Safe pressures for suction ureteric access sheaths: in vitro conversion of pressures to flow

To convert suction pressure settings into practical, size-specific operating limits for suction ureteric access sheaths (sUAS).

Using an 8.7-F flexible ureteroscope with a standard 3.9-F working channel, we measured irrigation inflow at various pressures with no instrument, 270-μm laser fibre, and 1.9-F basket. Outflow was measured using 10/12-, 11/13-, and 12/14-F sUAS with suction applied via Neptune 3™ (Stryker, Kalamazoo, MI, USA) at 30-s intervals and pressures ranging from 50 to 300 mmHg. Inflow and outflow were matched by linear interpolation deriving conservative size- and instrument-specific suction pressure limits for all evaluated sUAS configurations.

Ureteroscope irrigation inflow ranged from 30 to 130 mL/min as irrigation pressure irrigation increased from 50 cmH₂0 (gravity) to 300 mmHg. Working channel instruments reduced inflow: mean (± 2 × the standard error of the mean) change of -57.1% (1.6%) for the 1.9-F basket and -42.1% (2.1%) for the 270-μm laser fibre vs no instrument. Negative pressure via sUAS generated outflow as slow as 5 mL/min (10/12-F sUAS, 50 mmHg suction) and as fast as 700 mL/min (12/14-F sUAS, 300 mmHg suction). Outflow and negative pressure were linearly related (R² 0.94-0.99). Increased scope-to-sheath ratio (SSR) reduced irrigation outflow. Fully opening the valve significantly reduced outflow by 70.8% on average. The 40-cm sUAS modestly increased outflow by 13.3% on average compared to 50-cm sUAS of same diameter. Suction of 50-80 mmHg was ideal for maintaining flow equilibrium with 11/13- and 12/14-F sUAS. The small diameter and high SSR (0.87) of the 10/12-F sUAS required greater negative pressures (150-300 mmHg) to maintain flow equilibrium.

Suction pressures as low as 50-80 mmHg are sufficient for majority of sUAS cases. Increasing irrigation inflow with pressure bag systems may counter excessive sUAS outflow; however, introducing instruments can negate this effect.