In-vitro evaluation of intra-cavity pressures across various flexible ureteroscope and ureteral access sheath size combinations.

The primary aim was to assess how various flexible ureteroscopes and ureteral access sheath size combinations affected intra-cavity pressures, under excessive irrigation pressures with a pressure bag. The secondary aim was to determine the maximum irrigation pressure that could be applied while still maintaining safe intra-cavity pressures, when a certain sheath-scope combination had resulted in an unsafe intra-cavity pressure in the initial experiment. An in-vitro setup was used to measure intra-cavity pressures across different flexible ureteroscope (6.3-9.6 Fr; Hugemed, Innovex and WiScope) and sheath size (ClearPetra; inner lumen diameter 9.5-12 Fr, lengths 40 and 50 cm) combinations. A steady irrigation pressure was delivered via a pressure bag pressurised to 360 mmHg, and a urodynamic machine provided real-time readings of intra-cavity pressure. For each combination, the steady-state intra-cavity pressure was recorded. Combinations generating an unsafe intra-cavity pressure (i.e. intra-cavity pressure of more than 30 mmHg) were re-assessed by gradually reducing the irrigation pressure, until intra-cavity pressure recorded reached 30 mmHg. The corresponding irrigation pressure was then recorded, and the experiment was then repeated with this established safe irrigation pressure to confirm that a safe intra-cavity pressure had been achieved. An intra-cavity pressure of 30 mmHg was taken to be the determined safe threshold for intra-cavity pressure. High intra-cavity pressures were observed with smaller sheaths and larger scopes, exceeding 100 mmHg in some combinations. Longer sheaths tended to generate higher pressures than shorter ones. The use of 12 Fr sheaths or 6.3 Fr scopes appeared to be safe regardless of the paired scope or sheath, even under extreme inflow pressure. Scope and sheath combinations have a significant impact on intra-cavity pressure, and inappropriate combinations may lead to unsafely elevated pressures. Larger sheaths and smaller scopes offer safer pressure profiles, even under high flow or high-pressure irrigation.