Pressure and temperature: do high-power lasers pose a threat?
Pressure and temperature: do high-power lasers pose a threat?
In his state-of-the-art lecture, Prof. Evangelos Liatsikos (University of Patras, Greece) argued that both the high-power holmium laser and the superpulse thulium fibre laser can cause significant temperature and pressure damage in the absence of adequate irrigation, even with just 5 Watt (W). Using a ureteral access sheath (UAS) with a diameter of ≥10/12 French (Fr) helps regulating pressure, as well as improving visibility. To avoid damage, it is important to use 40 mL/min irrigation up to a power of 40 W, but in general, high laser power settings require high irrigation flows .
“For many years, we have been using both flexible laser lithotripsy and mini-percutaneous nephrolithotomy (PCNL) like video games without wondering what kind of pressures or what kind of temperatures we are generating inside the kidney,” Prof. Liatsikos stated. Previous research has shown that even with just 1 second of power dangerous temperatures can be reached, whereas the normalisation of the temperature typically requires 5 seconds . Moreover, the current state-of-the-art high-power holmium laser and the superpulse thulium fibre lasers both increase irrigation fluid temperature. The cellular injury threshold is 43°C, which is easily exceeded when low irrigation is combined with high power . Prof. Liatsikos stressed that even as little as 5 W could result in tissue damage if the irrigation is inadequate (≤30 mL/min) . Irrigation should be >100 mL/min for powers higher than 30 W. Laser power >100 W is never recommended.
While excellent at ablation, a recent study showed that the superpulse thulium laser is not safer with regard to temperatures when directly compared with the holmium laser . In this study, no differences were found between the inlet and outlet thermocouple readings when exposed to 8 W, 16 W, or 40 W. There was no substantial difference in the maximum temperature rise of water in the vicinity of the illuminated volume between the 2 laser systems. Thus, it can be concluded that irrigation is still a problem for superpulse thulium laser technology; a conclusion which was further supported in another study observing a rapid temperature above threshold in the absence of irrigation using a superpulse thulium laser .
Fortunately, the kidney can be protected from thermal damage by using a UAS, which kept the renal temperature increase borderline . This UAS buffering effect disappeared with the use of higher laser-power settings (i.e. 40 W and 60 W) with the maximal temperature exceeding 60°C. Moreover, laser activation at 60 W was associated with a very rapid increase in renal temperature within a few seconds. Under pump irrigation, laser activation at 60 W for 60 seconds was associated with a safe temperature <45°C, even without the use of a UAS. The authors of this study concluded that high power settings (>40 W) require high irrigation flow . The use of a UAS is advisable to reduce the temperature and balance any intrarenal pressure increase. Furthermore, Breda and collaborators showed that the use of a UAS provides increased irrigation inflow by 35-80% compared with the flexible scope alone, as well as improved intraoperative vision .
Other studies have investigated irrigation with temperature rates and showed that no irrigation leads to high temperature with as low as 5 W power [8,9]. Irrigation of 40 mL/min does not generate damage up to a power of 40 W.
Prof. Liatsikos asked what the right balance is between temperature and pressure in order to navigate completely on the safe side. Most stones are infected with bacteria, so pumping results in seeding bacteria into the bloodstream and can result in sepsis. The literature clearly states that intrapelvic pressure needs to be <30 mmHg. In the absence of a UAS, forced irrigation can get excessively high, up to 328 mmHg . Only when we have scopes with pressure sensors will we be able to avoid consequent renal extravasation, haematoma, urinoma, sepsis, post-operative pain, and renal scarring as a long-term impact, he noted.
Again, using an access sheath reduces sepsis from 15.2% to 4.3%, possibly attributable to decreased intrapelvic pressure . The access sheath keeps the pressures to a reasonable level. If the ratio of the ureteroscope diameter/inner diameter of the sheath is <0.75, then the flow and intrapelvic pressure is acceptable . In a recent publication, Prof. Liatsikos’ team recorded the maximal renal pelvic pressure during ureteroscopy using an animal model and learned that when using a pump, you need to ascertain that there is an outflow from the kidney. It is important to keep in mind that the threshold for pyelovenous backflow is 40-60 cm H2O or 30-45 mmHg, noting that the units here are important to keep straight .
Many centres will encourage flexible systems because of cost issues, but it is easy to thermoablate healthy parenchyma collaterally. A lot of technology and equipment is implemented, but we do not have enough information on how to use it safely. In the near future, pressure and temperature sensors will be integrated into the new scopes and we may look back at the flexible systems and realise how damaging these really were.
Keywords: Nephrolithotomy, Percutaneous; Lithotripsy, Laser; Kidney Calculi
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Top image: © Mohammed Haneefa Nizamudeen
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