Editorial Comment on: Effect of Laser Settings and Irrigation Rates on Ureteral Temperature During Holmium Laser Lithotripsy,an In Vitro Model by Wollin et al.

In the article written by Wollin and colleagues, the authors approach a hot topic, because of the essential and widespread use of holmium:yttrium-aluminim-garnet (Ho:Yag) laser in endourology. The same day of Wollin's group publication, Aldoukhi et al. published a similar article. ¹ Ho:Yag energy has been demonstrated to be absorbed by water and has short extinction length in tissue with a depth of penetration of 400 μm ^2,3 and urologists focused their attention on laser effectiveness in breaking stones or cutting tissue rather than on potential damages caused by laser energy. The low morbidity profile of flexible ureteroscopy (F-URS) is demonstrated in several studies. ⁴ However, the laser high density of power absorbed in irrigation solution and tissue results in a rough overheating. ⁵ As stated by the authors, 43°C is the threshold for exponential thermal cytotoxicity. Above this temperature, cellular functions may be altered as a consequence of protein denaturation.

Until now, there are a few studies on the effect of laser setting and irrigation system on intraluminal temperature during intraureteral surgery. ⁶ Ho:Yag laser has been used in the upper urinary tract not only to treat ureteral and kidney stones but also to cut ureteral strictures and ablate upper tract neoplasm. Many studies have been done, aiming to define the most efficient laser setting for different endourological procedures, in respect of patient safety avoiding damages of instruments used. Companies are launching on the market more and more powerful machines, up to 120 Watt often implemented with specific software for stone treatment. Irrigation is an issue of primary importance in endourology, allowing and providing a better vision and cooling the system. F-URSs are constantly downsizing because of technical advancements with working/irrigation channel diameter decreasing to 3.6F for majority of ureteroscopes making critical the irrigation when the working channel is occupied by instruments. Several systems to force irrigation are available ranging from the cheapest gravitational system to pumps able to define exactly temperature and pressure output. ⁷ Wollin and colleagues' article underlines how temperature control can help endourologists in a more conscious usage of laser power reducing potential ureteral injuries.

The authors decided to use a flow unit (0 cc/min, 50 cc/min, 100 cc/min). Flow is easy to calculate if you have a system able to measure exit flow or if flow is maintained constant throughout surgery, which in daily practice does not happen. Most urologists use gravity system, with or without the auxilium of different irrigation systems. It would be useful if authors, repeating the experiment in vivo, report the correlation pressure/flow. Aldouki et al. in their experiment report both flow and pressure. It is evident how two parameters are related. ¹ To obtain a flow of 15 cc/min in their model, pressure required was 100 cmH₂O; to obtain a flow of 40 cc/min, pressure was calculated to be 304 cmH₂O. Nowadays, surgeons have to consider the intrarenal pressure to prevent upper tract damages and postoperative infections, the technique preferred to treat the target that is the appropriate laser energy and now even the heating effect due to laser power. Wollin and colleagues' article shed some light on this topic and by correlating irrigation and temperature give some more freedom in laser setting choice to be effective respecting patient tissues. Moreover, looking at the primary results in vitro and according to Aldoukhi and colleagues' study too, beside the need of a proper irrigation, we can assume that high-power machines do not seem to be necessary for upper urinary tract treatment. In fact, high frequency (>40 Hz) results in blurred vision ⁸ and high power results in high intraluminal temperature. The logical question is: do we really need high power holmium laser machines for upper urinary tract disease during ureterorenoscopy? The answer, presently, is on debate. We congratulate the authors for identifying this outcome and for continuing to seek out temperatures in the future with in vivo study during lasing in ureterorenoscopy.