Beware the bolus size: understanding intrarenal pressure during ureteroscopic fluid administration

Background: Flexible ureteroscopy (FURS) and laser lithotripsy for ureteric and renal calculi requires adequate irrigation for visualisation. This study aimed to evaluate how bolus administration of irrigant fluid impacts intrarenal pressure (IRP) during FURS. We also investigated how ureteral access sheaths (UAS) of varying sizes mitigate elevated IRP. Methods: Using a porcine cadaveric model, IRP was evaluated using an arterial invasive pressure measurement system. Given a fluid column height (driving force) of 80 cm H2O, and varying bolus administration (1, 2, 3, 5, 10 ml), IRP was studied with and without a UAS. An IRP of < 40 mmHg was considered the cut off for “safe” FURS. The flow (drainage capacity) of UAS was also evaluated. At varying fluid column heights, three sizes of UAS were used, 10/12 French size (Fr), 11/13 Fr and 12/14 Fr, all 36 cm long. Results: Bolus administration with a UAS of < 5 ml with a starting fluid column height of 80 cm H2O was “safe” (< 40 mmHg). In contrast, where no UAS was used, bolus sizes as small as 2 ml produced “unsafe” peak pressures. The flow through a 10/12 Fr UAS was poor but improved greatly with larger UAS diameters. Conclusion: This study suggests that 10/12 Fr UAS may be inadequate to maintain drainage from the kidney at acceptable pressures. Bolus fluid administration produces “unsafe” (> 40 mmHg) elevated IRP in the absence of a UAS. When a UAS is used, a fluid bolus of < 5 ml is likely “safe”.