PD30-07 WHAT IS THE OPTIMAL URETEROSCOPE SHAPE TO REDUCE INTRAPELVIC PRESSURE?

Abstract

INTRODUCTION AND OBJECTIVE: Intrapelvic Pressure (IPP) during ureteroscopy is a function of fluid inflow and outflow. Ureteral access sheaths (UAS) have been shown to improve outflow and thereby reduce IPP during uretreoscope (US) use. Low IPP is thought to reduce pyelovenous backflow, bleeding, and sepsis. Given that maximal UAS diameter is limited by ureteral diameter, and US size is constrained by manufacturing technology and the size of the working channel, we sought to determine the optimal US cross section (CS) shape to reduce IPP. METHODS: Four US, each with the CS area of a 7.5 Fr circular US, were simulated within a 10 Fr UAS using COMSOL (COMSOL Inc., Stockholm, SWE) finite element analysis software. Volumetric flow rate in the US working channel is given by QUS=(Pirr-Pp)/RUS where Pirr is irrigation pressure at the scope tip, Pp is IPP, and RUS is hydrodynamic resistance. Assuming laminar flow, RUS is estimated by the Hagen-Poiseuille equation, RUS=128μlUS/(πdUS4). Here, μ is the viscosity of water and lUS and dUS are the length and the diameter of the scope. Flow from the pelvis occurs between the US and the UAS; expressed as QU=Pp/RUAS. Resistance when the cross-sections of both UAS and US are concentric and circular is well known (Figure, A). For other configurations, this resistance is solved by equations of motion in COMSOL. Steady state IPP is reached when the inflow equals outflow. RESULTS: For any given irrigation pressure, the lowest IPP and highest outflow was seen with an offset circular US (Figure, D) as this configuration had only 41% of the outflow resistance of a US in the center of a UAS. Time to steady state was roughly 5 minutes. By modifying the shape of the US to a circle with a 90° chord angle (Figure, C) the CS area of the US is reduced by 9% but the outflow resistance drops to only 60%. An elliptical CS provides more modest IPP reduction without sacrificing US CS area. CONCLUSIONS: The lowest IPP and highest outflow is seen with an offset circular US, followed by a US with a 90° chord angle. Small collapsible projections inside the UAS or on the shaft of a US could provide such an offset without affecting flow or stone basket extraction. Alternatively, a US could be constructed with a D shaped cross section. Modifying US shape is increasingly relevant as UAS diameter is limited to that of the ureter and modern US are so narrow that clinically meaningful diameter reduction is unlikely.Source of Funding: None