A simple fluid dynamic model of renal pelvis pressures during ureteroscopic kidney stone treatment

Abstract

Ureteroscopy is an endoscopic kidney stone removal procedure which increases the internal pressure in the renal pelvis, the kidney’s urinary collecting system. Elevated renal pelvic pressure may result in systemic absorption of irrigation fluid and urine, which can increase the risk of postoperative fever and sepsis. Urologists have investigated the effects of various surgical parameters on the renal pelvic pressure. However, it still remains unknown which surgical parameter has the most dominant effect on the renal pelvic pressure over time. Here we develop a physical model that computes the renal pelvic pressure as a function of time based on parameters that can be varied during ureteroscopy. The model is developed by applying pipe network analysis to the regions of the urinary tract that are involved in a representative ureteroscopic procedure. Our model unifies the findings of the previously published studies on this topic; an ex-vivo porcine study and an in-vivo human study. Furthermore it allows simulation of surgical procedures based on various techniques. Our simulation demonstrates that the two strong regulators of renal pelvis pressure during ureteroscopy are the size of the gap between ureteroscope and ureteral access sheath and the frequency and duration of ureteroscope withdrawal.

Effects of geometry on resistance in elliptical pipe flows

“…by improving visibility within the kidney (Williams et al 2019a). Minimising the flow resistance through the access sheath leads to lower kidney pressures during ureterorenoscopy (Williams et al 2019b;Oratis et al 2018) which is desirable as high pressures have been linked to post-operative complications, such as sepsis (Wilson & Preminger 1990). This application in particular motivates us to address questions of flow optimality in the sense of achieving the maximum flow rate for a prescribed axial pressure drop and fixed cross-sectional area available for fluid flow.…”

A lumped-parameter model for kidney pressure during stone removal

“…Through this, we can understand how geometric changes to particular components of the irrigation system will affect flow rate and kidney pressure. Fluid is driven through the working channel via a prescribed inlet pressure, and by modelling the kidney as a compliant body, and equating fluxes in through the scope and back out through the sheath, a lumped-parameter model for flow rate and kidney pressure as functions of time can be derived (Oratis et al, 2018). The resulting ordinary differential equation depends on the ratio between the viscous resistances through the working channel and back through the sheath, and the relationship 3 of 21 between changes in kidney pressure and volume (compliance).…”

“…The resulting ordinary differential equation depends on the ratio between the viscous resistances through the working channel and back through the sheath, and the relationship 3 of 21 between changes in kidney pressure and volume (compliance). In Oratis et al (2018), flow through the working channel and access sheath were modelled as Poiseuille flow through an unobstructed pipe and a concentric annular pipe of circular cross-sections, respectively. The importance of low outflow resistance to maintain low intrarenal pressures was discussed in Oratis et al (2018) only within the context of increasing access sheath size or intermittently withdrawing the scope.…”

“…In Oratis et al (2018), flow through the working channel and access sheath were modelled as Poiseuille flow through an unobstructed pipe and a concentric annular pipe of circular cross-sections, respectively. The importance of low outflow resistance to maintain low intrarenal pressures was discussed in Oratis et al (2018) only within the context of increasing access sheath size or intermittently withdrawing the scope. We will extend this work by varying the inflow and outflow resistances to represent: the addition of working tools, a non-concentric scope shaft, and modifying the cross-sectional shapes of the scope shaft and access sheath.…”

“…A second pipe, the access sheath, surrounds the shaft of the scope, allowing fluid to flow back out of the urinary system. We modify and extend a previously developed model (Oratis et al, 2018) through the use of an exponential, instead of linear, constitutive law for kidney compliance, and by exploring the effects of variable flow resistance, dependent on the presence of auxiliary working tools in the working channel and the cross-sectional shapes of the tools, working channel, scope shaft, and access sheath. We motivate the chosen function for kidney compliance, and validate the model predictions, with ex-vivo experimental data.…”