The Effect of Irrigation Power and Ureteral Access Sheath Diameter on the Maximal Intra-Pelvic Pressure During Ureteroscopy: In Vivo Experimental Study in a Live Anesthetized Pig

Introduction

The latest international guidelines on urolithiasis advocated endoscopic approaches for management of ureteral and renal calculi. ^1,2 Retrograde intra-renal surgery (RIRS) was recommended as the first-choice intervention for management of proximal ureteral and renal stones <2 cm because it has a comparable rate of stone-free status and lower complications compared with percutaneous nephrolithotomy. ^{1 –3} Since the introduction of flexible ureteroscopes into urology practice three decades ago, there have been revolutionary advancements in the diameter and deflection of scopes, types and power settings of laser devices, and design and diameter of laser fibers, ureteral access sheathes (UASs), basket tools, and irrigation devices to simplify the technique and improve the outcomes. ³ However, there are concerns about the changes in the intra-pelvic pressure (IPP) that might reach critical levels, resulting in pyelovenous, pyelolymphatic, and pyelointerstitial backflow with subsequent systemic inflammatory response syndrome and sepsis. ⁴ The main target for this study was to assess the effect of irrigation settings and the size of UAS on the maximal intra-pelvic pressure (IPPmax) during in vivo URS in a live anesthetized pig. Our hypothesis was that the larger the diameter of the UAS, the lower the IPPmax even under forced irrigation.

Materials and Methods

Operating room setup and technique

The pig was then put in supine position and tied to the operating table. Cystoscopy was then performed to identify the ureteral orifice (Fig. 1A), advance a 0.035″ hydrophilic guidewire (HiWire™ Nitinol Core Wire Guide, COOK Medical; Cook Ireland Ltd., Limerick, Ireland) up to the kidney (Fig. 1B), and insert a 6F open-end ureteral catheter (COOK Medical; Cook Ireland Ltd.) over the guidewire, leaving the tip of the ureteral catheter in the upper calix or the renal pelvis. In case ureteral tortuosity was encountered, which is frequent in pigs, a hydrophilic guidewire was advanced up the kidney and an angiographic catheter (Glidecath™; Terumo Europe N.V., Leuven, Belgium) was advanced over it to follow the ureteral tortuosity; then, the hydrophilic guidewire was withdrawn and replaced by a super stiff guidewire to straighten the ureter; and finally, the 6F open-end ureteral catheter was inserted and a contrast was injected to delineate the pelvi-caliceal system (PCS). A blue and a red marker was connected to the ureteral catheters to identify the right and the left kidney, respectively, and both were fixed to the external surface of the genitalia by using a 2/0 silk suture to prevent slippage (Fig. 1C). The pig was then turned to the prone position to establish a percutaneous access under fluoroscopy and insert a 10F nephrostomy tube to be connected to the Urodynamic device for real-time measurement of IPP (Fig. 2A). First, to delineate the PCS, a contrast dye was injected in the ureteral catheter. Employing the bull's eye technique, an 18G needle was used to puncture the PCS and a 0.035″ hydrophilic guidewire was inserted. An 8F dilator from an Amplatz percutaneous access tract dilation kit (COOK Medical; Cook Ireland Ltd.) was inserted over the guidewire into the collecting system, and the hydrophilic guidewire was replaced by a 0.035in super stiff guidewire (Amplatz Super Stiff™; Boston Scientific, Heredia, Costa Rica). Sequential dilatation was then performed, and a 10F nephrostomy tube was advanced in the renal pelvis and tied to the external surface of the skin by a 2/0 silk suture.

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FIG. 1.

(A) Endoscopic view of the right ureteral orifice in a female domestic pig. (B) Insertion of a hydrophilic guidewire through the right ureteral orifice. (C) Right and left ureteral catheters inserted in the ureters of both kidneys, fixed to the external genitalia with 2/0 silk sutures, and marked blue and red, respectively.

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FIG. 2.

(A) Flexible ureteroscopy in a domestic female pig in prone position. (B) The Urodynamic device connected to the nephrostomy tube.

Results

The median weight of the three pigs was 30 kg. Under normal irrigation while the 3 L bag was at 1 m above the pig's level (gravity irrigation), the recorded IPPmax while performing semi-rigid URS in the distal ureter was 0 cmH₂O and while performing semi-rigid URS in the renal pelvis it was 30 cmH₂O. Further, the IPPmax while performing flexible ureteroscopy (FURS) in the renal pelvis without UAS was 23 cmH₂O, FURS in the renal pelvis with UAS 9.5/11.5 was 6 cmH₂O, FURS in the renal pelvis with UAS 12/14 was 2 cmH₂O, and FURS in the renal pelvis with UAS 14/16 was 1 cmH₂O (Fig. 3). On the other hand, under manual pumping irrigation while the 3 L bag was at 1 m above the pig's level, the recorded IPPmax while performing semi-rigid URS in the distal ureter was 84 cmH₂O and semi-rigid URS in the renal pelvis was 105 cmH₂O. Further, the IPPmax while performing FURS in the renal pelvis without UAS was 45 cmH₂O, FURS in the renal pelvis with UAS 9.5/11.5 was 46 cmH₂O, FURS in the renal pelvis with UAS 12/14 was 18 cmH₂O, and FURS in the renal pelvis with UAS 14/16 was 1 cmH₂O (Fig. 3).

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FIG. 3.

IPPmax during semi-rigid and flexible ureteroscopy in female domestic pigs under different irrigation settings and different ureteral access sheath diameters. IPPmax = maximal intra-pelvic pressure.

Discussion

The use of UAS during RIRS, especially with the introduction of new digital flexible ureteroscopes, offers advantages such as decompression of the PCS, leading to decreased IPP, which might decrease the infectious complications, and potentially facilitating in and out passage of ureteroscopes, resulting in the shortening of operative time. ^{5 –7} Nevertheless, traumatic and ischemic complications were reported, especially when large-diameter UAS was used. ^8,9 This could be overcome by preoperative stenting of the ureter and selecting the appropriate UAS size depending on the state of ureteral lumen. ⁸

In our study, The IPPmax during semi-rigid URS under gravity irrigation in the distal ureter was zero. We think that this is due to the leakage of irrigation down to the bladder rather than going up to the kidney. However, it was increased to high levels during manual pumping because the diameter of the normal ureter cannot accommodate all irrigation to go down to the bladder; that is why most of the irrigation goes up to the kidney. When the semi-rigid URS advanced in the renal pelvis, the IPPmax was high during gravity irrigation because most of the irrigation goes to the renal pelvis. During pumping, the IPPmax reached critical levels due to the wide channel of the semi-rigid URS, which can allow too much irrigation fluid in the renal pelvis.

During FURS without UAS, the IPPmax was high during gravity irrigation and manual pumping. Nonetheless, it was still lower than the IPPmax during semi-rigid URS in the renal pelvis due to the narrower irrigation channel in the FURS and the smaller diameter of the FURS compared with the semi-rigid URS. However, it was clearly demonstrated that the presence of UAS during FURS decreased the IPPmax and the wider the diameter of the UAS, the lower the IPPmax during FURS.

Previous studies on pigs' kidney have demonstrated acute harmful effects in the form of flattening of the urothelial mucosa, edema of the submucosa, and vacuolization of renal tubules when exposed to high-irrigation pressure >150 mm Hg (≈204 cmH₂O). ¹⁰ In a cadaveric porcine model, RIRS was performed by using a Flex-X2 flexible ureteroscope and 10/12F and 12/14F Re-Trace™ UAS; the IPP was significantly lower while using the 12/14F UAS, and manual pumping was associated with significantly higher IPP (121 cmH₂O with 10/12F UAS and 29 cmH₂O with 12/14F UAS). ¹¹ These results were higher than our findings where the IPPmax was 18 cmH₂O and 46 cmH₂O, respectively. This might be due to the changes that occur in the ureter of pigs after death, which might result in loss of elasticity, leading to higher IPP values.

In an ex vivo study on human cadavers, Rehman and colleagues performed a similar study to ours where RIRS was performed by using a 7.5F flexible ureteroscope without UAS and with 10/12F, 12/14F, and 14/16F UAS and the IPP was recorded while the ureteroscope was in the distal ureter, middle ureter, and renal pelvis under irrigation pressure of 50, 100, and 200 cmH₂O, respectively. Interestingly, the IPP did not exceed 30 cmH₂O under all sizes of UAS and the IPP was almost similar between the 12/14F and the 14/16F UAS. ¹² These results were congruent with the results in our study where the IPPmax was almost similar between the 12/14F and the 14/16F UAS and it did not exceed 46 cmH₂O even under manual pumping using the 9.5/11 UAS. Also, another study in an ex vivo anatomical model highlighted the importance of using a large-diameter UAS. ¹³ The compatibility between the ureteroscopes and UASs was emphasized in a study by Dr. Traxer's group where they assessed IPP by using an artificial ex vivo kidney bench model using eight types of FURS and five types of UAS. ¹⁴ In a recent interesting study by Fang and colleagues using fresh porcine cadaver, the ratio of the inner diameter of UAS to the outer diameter of FURS was found to be a more important predictor of IPP rather than the diameter of the FURS, the size and the length of the UAS. ¹⁵ A ratio of ≤75% was accompanied with a potentially safe IPP during flexible URS. ¹⁵ According to our knowledge, this study was the first to assess the IPPmax in the live anesthetized pig under different irrigation settings and different UAS diameters. However, it has some limitations such as the small number of pigs. Nevertheless, this number was convenient to perform our experiment and obtain appropriate results. Another limitation is the presence of other studies in the literature that assessed the IPP during URS; however, the IPP was measured under nonideal circumstances in a cadaveric porcine model, ¹¹ ex vivo human cadavers, ¹² an ex vivo anatomical model, ¹³ an artificial ex vivo kidney bench model, ¹⁴ or fresh porcine cadaver. ¹⁵ Given the fact that the urologic anatomy and physiology of pigs are similar to humans and the IPP is affected by several physiological factors such as pyelovenous, pyelolymphatic, and pyelointerstitial backflow during endourologic procedures, ^{16 –19} we performed our study in a live anesthetized pig to ensure the normal physiologic conditions and avoid any bias of the results that might have occurred in previous studies. An extra limitation is that we did not obtain samples from the kidney for pathological examination to assess any deleterious effects of increased IPP during URS, but this study was just a proof-of-concept experiment and we plan to obtain samples for pathological examination in future studies. It is worth noting that the findings of this study were accepted for presentation during the upcoming 39th Congress of the Société Internationale d'Urologie in Athens.

Conclusion

Manual pumping can significantly increase the IPPmax to unsafe levels during URS. The UAS can significantly decrease the IPPmax, even under manual pumping. The larger the UAS, the lower the IPPmax. The use of UAS can render URS safer by acting as a safeguard against the consequences of increased IPP, even under forced irrigation.