Excessive heat can lead to ureteral strictures, damaged kidneys, and potential loss of function, so the short answer to the question is “Yes!” The concept of thermal dose is important to understand when thinking about the effect of heat in the urinary tract. Cell injury and death can occur at a temperature of 43 °C with an exposure time of 120 minutes. However, with ­sin­gle-digit increases in temperature, the exposure time for cell injury or death halves. At 44 °C, it takes only 60 minutes for it to occur, and at 45 °C, only 30 minutes. At 56 °C, cell injury or death can occur within 1 second. With an estimated 91% to 96% of laser energy being converted to heat, it is both the peak temperature and the duration of exposure that are of concern.

Modern holmium:YAG and thulium fiber lasers (TFLs) used in endourology are capable of both high pulse energy and, perhaps even more importantly, high pulse frequency. The equation of pulse energy (J) × pulse frequency (Hz) = power (W) is critical when thinking about the heat produced during a procedure. At a relatively low pulse energy setting of 0.5 J, a surgeon may think they are operating safely. However, if the pulse frequency is set to 80 Hz, still well below the maximum settings on a modern laser system, the power produced is 40 W. At these settings, dangerous temperatures of 60 °C could be reached within 10 seconds, causing cell death and tissue damage. While there was great enthusiasm related to high pulse frequency settings with the launch of modern lasers, some of this may have been misguided. During “popcorning” at high pulse frequency settings of 80 Hz, only 17.5% of pulses will hit the stone, and the remainder of the pulses will simply be absorbed by the surrounding fluid or hit the neighboring tissue, leading to increased temperature.¹

While the laser settings remain the most critical variable related to heat, other factors need to be considered. The duty cycle refers to the pedal on/off time. A 100% duty cycle implies the user is activating the laser 100% of the time vs a 50% duty cycle, where the user has the laser activated for 50% of the time and then has their foot off the pedal for 50% of the time. Modern lasers with long pulse duration or pulse modulation create less retropulsion of the stone when treated. This can lead to a longer duty cycle since the stone is more stable and can be lased continuously. In this scenario, it is important to take pauses in the laser activation to allow for cooling to occur. Similarly, the surgeon must be mindful of the flow rate of the irrigation fluid. Higher flow rates are protective as this helps to cool the environment by exchanging out the warm fluid that has been heated by the laser in the collecting system. It can be common practice to reduce irrigation flow to prevent the stone from moving, such as when lasering a ureteral stone, but this may inadvertently lead to increased temperature due to the lack of flow. Maintaining a flow rate of 15 to 30 mL/min will aid in heat mitigation at laser settings up to 20 W.²

The size of the laser fiber utilized during a procedure may impact temperatures. With many ureteroscopes, the laser fiber is passed through the same working channel that is also used for irrigation. A larger fiber is therefore going to have a greater impact on reducing flow rates, and lower flow rates are associated with greater heat. One study demonstrated a reduction of 5 °C on average when using a 150-μm core–sized fiber vs a 200-μm core–sized fiber with the TFL during laser lithotripsy. Therefore, the smallest diameter laser fiber that is feasible to use for laser lithotripsy is recommended for ureteroscopy to improve flow rates and aid in heat mitigation.

Along with increasing the inflow during a procedure, improving the outflow can also aid in reducing temperature while simultaneously helping to reduce intrarenal pressure. A properly sized ureteral access sheath will allow fluid to wash out alongside the ureteroscope, essentially venting the system of warm fluid. Cooler fluid is then brought into the kidney through the irrigation channel of the ureteroscope, creating a continuous circuit of flow. If a ureteral access sheath is not used, a smaller diameter ureteroscope may also work similarly by allowing fluid to drain out alongside it. With the recent introduction of a 6.3F flexible ureteroscope (HugeMed), more data are needed to determine if this does in fact improve outflow and thereby reduce temperatures, but the technology is promising. Another recent development is the influx of suction aspiration devices such as suction ureteral access sheaths, flexible and navigable sheaths, direct in-scope suction devices, and steerable ureteroscopic renal evacuation. All these devices should help with fluid turnover in the collecting system and, in theory, aid with reducing temperature. Future study is needed to better understand these potential benefits.

In summary, using the lowest laser pulse energy and frequency settings that efficiently break up the stone while limiting pulses that do not hit the stone is the most important factor in maintaining low temperatures in the collecting system. Our starting settings in the ureter with the TFL are 0.6 J, 6 Hz, and short pulse duration for a total power of 3.6 W. For intrarenal stones, we are using a starting setting of 1 J, 6 Hz, and short pulse duration for a total power of 6 W. Most stones respond well to these settings. Techniques to maintain adequate inflow and outflow for irrigation are important secondary considerations that can help increase the margin of safety. Future advancements including the placement of temperature sensors on devices such as ureteroscopes may allow for real-time monitoring of temperature during a procedure and alert surgeons when temperatures approach dangerous levels.