Under pressure: what we thought we knew about gravity irrigation in flexible ureteroscopy

This study revisits the physical principles of gravity-based irrigation in flexible ureteroscopy (fURS), assessing the effects of bag height, reference level, working channel occupancy, bag depletion, and drip chamber configuration on irrigation flow and pressure. An in vitro model was constructed with two 3-L saline bags connected to a Y-end system and a LithoVue™ fURS. Flow rates (mL/min) were measured at five heights (0-200 cm H₂O) using three reference levels: bag top, outflow, and drip chamber. Four working channel conditions were tested (free, 150 μm laser fiber, 200 μm fiber, and 1.9 Fr basket). Additional experiments examined bag volumes (3 L to near empty) and drip chamber content (air vs. fluid). Irrigation pressure at the ureteroscope tip was recorded via a water column. Flow increased proportionally with bag height and was lowest when referenced from the bag top. Channel occupancy markedly reduced flow, with no measurable flow at 0 cm when referenced from the top. Bag depletion and drip chamber air content had negligible influence, even in empty bags. Pressure recordings revealed a negative pressure in depleting bags, and the bag top correlated most closely with ureteroscope pressure. This confirms that gravity irrigation follows hydrostatic principles. Bag height and channel occupancy are the primary determinants of flow, whereas bag volume and drip chamber configuration exert minimal influence. The saline bag top best reflects irrigation pressure and should be the reference level. Negative pressures of 7-8 cm H₂O in depleting bags minimally affected flow but may impact intrarenal pressure.