Determination of Irrigation Flowrate During Flexible Ureteroscopy: Methods for Calculation Using Renal Pelvis Pressure

Background: Proper control of irrigation flowrate during ureteroscopy is important to manage thermal and pressure risks. This task is challenging because flowrate is not directly measured by commercially available ureteroscopic or fluid management systems. However, flowrate can be calculated using a hydrodynamic relationship based on measurable values during ureteroscopy. Objectives of this in vitro study were to (1) calculate inflow resistance for different working channel conditions and then using these values and (2) calculate irrigation flowrate and determine its accuracy across a range of renal pelvis pressures. Materials and Methods: A 16 L container was filled with deionized water and connected by irrigation tubing to a 9.6F single-use ureteroscope. Inflow resistance was determined by plotting flowrate (mass of fluid collected from ureteroscope tip in 60 seconds) vs irrigation pressure (range 0-200 cmH₂O). Next, the tip of the ureteroscope was inserted into the renal pelvis of a silicone kidney-ureter model and renal pelvis pressure was measured. In conjunction with the previously determined inflow resistance and known irrigation pressure values, flowrate was calculated and compared with experimentally measured values. All trials were performed in triplicate for working channel conditions: empty, 200 μm laser fiber, 365 μm laser fiber, and 1.9F stone basket. Results: Flowrate was linearly dependent on irrigation pressure for each working channel condition. Inflow resistance was determined to be 5.0 cmH₂O/(mL/min) with the 200 μm laser fiber in the working channel and calculated flowrates were within 1 mL/min of measured flowrates. Similar results were seen with a 365 μm laser fiber, and 1.9F basket. Conclusions: Utilizing renal pelvis pressure measurements, flowrate was accurately calculated across a range of working channel conditions and irrigation pressures. Incorporation of this methodology into future ureteroscopic systems that measure intrarenal pressure could provide a real-time readout of flowrate for the urologist and thereby enhance safety and efficiency of laser lithotripsy.