The Effect of Ratio of Endoscope-Sheath Diameter on Intrapelvic Pressure During Flexible Ureteroscopic Lasertripsy

Abstract

Purpose:

To clarify appropriate combinations of flexible ureteroscope (FURS) and ureteral access sheath (UAS), which can maintain a low intrapelvic pressure (IPP) during FURS with lasertripsy.

Methods:

Four kinds of FURSs (Polyscope, Olympus URF-P6, Stroz Flex-X^c, and Olympus URF-V) and six sizes of UASs (11/13F × 36 cm, 11/13F × 46 cm, 12/14F × 36 cm, 12/14F × 46 cm, 13/15F × 36 cm, and 13/15F × 46 cm) were tested. A novel gas-liquid-pressure-control system (GLPCS) was used to maintain a stable and adjustable irrigation pressure form 50–500 cmH₂O. IPPs and flow rates were recorded with different combinations of FURSs and UASs by using an in vitro fresh cadaveric porcine urinary system.

Results:

The GLPCS can provide stable and adjustable water pressure. The ratio of outer diameter of FURSs and inner diameter of UASs (Ratio of Endoscope-Sheath Diameter—RESD) is essential to the IPP and flow rate. The highest IPP group is the combination of Olympus URS-V and 11/13F UAS with RESD of 0.87–0.88, the IPP is over 40 cmH₂O when irrigation pressure comes to 250 cmH₂O; Olympus URS-V in 12/14F UAS is at the middle group, RESD values of 0.81–0.82, IPP reaches 40 cmH₂O at 500 cmH₂O of irrigation pressure. Combinations with RESD ≤0.75 enjoy low IPPs (<13 cmH₂O). Polyscope with 13/15F UASs has the highest flow rate (74.3–77.7 mL/min), whereas Olympus URS-V with 11/13F UASs has the lowest flow rate (17.0–17.3 mL/min). UAS length has less effect on the IPP and flow rate.

Conclusions:

To maintain a low IPP and acceptable flow rate during flexible ureteroscope lasertripsy, RESD should be kept below 0.75. UAS length has less effect on the IPP and flow rate.

Introduction

Flexible ureteroscope (FURS) has been recommended as the first-line treatment option for proximal and renal stone <2 cm.¹ It provides a minimal invasive treatment through nature channel of urinary tract with similar stone-free rate and less complications compared to percutaneous nephrolithotomy. Ureteral access sheath (UAS) offers a convenient working channel for retrograde intrarenal surgery (RIRS) and thus decreases the damage of ureter by facilitating passage of the ureteroscope in and out,² reduces the intrapelvic pressure (IPP),³ lessens operative time,⁴ increases stone-free rate,⁵ and minimizes the morbidity.⁶ The combination of FURS and UAS has become a standard operation procedure so far.

Despite those advantages, risks and drawbacks of UAS using have been reported. When acute ischemic effects of UAS were evaluated by a porcine model, ureteral blood flow was found to decrease as much as 65% as the lager diameter of UAS was used.⁷ The reperfusion that occurs after sheath removal might expose the ureteral wall to free radicals and subsequent tissue damage.⁸ Injury of the ureteral wall involving ureteral smooth muscle layers is as much as 13.3% after access of UAS of 12/14F.⁸ Smaller diameter of UAS may reduce the damaging incidence, whereas a narrow UAS's channel may lead to a higher IPP,⁹ lower irrigate flow rate, and subsequently result in postoperative hemorrhage, renal extravasation, and urosepsis.¹⁰

What is the minimal size of UAS used for a certain FURS so that the IPP would remain low during RIRS, even with a high irrigation pressure? Is there a basic rule for combinations between FURSs and UASs?

We tested IPP of fresh cadaveric porcine kidney with six sizes of UASs and four kinds of FURSs with laser fiber in channel at irrigation pressure of 50–500 cmH₂O.

Materials and Methods

An in vitro fresh cadaveric porcine urinary system with intact kidneys, ureters, and bladder was used to simulate the condition of endoscopy lithotripsy. The animals were treated with humanity. All procedures performed were in accordance with the local ethical standards. A novel gas-liquid-pressure-control system (GLPCS) was used to maintain a stable and adjustable irrigation pressure. Instruments were connected as follows (Fig. 1): an air compressor (OTS-600W-8L; Taizhou Outstanding Industry and Trade Co., Ltd.) provided a high gas pressure as much as 1.0 Mpa and a precision pressure reducing valve (IR1000-1, set pressure range 0.005–0.2 Mpa; SMC Corporation) connected the air compressor to a pressure water tank (AR-32; Jiu Mu Water Purification Equipment Co., Ltd), which can transform gas pressure to liquid pressure. Three liters of 0.9% sodium chloride was drip into the tank in advance. Urodynamics system (Laborie) was powered on, and the sensors and software were tested. A three-way stopcock was connected to the urodynamics system (channel A), pressure water tank (channel B), and working channel of FURS (channel C). Two hundred micrometers of laser fiber (Lumenis SlimLine™) was inserted into the working channel of FURS to simulate the condition of lasertripsy. A UAS was placed at the level of ureteropelvic junction. An FURS combined with laser fiber was placed into the pelvis. A pressure probe was punctured into the renal calix on direct vision of endoscope and attached to urodynamics system (channel E) to detect the IPP. The flow rates were measured by counting the volume of flow back irrigant from UAS in 60 seconds.

FIG. 1.

Instrument connection and experimental procedure. Step 1: Connect system and leveling. Switch three-way stopcock in position ➀. Connect all the instruments. Make sure that three-way stopcock, FURS, UAS, pressure probe, and renal pelvis are at same horizontal plane. Minimize the pressure of pressure reducing valve, switch on air compressor and exhaust air from the pipes connected with endoscope, urodynamics system, and in vitro porcine urinary system. Step 2: Set zero. Switch three-way stopcock in position ➁. Wait 60 seconds till the pressure balanced and then set zero of urodynamics system. Step 3: Set irrigation pressure. Switch three-way stopcock in position ➂. Adjust the pressure reducing valve till the water pressure in channel D achieved the set point and stabilized for 60 seconds. Step 4: Measure IPP and flow rate. Switch three-way stopcock in position ➃. After the IPP of channel E is kept constant for 60 seconds, record the IPP and flow rate for 60 seconds. Step 5: Set another water pressure and repeat steps 2 to 4. Step 6: Change the FURS and UAS combinations and repeat steps 1 to 5. FURS = flexible ureteroscope; IPP = intrapelvic pressure; UAS = ureteral access sheath.

Four kinds of FURSs (Polyscope, Olympus URF-P6, Stroz Flex-X^c, and Olympus URF-V) and six sizes of UASs (Navigator™ HD 11/13F × 36 cm, 11/13F × 46 cm, 12/14F × 36 cm, 12/14F × 46 cm, 13/15F × 36 cm, and 13/15F × 46 cm; Boston Scientific Corporation) were available and be tested in the study.

The study was performed strictly according to the following procedure:

Step 1: Connect system and leveling. Switch three-way stopcock in position ➀. Connect all the instruments as shown in Figure 1. Make sure that the three-way stopcock, FURS, UAS, pressure probe, and renal pelvis are at the same horizontal plane. Minimize the pressure of pressure reducing valve, switch on air compressor, and exhaust air from the pipes connected with endoscope, urodynamics system, and in vitro porcine urinary system.

Step 2: Set zero. Switch three-way stopcock in position ➁. Wait 60 seconds till the pressure balances and then set zero of urodynamics system.

Step 3: Set irrigation pressure. Switch three-way stopcock in position ➂. Adjust the pressure reducing valve till the water pressure in channel D achieves the set point and is stabilized for 60 seconds.

Step 4: Measure IPP and flow rate. Switch three-way stopcock in position ➃. After the IPP of channel E is kept constant for 60 seconds, record the IPP and flow rate for 60 seconds.

Step 5: Set another water pressure and repeat steps 2 to 4.

Step 6: Change the FURS and UAS combinations and repeat steps 1 to 5.

Step 7: Repeat steps 1 to 6, and double check the result with other ureters and kidneys for three times.

The irrigation pressure was set to 50, 100, 150, 200, 250, 300, 350, 400, 450, and 500 cmH₂O. The precise outer diameters of FURSs on the shaft were measured by micrometer (Fig. 2a) and the inner diameters of UASs were measured by microscopic image (Fig. 2b)⁹; the accurate diameters and the combination of FURS and UAS are shown in Table 1.

FIG. 2.

Measurement of outer diameters of FURSs (a) and inner diameters of UASs (b).

Table 1.

Combination of Endoscope and Access Sheath Shows Different Ratio of Endoscope-Sheath Diameter

FURS = flexible ureteroscope.

Data were recorded and mean values were reported.

Results

Outer diameters of FURS vary from 7.38F (Polyscope) to 9.69F (Olympus URF-V) and inner diameters of UAS range from 11.07F (11/13F × 36 cm) to 12.96F (13/15F × 46 cm). The combinations among different FURSs and UASs are listed in Table 1; the ratios of the outer diameter of FURS to the inner diameter of UAS (Ratio of Endoscope-Sheath Diameter—RESD) are marked below. The RESD increased from the bottom left to the top right and varied from 0.57 (Polyscope in 13/15F × 36 cm and 46 cm UAS) to 0.88 (Olympus URF-V in 11/13F × 36 cm UAS).

The GLPCS could provide a very stable and adjustable water pressure from 50 to 500 cmH₂O; flow rate and IPP had a linear correlation with the irrigation pressure.

IPPs in different combinations of FURSs and UASs at different irrigation pressure are shown in Figure 3. Three groups could be identified, the highest IPP group was the combination of Olympus URS-V and 11/13F UASs with RESD of 0.87–0.88, the IPP was over 40 cmH₂O when irrigation pressure came to 250 cmH₂O; Olympus URS-V in 12/14F UASs was at the middle group, RESD values of 0.81–0.82, it reached the secure pressure cordon of renal at 500 cmH₂O of irrigant. The rest of the combinations were at the bottom, RESD ≤0.75, IPPs were not >13 cmH₂O.

FIG. 3.

IPP in different combinations of FURS and UAS at different irrigation pressure.

Figure 4 showed the flow rates in different irrigation pressures. Olympus URS-V with 11/13F UASs had the lowest flow rate (17.0–17.3 mL/min), whereas Polyscope with 13/15F UASs had the highest flow rate (74.3–77.7 mL/min). With the same size of access sheath, the Polyscope enjoyed the highest flow rate, whereas the Olympus URS-V had the lowest.

FIG. 4.

Flow rates in different combinations of FURS and UAS at different irrigation pressure.

When the relationship of flow rate and RESD was concerned at the same IPP level, flow rate was growing as the RESD was decreasing. At 40 cmH₂O IPP, the flow rate of Olympus URS-V with 11/13F UASs (RESD = 0.87–0.88) was only 10 mL/min, and the flow rate of Olympus URS-V with 12/14F UASs (RESD = 0.81–0.82) was about 20 mL/min. Other combinations of FURSs and UASs (RESD ≤0.75) kept a low IPP less than 13 cmH₂O and enjoyed a high flow rate of 53.0–77.7 mL/min.

The IPP and flow rate were similar between 36 and 46 cm UAS when the same endoscope and diameter of the UAS were tested.

Discussion

Routine IPP monitoring during FURS is inevitable up to now. Although urologists focus on lithotripsy, it is difficult to divert their attention to the IPP; they may increase irrigation pressure as much as 300 mm Hg occasionally¹¹ for maintaining a clear vision. When the IPP comes as much as 40 cmH₂O, pyelosinus, pyelovenous, and/or pyelolymphatic backflow may occur and consequently morphologic and physiologic changes in the kidney.^3,12,13 Maintaining low IPP may prevent complications such as infection, urosepsis, and subcapsular and perinephric hematoma, and it becomes more important for dealing with larger intrapelvic calculi.

One of the convenient ways to maintain low IPP is to use UAS, it may provide reduction of IPP by 57% to 75%.³ UAS offers a channel around FURS; the larger size sheath allows irrigant to flow back more easily than the smaller one, thus reducing the IPP as well as increasing irrigation flow rate.^3,9,14 Actually, the average size of the human ureter is estimated at 10F (3–4 mm),¹⁵ which is why for those patients who underwent RIRS, UAS of 12/14F was responsible for ureteral injury as much as 46.5%.⁸ The higher grade UAS-related injury was connected with higher incidence of postoperative pyelonephritis.⁸ A prospective randomized comparison of UASs had found that the device failure rate was 0% for the 12/14F and 44% for the 12/15F. Failures includes buckling (25%), kinking (25%), and difficulty passing instruments (13%).¹⁶ The larger external diameter may contribute to difficult placement. It is important for urologists to keep in mind that a bigger UAS does not always mean safety.

The appropriate UAS should be small enough to reduce ureter injury and big enough to maintain low IPP, and the low IPP is not only decided by UAS but also by FURS. Al-Qahtani and coworkers¹⁵ investigated the compatibility between 21 different UASs and 12 different FURSs. They concluded that UAS of 12/14F was considered a universal UAS that accepts all available FURSs, and 10/12F UAS will become a new standard UAS that accepts all endoscopes with the advances in minimizing size of the FURSs. However, they are only concerned about the size matching, IPP was not taken into account. Sener et al.⁹ used a bladder evacuation device enforced by two rubber caps to create a closed-system artificial kidney model and tested eight different FURSs and five sizes of UASs; they found that with an irrigation of 60 cmH₂O, most of the FURSs were not compatible with 9.5/11.5F UAS because of the high IPP and not fitting inside, excepted for Olympus P6. However, the irrigation pressures in this study were much lower than the real practice of RIRS.¹¹ When the irrigation pressure reached 100 mm Hg (133 cmH₂O), the IPP was proven to be >40 cmH₂O in combination of Stroz Flex-X^c and 10/12F UAS.¹⁷ Furthermore, the irrigation flow in Sener et al.'s study was too slow to maintain a clear endoscopic vision, which should be 21–112.8 mL/min depending on different irrigation system.⁹

The fundamental purpose of irrigation is to provide a clear vision during endoscope lithotripsy, and the clear vision is decided by irrigation flow rate. To provide a high irrigation flow rate, it is a common practice to increase the pressure of irrigant by gravity, irrigation pumping, or syringe injecting with hand/foot. Neither of them can provide a stable irrigation pressure. Our novel GLPCS is more stable and can provide a higher water pressure. The air pressure was automatically controlled by the precision pressure reducing valve, and then the stable gas pressure was transformed to a stabilized liquid pressure by the pressure water tank.

According to Blew et al.'s study,¹⁸ with 200 μm laser fiber in the working port of the FURS, a handheld 60-cc syringe could create almost six times of mean flow than pressurized irrigant bag at 300 cmH₂O. Handheld 60-cc syringe is the most common irrigation method for RIRS in our department, and maximal pressure of 60-cc syringe by hand (>600 cmH₂O) was beyond the range of our urodynamics system. Considering the limitation of pressure measuring system and the maximal simulating of real conditions, we set 500 cmH₂O as the highest irrigation pressure, which was much higher than previous studies.^3,9,17

Because of the time-consuming work, we only tested FURSs with laser fiber inserted. Although FURS with free channel had higher IPP than with laser fiber inside,⁹ during RIRS, the majority of the time is spent with the laser fiber inside. The 200 μm laser fiber is the most common laser fiber for flexible ureteroscope lasertripsy (FURSL), and it is smaller than other instruments (basket, guidewire) put in working channel. Therefore, knowledge of the pressure and irrigation flow in this condition is essential to decide the appropriate FURS and UAS combinations.⁹

In consideration of the manufacturing error, the marked size of the FURSs is not the same as the actual value, and the diameter of same marked size of UASs from different companies may have great difference.^9,15,17 We measured the actual diameter of the FURSs by micrometer and UASs by microscopic image. Polyscope was the thinnest FURS (F7.38), smaller than Olympus URF-P6 (F7.92), despite its marked diameter (F8.0) being larger than Olympus URF-P6 (F7.95). UASs' diameters were closed to marked size.

In this study, we try to find secure combinations of FURSs and UASs, which can maintain a low IPP during FURSL even at a high irrigation pressure. As we draw a picture of diameter of UASs and FURSs (Table 1) in a certain order, an obvious phenomenon was found: as the diameter ratio of FURS to UAS decreased, the outflow channel between UAS and FURS increased. We realized that it may not be the size of UAS or the diameter of FURS that affects IPP individually, but the ratio of them!

When the relationship of irrigation pressure and IPP was considered (Fig. 3), in the condition of RESD ≥0.87 (Olympus URS-V in 11/13F UAS), the IPP increased dramatically as the rise of irrigation pressure, and it was over 40 cmH₂O at 250 cmH₂O of irrigation pressure; when RESD ≤0.75, the IPP remained <13 cmH₂O. If we take 21 mL/min as the lowest irrigation flow that could keep a clear vision,¹⁹ IPP of Olympus URS-V and 11/13F UASs (RESD = 0.87–0.88) must be >70 cmH₂O, IPP of Olympus URS-V and 12/14F UASs (RESD = 0.81–0.82) was about 20 cmH₂O, and other combinations (RESD ≤0.75) were <5 cmH₂O. In the case of RESD = 0.81–0.82, IPP was not beyond 40 cmH₂O and the flow rate was tolerable. However, we need to keep in mind that the conclusion was drawn from the idealized conditions, without blood or fragments of stone; any blockage of the outflow channel would dramatically increase the IPP and thus threaten the safety of patients.

When the RESD ≤0.75. the outflow channel between UAS and FURS is more than four times bigger than the FURS irrigation channel (3.6F in common); it is the irrigation channel instead of outflow channel that affects the flow rate and that explains why the IPP and flow rate remained stable for the same FURS when RESD ≤0.75.

We tested different length of UASs at the same diameter, but the length of UAS had little effect on the flow rate and IPP. We presume that the main factors that impact IPP are inflow and outflow channel area. Increasing the length of UAS, as matter of fact, is increasing the outflow channel length, which might raise the IPP theoretically. However, the added length (10 cm) could not make the difference significantly. It is noteworthy that, when the outflow end of the UAS is elevated, which is very common during operation especially for the male patients, the IPP will increase by 20 cmH₂O, according to the altitude between renal pelvic and the opening end of UAS.

We estimate scrupulously that during FURSL with UAS, RESD ≤0.75 is recommended and RESD ≥0.85 is not recommended; urologists should be very careful and pay much attention to the IPP when 0.75 < RESD <0.85.

Unfortunately, we did not assess the smallest UAS (F9.5) for its unavailability, and only have UASs from one company. We only tested the pressure with a laser fiber inside the scope and missed to do the experiment without an instrument or with a basket because of the time-consuming work. We only test the ideal situations with irrigant of 0.9% sodium chloride; however, the fragments of stone and blood clot during real operation may block the outflow channel and increase the IPP.

Conclusions

To maintain a low IPP and acceptable flow rate during FURSL, an appropriate combination of FURSs and UASs is very important. We estimate that during FURSL with UAS, RESD ≤0.75 is recommended and RESD ≥0.85 is not recommended; urologists should be very careful and pay much attention to the IPP when 0.75 < RESD <0.85. The length of UAS has less effect on the IPP and flow rate.

Footnotes

Acknowledgment

Source of financial support: Medical scientific research projects of Zhejiang Province (2019326541).

Author Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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