Ureteroscope‐sheath size mismatch affects intrarenal pressure during suction‐assisted flexible ureteroscopy: a human ex vivo study

Intrarenal pressure (IRP) control during flexible ureteroscopy (FURS) is crucial to minimise the risk of pyelovenous backflow and its potential harmful consequences in terms of infectious complications and renal injury [1]. In recent years, flexible and navigable suction ureteric access sheaths (FANS) have been introduced to support safer and more efficient FURS. By enabling continuous aspiration of irrigation fluid and stone fragments, suction sheaths may improve visualisation, reduce stone retropulsion, facilitate efficient fragment evacuation and help maintain low IRP, potentially enhancing both procedural efficacy and patient safety [2]. Despite the growing adoption of FANS in clinical practice [3-5], comparative data on pressure and flow dynamics across different ureteroscope–sheath combinations remain limited to in vitro studies [6-8]. We therefore conducted an ex vivo human kidney study to evaluate the effect of different ureteroscope–sheath calibre combinations on IRP control during suction-assisted FURS. An ex vivo human kidney was obtained from a patient undergoing nephrectomy for a functionally excluded renal unit secondary to nephrolithiasis, following written informed consent and review board approval (NCT06187870). During explantation, the ureter was preserved up to the iliac vessel bifurcation. The specimen was submerged in saline solution and connected to an open irrigation–outflow system allowing controlled hydrostatic irrigation pressure delivery. The FURS was performed using three ureteroscopes with increasing outer diameters: 6.3 F (HU30M; HugeMed, Shenzhen, Guangdong, China), 7.5 F (PU3033A; Pusen, Zhuhai, Guangdong, China), and 8.4 F (URF-V3; Olympus, Tokyo, Japan). Each ureteroscope was tested, when technically feasible, with three FANS calibres: 9/11 F and 10/12 F (Elephant II; YiGao Medical Technology, Hangzhou, Zhejiang, China) and 11/13 F (Clear Petra; Well Lead Medical, Panyu, Guangzhou, China) (Fig. 1). Irrigation was delivered using a gravity-based system set at 40 cmH2O and 100 cmH2O, without hand-assisted pumping. Mean IRP was continuously measured using a 4.8-F pressure-sensing catheter positioned in the renal pelvis alongside the access sheath and connected to a standard arterial pressure transducer. Measurements were recorded after pressure stabilisation for at least 30 s, both at baseline (no suction) and after suction activation. Suction was applied at a fixed target negative pressure of approximately −200 cmH2O using the full-range setting of the FANS. For clarity, only results obtained at the lowest (40 cmH2O) and highest (100 cmH2O) irrigation pressures are reported. The 9/11-F sheath did not accommodate the 8.4-F ureteroscope and this configuration was therefore not tested. At an irrigation height of 40 cmH2O, mean IRP measured at baseline with the HugeMed 6.3-F scope was 24 cmH2O, which decreased to 8.2 cmH2O when activated the 9/11-F FANS (mean difference: 15.8 cmH2O). At 100 cmH2O, IRP similarly decreased from 41 cmH2O at baseline to 5 cmH2O with suction assistance (mean difference: 36 cmH2O). Conversely, when the Pusen 7.5-F ureteroscope was combined with the 9/11-F sheath, suction failed to achieve meaningful pressure reduction (mean difference: 0 cmH2O), with mean IRP values persisting at 25 and 40 cmH2O at irrigation heights of 40 and 100 cmH2O, respectively. In contrast, the 10/12-F sheath provided significant IRP reduction with both the HugeMed and Pusen ureteroscopes, lowering mean IRP from 24 to 6 cmH2O at 40 cmH2O irrigation (mean difference: 22 cmH2O) and from 41 to 4 cmH2O at 100 cmH2O (mean difference: 37 cmH2O). The combination of the Olympus URF-V3 ureteroscope with the 10/12-F sheath did not result in effective pressure reduction, with mean IRP plateauing at baseline values, despite continuous suction over 1-min interval across all tested irrigation heights (mean difference: 0 cmH2O). Finally, the 11/13-F sheath consistently achieved perceptibly rapid and complete decompression of the intrarenal collecting system. Specifically, baseline values ranged from ~24 to 28 cmH2O at 40 cmH2O irrigation and from 41 to 45 cmH2O at 100 cmH2O with the HugeMed, Pusen, and URF-V3 ureteroscopes, progressively. Upon suction activation, IRP promptly decreased to 0 cmH2O across all ureteroscopes and irrigation settings, often resulting in collapse of the intrarenal cavity within seconds. The principal finding of this study is that effective suction-assisted IRP control during FURS is strongly dependent on the calibre gap between the ureteroscope and the access sheath. In our ex vivo human kidney model, reliable pressure reduction was observed only when a minimum gap of ~2.5 F between the ureteroscope outer diameter and the sheath inner lumen was preserved. When this condition was met, suction activation consistently resulted in marked IRP reductions, even at high irrigation pressures (up to 100 cmH2O), whereas narrower configurations failed to achieve meaningful decompression despite continuous suction. These findings are consistent with prior bench and in vitro studies highlighting the importance of endoscope–sheath dimensional relationships. In an in vitro renal model, Shi et al. [7] showed that negative-pressure ureteric access sheaths maintained safe IRP (<40 cmH2O) only when the endoscope-to-sheath diameter ratio (RESD) remained below ~0.85, while higher ratios (≥0.89) resulted in loss of pressure control despite suction. Translating these ratios into absolute dimensions corresponds closely to the ~2–3 F effective clearance required in our study. Similarly, Ding et al. [8] demonstrated that configurations with smaller RESD values (~0.62–0.64) not only maximised aspiration efficiency but also maintained low intrapelvic pressures across a wide range of irrigation flows and negative pressure settings, whereas narrower outflow spaces reduced functional benefit. Our study extends these observations by demonstrating, in a biologically compliant ex vivo human kidney model, that borderline combinations, such as an 8.4-F ureteroscope within a 10/12-F sheath, remain ineffective despite active suction. From a clinical perspective, these data emphasise that suction alone is insufficient to ensure IRP control. While smaller-calibre access sheaths may reduce ureteric trauma, pairing them with larger ureteroscopes can negate the pressure-lowering effect of suction. Expressing this relationship as an absolute calibre gap, rather than a ratio, may therefore provide a more intuitive and clinically practical framework for device selection during suction-assisted FURS [7, 8]. Our study is not devoid of limitations. The ex vivo setup does not allow full control of fluid losses, and repeated measurements may alter collecting system compliance over time; for this reason, repeated testing to quantify measures of dispersion for each configuration was not possible. These factors introduce unavoidable variability that should be acknowledged when interpreting the results. Nonetheless, the internal consistency of our results across irrigation pressures and device combinations supports the robustness of the observed relationship. In conclusion, our findings should be considered for clinical decision-making in device selection to optimise IRP control during suction-mediated FURS, by providing an easily interpretable and clinically applicable threshold of 2.5-F gap between the flexible ureteroscope outer diameter and FANS inner lumen to ensure adequate suction and IRP control. When this condition was met, FANS effectively reduced IRP even under high irrigation inflow conditions (up to 100 cmH2O). Olivier Traxer is consultant for Coloplast, Rocamed, Olympus, EMS, Boston Scientific, Quanta System, and IPG. Eugenio Ventimiglia is consultant for Quanta System. Guido Giusti and Silvia Proietti are consultants for Dornier MedTech Systems. The remaining authors have no conflicts of interest to declare.