Holmium laser is the most powerful energy tool for stone fragmentation during ureteroscopic lithotripsy . Even though it is regarded as a minimally invasive surgical approach, it still brings troublesome complication, such as ureteral stricture . Ureteral stricture is one of common complication post ureteroscopic surgery, especially for the impacted ureteral stone . Previously, mechanical injury is considered as the main cause for ureteral stricture. However, more and more evidence indicates ureteral stricture can also occur even without mechanical injury. The issue of the intra-opertative holmium laser thermal effect has gradually gained attention [3, 10]. Thermal effect stemmed from holmium laser is considered the hidden etiology for ureteral stricture.
The holmium laser is a long-wavelength pulsed laser, which can transform the energy into optomechanical/photoacoustic and photothermal effect during calculi fragmentation [11, 12]. The estimated mechanical effect to turn a stone into fragments is small. Most of energy will turn into the thermal effect boiling the fluid around the laser tip. Numerous studies have confirmed that significant temperature rise can occur around the fiber. Once, the temperature exceeds the “threshold” (43 °C), it can lead to cell damage, protein coagulation and tissue injury , which subsequentially progresses to scar formation and ureteral stricture. According to the relationship of the tissue thermal damage and temperature, for every 1 °C exceeds the threshold temperature, the time required to cause damage to the same number of cells will decrease by half . Therefore, when the temperature reaches 56 °C, it takes only 1 s to cause thermal damage to the tissue. So, the thermal injury towards ureter will be easily neglected during lithotripsy procedure.
In our current study, we established an in vitro model to evaluate the thermal effect of holmium laser and the protection effect of fluid irrigation during ureteroscopic laser lithotripsy. According to our model, the temperature rise causing by laser firing depends on the laser power, working time and fluid irrigation. Higher power laser can increase temperature much more quickly. The 20w laser setting can increase the temperature as high as 73.5 °C if irrigation is insufficient. Meanwhile, the high-frequency dusting model of holmium laser is much more efficient in heat production. Fluid irrigation plays an important role in regional heat dispersion. The flow rate up to 20 ml/min seems to be the optimal requirement during the laser lithotripsy while considering the intra-renal pelvis pressure and stone retropulsion at the same time. When the flow rate reaches 30 ml/min, the temperature can not surpass the threshold.
Thus, we provide several feasible suggestions to prevent thermal injury during endoscopic laser lithotripsy, especially for impacted ureteral calculi. First, lithotripsy is advised start with low power setting. Generally, low power setting is sufficient to fragment majority of calculi. We recommend that the power setting should not exceed 20w for most devices. Second, apply “high-frequency” setting with more caution. Previous study have showed that dusting maneuver with higher frequency can acquire better stone free rates for renal calculi . Even though the dusting maneuver is more time-consuming and will produce more heat, the renal pelvis can disperse the heat quickly because of its larger space comparing with ureter. So, try to avoid the “high-frequency” laser parameter for stone dusting in the ureter. Third, ensure the fluid irrigation during the procedure. Irrigation flow should be sufficient to clear field of view and cool down the regional temperature. Using a smaller caliber ureteroscope accompanied with fine laser fiber can facilitate a better fluid irrigation and counter-flow. Last, for the impacted calculi, in-situ lithotripsy will be clumsy . It can impair the fluid irrigation by blocking fluid circulation and cause the direct injury towards the ureter mucosa at the impacted site. So, we suggest displace the calculi to proximal portion of ureter prior to calculi fragmentation, since the dilated ureter will provide much more space for fluid irrigation and have less chance of direct injury by the laser fiber.