically, it does not cause direct thermal damage as long as the laser fiber is >0.3 mm away from human tissue.[10] In the human urinary tract there is only a small amount of fluid. To avoid complications such as stone regression or intrapelvic hypertension, the operator will be able to reduce the irrigation flow during the procedure, which may cause thermal damage to the tissue due to insufficient local irrigation and heat accumulation beyond the safe tissue temperature.[11] The literature suggests that the thermal effect of holmium lasers has become a major cause of intraoperative urological tract injury. Through in vitro experiments, we found that the laser was continuously excited for 9.2 ± 3.0 s in the absence of perfusion, and the temperature of each group exceeded 43°C. Considering that the perfusion fluid we used during surgery was generally approximately 22-25°C at room temperature, which was consistent with the temperature of the perfusion fluid used in our experiments, this suggests that in clinical surgery, with an output power ≥ 10 W and no or poor perfusion, continuous laser excitation should not exceed 9 s, and the intermittent excitation mode should be adopted for lithotripsy. By observing the process of continuous laser excitation to the plateau temperature, we found that under the same conditions, the dusting mode produced the greatest thermal effect and the highest plateau temperature; while the fragmenting mode produced the least thermal effect and the lowest plateau temperature. This suggested that in the case of ureteral stone obstruction, the fragmenting mode could be used preferentially to break up the stone, release the obstruction, and protect kidney function, while the laser work time is short and a postoperative stone residual rate is lower[4].
Compared with Chinese scholars’ studies on the thermal effects of holmium lasers, [12, 13] their models lacked perfusion and did not correspond to actual clinical practice. Some foreign scholars have also conducted similar studies.[9] Therefore, based on the recommendations of other scholars, we used a 3D printed kidney model to simulate a real human ureter, a 37°C thermostatic water bath to simulate a human thermostatic environment, and a bilateral thermometry probe for temperature measurement to make the holmium laser thermal effect study model more realistic and the measurement results more accurate. During the experiment, we found that the temperature of the plateau period gradually decreased with the gradual increase of the perfusion volume and that when the perfusion volume was above 20 ml/min, the temperature around the fiber dropped to below 43°C. Maxwell et al.,[14] in an in vitro model, found an intra-ureteral holmium laser temperature of 42°C at a perfusion volume of 15 ml/min, while Aldoukhi et al.[15] found a local temperature of approximately 43.1°C in the in vitro model when using a 20 W power powdered lithotripsy mode, maintaining a perfusion volume of approximately 10 ml. The safe perfusion volume varied between models with a large variability in the distance between the temperature probe and the head end of the fiber, but maintaining a perfusion volume above 20 ml/min was safe for all. As the perfusion volume decreases, the plateau temperature exceeds 43°C when 10 ml/min, which indicates that the heat that can be taken away by the perfused liquid is much less than the heat released by the laser, and the perfusion volume needs to be increased to remove the excess heat to keep the plateau temperature at 43°C. In conclusion, under the experimental conditions of this study, the infused fluid can maintain a safe local temperature by effective heat convection and diffusion when the infusion volume is ≥20 ml/min.
This study could not fully model the real scenario of in vivo lithotripsy because it was an in vitro model of lithotripsy, and there were some limitations. However, the preliminary findings provide a valuable reference for in vivo testing and clinical application, and subsequent in vivo testing on animals is needed to further validate the findings.
In summary, a local thermal effect caused by holmium laser excitation will occur during ureteroscopic holmium laser lithotripsy. Locally high temperatures caused by the thermal effect and the thermal damage can be reduced using a lithotripsy power ≤20 W with a perfusion flow rate ≥20 ml/min. The high-energy low-frequency fragmenting mode has the lowest local thermal effect, whereas the low-energy high-frequency dusting mode has the largest.
The datasets generated and/or analysed during the current study are not publicly available due the mechine is not appear on the market, all the data are should be privated, but are available from the corresponding author on reasonable request