An important factor in the increase in flexible ureteroscopy is the greater availability of high-power lasers, which have allowed effective pulverization of kidney stones. With the idea of further improving the efficiency of the laser, the option to modulate the pulse arose. In 2017, the first laser was launched in the market that allowed the delivery of two pulses with different peak powers (Moses™ Technology, Lumenis®) that can reach up to 80 Hz. The latest update of this technology is Moses™ 2.0, which reaches up to 120 Hz. However, evidence on the usefulness of this technology for urinary lithiasis is scarce. One study showed that the extended frequency rate of Moses™ 2.0 had a superior ablation volume to that of the Moses™ Distance of Moses™ 1.0 across all pulse energies at a stone distance of 0 mm, resulting in greater efficiency because of its lower retropulsion with low pulse energies and higher pulse frequencies [13]. The main holmium laser mechanism of action is related to the photoacoustic and photothermal effects generated during intracorporeal lithotripsy [14, 15]. A large proportion of the energy delivered will have a thermal effect, boiling the fluid around the laser tip. One of the most important issues related to laser usage is the potential harm caused by temperature increases. However, the question remains: How hot is too hot? Several studies have attempted to answer this question. Recently, in vitro and in vivo studies have focused on the optimal laser settings and operational parameters for laser firing in the renal collecting system [9, 10, 17]. The thermal dose is the main factor because its biological effects determine cell death and tissue injury. This requires consideration of temperature as well as duration at a given temperature [12]. A clinical observation study showed that even when using 10 W, the lavage solution achieved a threshold of 43°C in 100% of cases [16]. In our study, the threshold was exceeded by over 25 W in the renal pelvis. Furthermore, ureteral temperatures exceeded 43°C when 30 W was used, regardless of the parameter combination. Therefore, it is important to note that backflow from the renal pelvis to the ureter retains heat, having less capacity to dissipate heat because of the lack of space in the ureter in comparison to the renal cavities. The thermal dose is also affected by the pattern of laser activation, specifically by the lasing time (the duration for which the laser is activated by pedal depression). In a recent publication, Aldoukhi et al. showed that 9 s of activation at 40 W was sufficient to cross the threshold with laser activation patterns of 30 s on/off and 15 s on/off [17]. The settings selected for this study were based on activation patterns that are similar to those used in our clinical practice when using a high-power laser. Although it seems logical to think that the time the laser remains active should have an independent influence on temperature, multivariate analysis did not reveal any such influence, maintaining the power used and the irrigation flow rate as the main factors influencing temperature increase. Most studies on temperature and laser use have identified the flow volume infused through the working channel of the endoscope as a fundamental factor. In an ex vivo model evaluating the temperature of the ureter during laser lithotripsy for three seconds, a maximum temperature of 49.5°C was reached when no irrigation was used. In the same scenario, but with an irrigation of 8 mL/s, the peak temperature was 37.4°C [18]. Another in vitro assessment showed that fluid outflow rates of 20 and 30 mL/min were sufficient to maintain the temperature below the threshold (43°C) when using 40 W and 60 W in an intermittent activation fashion [19].
In our study, all the possible parameter combinations were directly affected by the increase in flow rate, with a flow rate of 20 mL/min decreasing temperature by a maximum of 9°C for both the kidney and the ureter. The presence of an access sheath could facilitate more stable temperature control because the continuous outflow would help eliminate the excessive heat caused during intracorporeal lithotripsy. A study performed in a porcine model showed that, under gravity irrigation, flexible ureteroscopy was associated with hazardous intrarenal temperatures at laser powers as low as 20 W. When a ureteral access sheath was used, the temperature remained safe; however, the protective effect disappeared when the laser power was increased to 40 and 60 W [20]. Most of the studies conducted to date have been performed in a simulated environment. The only study that evaluated temperature change in a real clinical setting showed that the majority of laser parameter combinations were associated with potentially harmful temperatures, especially when incarcerated ureteral calculi were treated [16].
Our study had some limitations. First, although the study was carried out in a high-fidelity simulator, it was difficult to determine the real clinical impact of the temperature increase observed in our evaluation. Second, the selection of times during which the laser was active was predetermined based on our usual clinical practice, which is not necessarily the same worldwide. Third, the most appropriate combination of parameters for Moses™ 2.0 technology in urinary stone lithotripsy is still a matter of debate because of the lack of validating publications.