Renal Pelvic Pressure in Percutaneous Nephrolithotomy: The Effect of Multiple Tracts

Abstract

Introduction:

During percutaneous nephrolithotomy (PCNL), elevated renal pelvic pressures (RPPs) may spread infection through pyelovenous backflow whereas decreased pressures can hinder observation and increase bleeding. The purpose of this study was to evaluate the effects of multiple access tracts and different sized endoscopic equipment on RPP in a porcine model.

Materials and Methods:

RPP was measured in one- vs two-tract access, rigid vs flexible nephroscopy, and suction vs no suction. Twenty trials were performed for each condition. An independent samples Mann–Whitney U-test was used to compare parameters, with p < 0.05 considered significant.

Results:

With one tract, rigid nephroscopy resulted in higher mean pressures (31.35 mm Hg) than flexible nephroscopy (11.1 mm Hg; p < 0.001). The RPP was higher with rigid nephroscopy in one tract (31.35 mm Hg) than when two tracts were present (9.35 mm Hg; p < 0.001). In contrast, there was no difference in pressure during the use of a flexible nephroscope in one (11.1 mm Hg) vs two tracts (10.7 mm Hg; p = 0.63). Use of suction with the rigid nephroscope resulted in significantly lower pressures with one (−1.3 mm Hg) than with two tracts (1.8 mm Hg; p = 0.004).

Conclusion:

In PCNL, RPP is significantly affected by an additional tract during rigid nephroscopy and suctioning but not when using a flexible nephroscope. Understanding the effects of multiple tracts and equipment type on RPP may improve the safety of PCNL.

Introduction

Percutaneous nephrolithotomy (PCNL) is the gold standard for the removal of large renal calculi because of its ability to achieve high stone-free rates with minimal complications.¹ Despite the success of the procedure, some questions still remain regarding its physiologic effects on the urinary tract, in particular its association with postoperative infection. During PCNL, the renal pelvic pressure (RPP) has been shown to vary widely, from <20 mm Hg up to 80 mm Hg.² Hinman and Redewill showed in their classic canine study that elevated RPP >30 mm Hg was associated with pyelovenous backflow and the potential spread of infection.³ In a contemporary study, Zhong et al. found a positive correlation between RPP persistently >30 mm Hg and postoperative fever.⁴

During PCNL, variations in RPP are in large part because of changes in nephrostomy tract outflow; for example, the use of wider or narrower nephroscopes may decrease or increase outflow, respectively.² The number of access tracts may also alter the RPP by affecting the degree of outflow. Other factors that may affect RPP include endoscopy through a narrow infundibulum and a nephrostomy sheath incompletely positioned in the collecting system.² To our knowledge, no study to date has examined how the addition of a second tract might affect the RPP. To address this question, we measured the RPP in a porcine model in one and two tracts simulated PCNL while inserting a rigid nephroscope, a flexible nephroscope, and applying suction.

Materials and Methods

Porcine percutaneous nephrostomy

The effect of one- vs two-tract PCNL was investigated in a domestic farm pig model (Fig. 1), after approval by the Institutional Animal Care and Use Committee. Each animal was placed under general anesthesia in the prone position using 2.5% isoflurane. The collecting system was opacified in a retrograde manner, and, using the bulls-eye technique, an 18-gauge needle was used to obtain renal access by the same fellowship-trained endourologist. One access was placed in the upper pole and the other access was placed in the lower pole. After balloon dilatation, a 30F Amplatz access sheath (Boston Scientific, Natick, MA) was used to establish the first nephrostomy tract. Retrograde placement of a 9.9F flexible ureteroscope (Cobra, Richard Wolf, Vernon Hills, IL) into the renal pelvis served to transmit pressures from the kidney through its working channel to an arterial line pressure transducer. Renal pressures were measured during rigid nephroscopy with and without suction, and flexible nephroscopy. Subsequently, the second tract was dilated by the same method, and the nephroscopes and suction moved to the new tract to determine the pressures with two tracts present. This procedure was performed in four renal units. Twenty measurements were performed with each condition.

FIG. 1.

Single nephrostomy tract in place in the porcine model.

Suction was generated through a Neptune 2 waste management system (Stryker, Kalamazoo, MI) at a negative pressure of 525 mm Hg, and was delivered to the kidney through a 3.5 mm sonotrode ultrasonic lithotripter (Karl Storz, Tuttlingen, Germany) deployed through the 26F rigid nephroscope (Karl Storz). A Level One infusion device (Smiths Medical, London, England), with the drip chamber set to a height of 1.28 m was used to provide gravity saline irrigation identical to the settings used during clinical PCNL.

Measurement of RPP

Measurement of RPP was obtained using an arterial line pressure transducer (Medical Data Electronics, Arleta, CA). The transducer was zeroed to atmospheric pressure and placed at the level of the renal pelvis, and was connected through tubing to the luer-lock fitting of the ureteroscope's working port. The RPP was measured in three conditions with both one tract and two tracts established: the use of a 26F rigid nephroscope, a flexible 16F nephroscope (Karl Storz), and using suction applied to a 3.5-mm ultrasound probe deployed through the rigid nephroscope. Measurements were taken sequentially until five measurements were recorded in each condition. The RPP was recorded when “systolic” and “diastolic” pressures read equally on the monitor, that is, after the pressure was allowed to equilibrate.

Statistics

A total of 120 measurements were taken, 20 trials for each condition, with four renal units tested. Statistical analyses were performed using an independent samples Mann–Whitney U-test, with p < 0.05 considered statistically significant.

Results

All pigs were female and weighed from 47 to 50 kg at the time of testing. During single-tract simulated PCNL through a 30F inner diameter and 34F outer diameter rigid sheath, the pressures ranged from 14 to 65 mm Hg during rigid nephroscopy with a mean of 31.35 mm Hg (Fig. 2). When utilizing the flexible nephroscope, the pressures ranged from 9 to 15 mm Hg, and the mean RPP was 11.1 mm Hg. When suction was applied through the sonotrode, in the one-tract model, the range of pressures fell to −9 to 2 mm Hg. The mean RPP with suctioning was −1.3 mm Hg.

FIG. 2.

Mean pressures (mm Hg) during the use of various instruments, organized by tract.

With the addition of a second 34F outer diameter tract, the pressures were different. During rigid nephroscopy, the range of pressures was 7 to 13 mm Hg, with a mean of 9.35 mm Hg. During flexible nephroscopy, pressures ranged from 9 to 13 mm Hg, with a mean of 10.7 mm Hg. Finally, when suction was applied to the rigid nephroscope, the pressures ranged from −1 to 6 mm Hg. The mean pressure during suctioning was 1.8 mm Hg.

During rigid nephroscopy, the two-tract model had significantly lower pressures than the one-tract model (difference of 22.0 mm Hg, p < 0.001) (Table 1). During suctioning, the two-tract model had significantly higher pressures than the single-tract model, albeit by a small difference (difference of 3.1 mm Hg, p = 0.004). During flexible nephroscopy, single-tract RPP was higher than two-tract RPP; however, the difference was small and not statistically significant (difference of 0.4 mm Hg, p = 0.633). A three-way comparison of each type of condition (i.e., rigid nephroscopy vs flexible nephroscopy vs suctioning) yielded significantly different pressure, in both the one- and two-tract models (Table 2).

Table 1.

Renal Pelvic Pressure in a Porcine Model, Organized by Instrument

Instrument	No. of tracts	Pressure (mean ± SD in mm Hg) ^a	p^b,c
26F rigid nephroscope	One tract	31.35 ± 7.47	<0.001
	Two tracts	9.35 ± 0.42	<0.001
16F flexible nephroscope	One tract	11.1 ± 0.60	0.633
	Two tracts	10.7 ± 0.29	0.633
Suction through 3.5-mm ultrasound probe	One tract	−1.3 ± 1.49	0.004
	Two tracts	1.8 ± 0.72	0.004

Values are presented as mean ± 95% confidence interval.

Mann–Whitney U-test.

Exact significance is displayed for all tests.

Table 2.

Renal Pelvic Pressure in a Porcine Model, Organized by Number of Tracts

No. of tracts	Instrument	Pressure (mean ± SD in mm Hg) ^a	p^b,c
One tract	26F rigid nephroscope	31.35 ± 7.47	<0.001
	16F flexible nephroscope	11.1 ± 0.60
	Suction through 3.5-mm ultrasound probe	−1.3 ± 1.49
Two tracts	26F rigid nephroscope	9.35 ± 0.42	0.001
	16F flexible nephroscope	10.7 ± 0.29
	Suction through 3.5-mm ultrasound probe	1.8 ± 0.72

Values are presented as mean ± 95% confidence interval.

Mann–Whitney U-test.

Exact significance is displayed for all tests.

Discussion

PCNL is the treatment of choice for renal stones >2.0 cm in diameter.⁵ Although PCNL is associated with a much lower complication rate than open surgery,⁶ it still carries risks, including fever, sepsis, renal hemorrhage, lung/pleural injury, and bowel injury.^7,8 With regard to fever and sepsis, several risk factors have been established, including the presence of an infectious stone, size of stone burden, and operative time.⁹ In an attempt to lower the risk of PCNL, urologists have sought to identify additional risk factors to help reduce the incidence of infection. RPP is thought to be one such risk factor.¹⁰

Normal RPP varies between 1.47 and 4.41 mm Hg.¹¹ During PCNL, RPP is altered by the use of access tracts, irrigation, and endoscopic equipment, and elevations in RPP have been shown to lead to untoward consequences. Zhong and colleagues demonstrated that 50 seconds of an RPP >30 mm Hg during minimally invasive PCNL was associated with postoperative fever.⁴ One mechanism could be pyelovenous backflow, which Hinman and Redewill first determined to occur at pressures >30 mm Hg in a canine model.³ More recent studies have found that pyelovenous backflow may occur at pressures <30 mm Hg, especially in kidneys with purulent infection.¹² In addition to infection risk, an acutely elevated RPP has also been associated with a number of alterations in renal physiology. Suki et al., using a canine model of acute and chronic hydronephrosis, demonstrated that ureteral obstruction led to distal nephron underperfusion.¹³ Additional animal studies have demonstrated similar harmful effects of elevated renal pressures, including decreased nephron population, as well as renal cellular injury.^14,15 The effects of excessively low RPP, by contrast, have not been previously investigated. However, it would be logical to assume that lower pressures would result in collapse of the renal collecting system, poorer observation, and the potential for increased bleeding because of the absence of pressure tamponade effect on open venules.

Variations in RPP during PCNL are largely modulated by the degree of outflow from the renal pelvis. Troxel and Low found that endoscopy through a narrow infundibulum and a nephrostomy sheath incompletely positioned in the collecting system would lead to elevated pressures by obstructing outflow through the nephrostomy sheath.² Our results corroborate this finding in one-tract PCNL. The use of a 26F rigid nephroscope yielded an RPP 20 mm Hg higher than the use of a 16F flexible nephroscope, which allows for much greater irrigant outflow. Additional causes of increased RPP may include cough or other rises in intraabdominal pressure; however, these causes are unlikely to be sustained long enough to cause significant negative effects on the kidney.¹⁶ The use of ureteral access sheaths or ureteral catheters may also increase outflow through the ureter, thereby lowering the RPP.¹⁷ However, ureteral catheters alone may not be sufficient to lower intrarenal pressures below the limit for pyelovenous backflow.¹⁸ Irrigation pressure is another variable, which may affect RPP. Hand irrigation was not employed in this study. We used the same irrigation source (Level 1) for both rigid and flexible nephroscopy, to ensure consistency throughout the procedure. Hand irrigation, by its nature, may generate inconsistent pressures, and would have added variability to the RPP measurements.

Until now, the effect of multiple tracts on the RPP had not been investigated. Our study determined that the RPP in the presence of two tracts was significantly altered in comparison with a single tract during the use of a 26F rigid nephroscope and during suctioning. In one tract, the rigid nephroscope caused significantly elevated pressures (31.35 mm Hg), and the use of suction caused negative pressures (−1.3 mm Hg); the magnitudes of these effects were diminished when two tracts were present (9.35 mm Hg; 1.8 mm Hg). This indicates that the second tract has a significant effect on the RPP by providing an additional outlet for irrigant outflow during the use of the rigid nephroscope, and by allowing a channel for the equalization of pressure during suction. However, the equalization effect of the second tract on the RPP only appears to be significant when the instrument was sufficiently large to cause outflow obstruction. For example, the use of the 16F flexible nephroscope, whose obstructive effect on the outflow through the nephrostomy sheath is much less than that of the rigid nephroscope, did not yield significantly different pressures in one tract vs two tracts. This is concordant with Troxel and Low's findings that elevated pressures only occur during the use of a rigid nephroscope.²

Multiple tracts may be utilized in up to 28% of cases during PCNL.¹⁹ Indications for placing additional tracts are large stone burden and inability to reach the stone through the current access. Conflicting data exist as to the safety of multitract PCNL. Cho and colleagues examined 109 consecutive PCNL cases, 30 of which required multiple tracts.¹⁹ They found no difference in complication rates or rise in creatinine between single- and multiple-tract access. However, Fayad et al. found that patients with baseline renal impairment and diabetics suffered further decline in function after multitract PCNL.²⁰ Studies have found multiple tracts associated with greater blood loss,^21
–23 although this association has not been consistent.^24,25 Of note, however, in a large retrospective study and literature review, Ganpule and colleagues found that patients requiring multiple tracts had significantly higher transfusion rates, in some cases 5–10 times higher than single access PCNL.²⁶ Given these results, it would seem prudent that the use of multiple tracts, although necessary at times, should be employed only when necessary because of an increased risk of bleeding. This is likely because of a significant drop in mean RPP, which decreases the venous tamponade effect. The influence of multiple tracts on the risk of infection is more difficult to define. On one hand, multiple tracts may lower the RPP and, therefore, lower the risk for pyelovenous backflow and sepsis. However, multiple tracts also result in additional portals for bacteria to enter the collecting system and additional damage to vascular structures, which could open entry points for bacteria to enter the bloodstream. Not surprisingly, the evidence of the effect of multiple tracts on the risk of infection is conflicting in the literature, with Sharma et al. and Korets et al. citing increased risk,^27,28 and Netto and colleagues²⁹ citing no change in risk.

Although the predominantly detrimental effects of elevated RPP are well represented in the literature,^3,4,13
–15 the effects of low RPP remain largely undescribed. Our results demonstrate that RPP is much lower in two-tract vs one-tract PCNL with the use of the 26F rigid nephroscope. Although an excessive amount of backflow may cause fever and septicemia,^3,4 low RPP may cause the collecting system to collapse, impairing observation and leading to risk of trauma to the urothelium. Increased bleeding may result in further impairment of observation of the collecting system. During laparoscopy, pneumoperitoneum is routinely raised from 15 to 20 mm Hg to effectively control troublesome venous bleeding. Similarly, in the kidney, normal renal venous pressure ranges from 1.3 to 10 cm H₂O (0.9–7.4 mm Hg).³⁰ During conversion from rigid to flexible nephroscopy, the drop in RPPs may decrease the tamponade effect on the small blood vessels. In addition, the collecting system may collapse at low pressures, making observation of the infundibula more difficult. This represents a real concern for urologists. One qualitative analysis of surgical experience in PCNL found that a common complaint was that greater blood loss in multiple tract PCNL impaired visual acuity.²² Understanding the physiologic consequences of multiple tracts may allow surgeons to take steps to avoid these potentially adverse consequences. Adequate pressures allow distention of the collecting system and may permit easier identification of renal calices and ultimately improve stone-free rates. To achieve adequate pressures during rigid nephroscopy with multiple tracts, surgeons could raise inflow pressure, decrease outflow from unused tracts, limit suction duration or intensity, limit operative times with a second tract in place, or potentially use smaller access sheaths. There may be significant variation in the ideal RPP based on patient size, anatomy, and stone characteristics. We suspect that the ideal pressure would be high enough to result in distention of the collecting system and venous tamponade, without causing pyelovenous backflow. A priority of future studies should be to elucidate this ideal RPP.

There were several limitations in this study. First, there may be subtle differences in human and porcine renal physiology, such as amount and density of perinephric fat, tract length, and renal size and orientation that may subtly alter collecting system pressures compared with a human. An additional limitation of the porcine model is the diminutive collecting system relative to an adult human. Therefore, the magnitude of the effects seen in this porcine model may be less pronounced in a clinical setting. Although stones are routinely present in the collecting system and may also be present in the ureter during PCNL, the presence of stones was not simulated in our model. Large obstructing stones in the collecting system may limit the ability of fluid to equilibrate throughout the collecting system, thereby preventing pressure equilibration. In addition, the effect of infundibular stenosis on RPP could not be easily recreated in this porcine model, and study of this effect would require further investigation. In this study, we employed a 26F rigid nephroscope, and, therefore, did not evaluate the effect of varying nephroscope size on RPP. Future studies with different sized nephroscopes would be useful to confirm the findings of this study. Lastly, our experimental model did not determine the RPP that optimizes observation while minimizing pyelovenous backflow. It is possible that if surgeons could maintain the RPP within a certain range, they could balance the beneficial effects of an elevated RPP (bleeding tamponade and collecting system distention) while keeping the pressure below the threshold that would cause pyelovenous backflow. The elucidation of this “safe zone” should be a major priority of further studies in this domain.

Conclusions

In an animal model of simulated PCNL, mean RPP was above the threshold for pyelovenous backflow during rigid nephroscopy using a single tract. The addition of a second tract significantly lowered RPP during rigid nephroscopy, such that pyelovenous backflow would be less likely to occur. However, outflow through this second tract resulted in significantly lower RPPs, which could result in inadequate distention, increased risk for collecting system injury, and a decreased tamponade effect on venous bleeding. Addition of a second access tract had little impact when performing flexible nephoscopy. Knowledge of the influence of an additional renal access tract on RPP could allow the surgeon to manipulate intraoperative parameters to improve the safety and efficacy of multitract PCNL.

Footnotes

Author Disclosure Statement

Dr. D. Duane Baldwin is a consultant for Olympus and Bard^®, a speaker for Cook Medical, and serves on the scientific board for DARRT. No competing financial interests exist for any of the other authors.

Abbreviations Used

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