Re: “Toward Respiratory-Gated Retrograde Intrarenal Surgery: A Prospective Controlled Randomized Study” by Kourmpetis et al.

Dear Editor:

In this randomized controlled trial, the authors sought to improve the surgical performance of retrograde intrarenal surgery (RIRS) by reducing the respiratory kidney motion (RKM) with low ventilation (LV). They came to a conclusion that surgical performance was significantly improved with LV, as well as endourologists in training had similar surgical performance as experts compared with setting of conventional ventilation (CV). Furthermore, no intraoperative complications reported and postoperative complications rate were comparable. In other words, LV did not contribute to a higher risk of surgery and ventilation-related complications even the ventilation parameter different from CV. ¹

However, an average of 50 mm Hg of end-tidal carbon dioxide (EtCO₂) level in LV meant hypoventilation during RIRS, compared with a normal of 36 mm Hg in CV. Obviously, special situations such as longer duration of surgery and elderly patients with chronic comorbidities such as coronary heart disease and pulmonary diseases would require further investigation. To minimize the potential risk of the high level EtCO₂, a technique named respiratory support system (CRS), which combined small volume mechanical ventilation with high-frequency jet ventilation (HFJV), was reported by Gadzhiev et al. in 2019. ² But with the concern of ventilation parameter and no objective criteria to assess the RKM in this study, it was hard to draw a conclusion that CRS was better than CV. In contrast, CV did not present a significantly adverse effect on lithotripsy, especially when a steady anesthetic status was attained.

Apnea as a valuable technique to stabilize the stone during laser lithotripsy was introduced by Traxer and colleagues in 2016. ³ With scarce studies focused on this issue, our question is that why RKM was not a concern until recently since RIRS has been developed for half of century? Under monitoring by ultrasonography or fluoroscopy, an average kidney movement was 12 mm (range 5–19 mm) under free breathing, 34.3 ± 4.3 mm during CV, and 4.1 ± 1.9 mm during HFJV. ^{4 –6} Given other factors were closely associated with performance and outcomes of RIRS, such as retropulsion caused by energy emission, influence of infundibulopelvic angle and experience of surgeon, present urologist seemed a little picky. An overall 96% of stone-free rate was better than previous reports, but no significant differences were found between LV and CV. Maybe the influence of normal RKM is not too much to alter final stone-free rate or increase complications after RIRS. The issue of novel mechanical ventilation in RIRS is new and also controversial. With no better designed equipment and large-scale RCTs available, it is still hard to tell the safety and effectiveness of respiratory control in RIRS in present.