Ex Vivo Porcine Kidney Model for Evaluating Flow-Pressure Dynamics and Stone Retrieval in Suction-Assisted Retrograde Intrarenal Surgery

Summary

This protocol establishes an ex vivo porcine kidney platform for controlled, repeatable quantification of intrarenal pressure and stone pull-out time across different irrigation pressures, suction pressures, and ratios of endoscope-sheath diameter.

Abstract

Retrograde intrarenal surgery (RIRS) using a suction ureteral access sheath (UAS) has become an effective approach for renal stone management. However, standardized experimental models capable of simultaneously controlling and evaluating relevant parameters, including irrigation pressure, suction pressure, and the ratio of endoscope-sheath diameter (RESD), remain limited. The aim of this study is to propose an ex vivo porcine kidney model designed to enable controlled and repeatable evaluation of flow–pressure conditions with suction UAS. The protocol utilized fresh cadaveric porcine kidneys placed in a fixed container system, with the suction UAS positioned at the upper pole calyx, and artificial stones mimicking calcium oxalate monohydrate introduced into the collecting system. An automated pressure-control pump system enabled precise regulation of irrigation and suction pressures. Intrarenal pressure (IRP) was continuously recorded for 1 min, and pull-out time (POT) was measured repeatedly under each RESD and pressure setting. The coefficient of variation (CV) and intraclass correlation coefficient (ICC) were calculated to assess measurement variability and reliability. The model demonstrated distinct IRP responses across different RESD and pressure conditions, including values exceeding 40 mmHg. POT measurements varied according to pressure settings, reflecting sensitivity to changes in flow–pressure conditions. CV analysis showed relatively narrow variability across most conditions, while ICC analysis demonstrated moderate reliability for single measurements and high reliability for averaged repeated measurements. Overall, this ex vivo porcine kidney model provides a standardized ex vivo reference for RIRS by presenting objective and reliable repeated measurement data using a suction UAS and a flexible ureteroscope.

Introduction

Retrograde Intrarenal Surgery (RIRS) stands as a primary minimally invasive approach for managing renal stones^1,2,3. A recent technical advancement in RIRS is the development of the suction ureteral access sheath (UAS), which has introduced an adjunctive approach to stone retrieval based on controlled irrigation and suction^4,5,6,7.

The primary challenge in utilizing suction UAS is balancing stone extraction speed with the risk of high intrarenal pressure (IRP)^8,9. IRP exceeding 30–40 mmHg can trigger pyelovenous backflow, potentially leading to systemic infection and sepsis^10,11,12. Factors involved in regulating this balance include irrigation, suction, and the ratio of endoscope-sheath diameter (RESD).

Although numerous clinical studies have investigated suction UAS and IRP^13,14,15, standardized experimental methodologies and validated models enabling the simultaneous control, measurement, and analysis of relevant factors remain limited.

This protocol provides a step-by-step guide for establishing an ex vivo porcine kidney model with acceptable reproducibility, specifically designed to standardize the evaluation of fluid dynamics and stone retrieval in suction-assisted RIRS.

Protocol

This experimental protocol using a cadaveric porcine kidney was deemed exempt from the Institutional Animal Care and Use Committee (IACUC) of Seoul National University Hospital. The reagents and the equipment used are listed in the Table of Materials.

1. Preparation of the porcine kidney model

NOTE: See Figure 1.

1. Obtain fresh kidneys from deceased pigs at an authorized slaughterhouse within 6 h of death. Transport and store the kidneys at room temperature until the experiment.
   NOTE: Cadaveric porcine kidneys should be used within 24 h of death to avoid corruption and further renal perfusion decrease.
2. Select kidneys with a partially remaining ureter to allow stable fixation of the UAS.
   NOTE: The length of the ureter must enable a fixation suture to immobilize the UAS, and the kidneys containing a longer remnant ureter should be preferred.
3. Gently fill the renal collecting system with normal saline using a syringe in a tension-free manner. Determine the calyceal volume when reflux occurs, and saline leakage outside the ureter is observed. Exclude porcine kidneys with a calyceal volume of less than 10 mL.
4. Prepare artificial stones mimicking calcium oxalate monohydrate stones.
5. Crush the stones into fragments of an appropriate size of 2 mm to 2.5 mm using a pestle. Use a digital caliper to select appropriately sized stones.
   NOTE: The stone size must be large enough to prevent direct in-scope suction or spontaneous escape through the gap between the scope and the sheath.
6. Place an artificial stone fragment into the upper pole calyx of the porcine kidney. Use saline to push the stone into the UAS and position it in the collecting system.
   NOTE: Use an endoscope to visually verify that the UAS is consistently fixed in the proper position during the experiment.
7. Place the porcine kidney in a metal box of appropriate size and put the box in a plastic container.
8. Fix the long plastic pipe to the ureter using 3-0 polyglactin sutures to ensure the system remains stable without movement during the experiment.
9. Insert the suction UAS inside the pipe and position the distal tip at the upper pole calyx, at the entrance to the renal pelvis.

2. Experimental system configuration

1. Select an appropriate flexible ureteroscope (outer diameter with 7.5 F, 9.0 F, or 9.5 F) and pair it with a 50 cm suction UAS of a specific size (inner/outer diameter with 10/12 F, 11/13 F, or 12/14 F).
2. Measure the outer diameter of the flexible ureteroscope using a digital caliper at the distal tip and midshaft. Calculate RESD based on measured data (0.68, 0.75, 0.86, or 0.95).
3. Connect the irrigation line from an automated pressure-control pump system to the flexible ureteroscope.
4. Connect the suction line from an automated pressure-control pump system to the suction side port of the UAS.
5. Connect the pressure sensor inside the UAS to an automated pressure-control pump system.
6. Calibrate the pressure sensor to atmospheric pressure without priming, according to the manufacturer’s guidelines.
7. Insert a 200 µm laser fiber into the working channel of the ureteroscope to mimic the real environment of retrograde intrarenal surgery.

3. Intrarenal Pressure (IRP) data collection

1. Put the flexible ureteroscope inside the UAS. Confirm that the endoscope has passed beyond the distal end of the UAS and reached the porcine kidney.
2. Enter flexible endoscopy mode on the automated pump device.
3. Select the specific pressure settings of irrigation and suction.
4. Ensure that other settings are controlled and kept constant. Fix irrigation flow at device’s baseline setting of 200 mL/min.
5. Record IRP values every 5 s for a continuous duration of 1 min under each designated pressure setting.

4. Pull-out Time (POT) measurement

1. Set the irrigation pressure (100 mmHg, 200 mmHg, or 300 mmHg) and the suction pressure (100 mmHg, 200 mmHg, 300 mmHg, or 400 mmHg) on the control unit.
   NOTE: A fixed waiting period of 3 s should be kept after each parameter adjustment to allow the pressure to stabilize before recording any measurements.
2. Visualize the stone fragment using the flexible ureteroscope and bring it to the distal opening of the UAS.
3. Initiate the suction and begin recording the POT exactly when the stone enters the distal tip of the sheath.
   NOTE: When a stone fragment is retrieved through the UAS, it must always be visible in the endoscopic view.
4. Stop the timer when the stone fragment is completely evacuated through the proximal sideport of the UAS. Record the measured POT.
5. Repeat this measurement 10 times for every pressure and RESD setting.
   NOTE: Time measurement should be performed blindly to avoid operator bias.

5. Statistical analysis

1. Input all gathered experimental data into a specialized statistical software package.
2. Present all measured time data as the mean ± standard deviation for each setting.
3. Use Spearman’s rank correlation test to identify the tendency across settings. Check the coefficient of variation and intraclass correlation coefficient for repeatability analysis.
4. Establish the threshold for statistical significance at a p-value of less than 0.05 for all comparative analyses.

Results

The outer diameter of the flexible ureteroscope was measured at both the distal tip and midshaft using a digital caliper. The measured outer diameters were 7.5 F, 9.0 F, and 9.5 F, respectively, consistent with the product catalog. No difference was observed between distal tip and midshaft measurements, confirming that the instrument dimensions were uniform and suitable for reliable RESD calculation (see Figure 2).

IRP was recorded continuously for 1 min at 5 s in...

Discussion

This protocol was designed to establish a controlled experimental platform for evaluating flow–pressure conditions during RIRS using a suction UAS. This study provided an ex vivo porcine kidney model capable of systematically controlling irrigation pressure, suction pressure, and RESD settings, while enabling consistent measurement of IRP and POT with statistical validation.

The most critical step in this protocol is to maintain the remnant control settings constant to ensure co...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Flexible-Y Type Negative Suction Sheath	ZSR System Biomedical Technology Co.	ZSR-UAY-1035 ZSR-UAY-1134 ZSR-UAY-1235	Suction ureteral access sheath
FLEX-XC1	KARL STORZ SE & Co.	091271-06	Flexible ureteroscope
HM30S	HugeMed Medical Technical Development Co.	HM30S	Flexible ureteroscope
i-MIMER-sys	ZSR System Biomedical Technology Co.	ZSR-ISP100	Automated pump
JW Digital Calipers	Jongwon Tooling Co.	404-200	Digital caliper
URUS	Dyne Medical Group Inc.	URUS SU 100R	Flexible ureteroscope