Characterization of renal pelvis compliance and outflow resistance during ureteroscopy of the ex-vivo human kidney

INTRODUCTION: Elevated intrarenal pressure during ureteroscopy has been linked to complications during recovery from stone removal surgery. Intrarenal pressure depends on irrigation flow rate, inflow and outflow resistance, and kidney compliance. These have been examined for porcine models in the literature. In this study, we report on the experimental method and results of characterizing compliance and outflow resistance in human cadaveric kidneys. METHODS: To characterize compliance a catheter is inserted retrograde into the renal collecting system and sealed at the UPJ. Pressurized irrigation fluid is then allowed to flow into the renal pelvis. Pressure and flow are recorded over time, representing a measure of compliance. To characterize outflow resistance a UAS is inserted into the ureter and a ureteroscope placed. Inflow is increased over time and pressure and outflow recorded. RESULTS: The resulting compliance curves show a high variability between specimens. Anatomical differences and initial volume are significant contributing factors. We show that non-dimensionalizing the data with respect to a measure of volume allows us to fit a single non-linear compliance model to the data. Our outflow testing shows that with consistent placement of devices, outflow resistance is primarily a function of device parameters and largely independent of the specimen anatomy. We determined an outflow resistance of 8.1 mmHg/(ml/min) (σ = 1.6) for UAS size 10/12 F, 0.73 mmHg/(ml/min) (σ = 0.05) for size 11/13 F and 0.18 mmHg/(ml/min) (σ = 0.04) for size 12/14 F. CONCLUSION: The compliance results for human kidneys add a critical data set that enables more accurate modelling of the mechanics of processes involved in stone removal. The outflow data similarly provides critical parameters for modelling the mechanics of ureteroscopic procedures and confirms that this characteristic is primarily driven by surgical device parameters.