There is a fine balance between being able to see and elevated intrapelvic pressures and sepsis risks in endoscopic stone surgery.¹ Increase in intrapelvic pressures can lead to pyelovenous backflow which, especially in the setting of bacterial load, can increase risk of bacteremia and in turn sepsis. Lee-Brown and Laidley reported in 1927 that pyelovenous backflow began at intrarenal pressures of 25 mmHg and significant backflow occurred at sustained pressures over 30 mmHg. Thomsen et al in 1984 showed that the threshold for pyelovenous backflow was 30–45 mmHg.² As such, ensuring stable pressures in the collecting system during endoscopic stone surgery is essential. Moreover, stones may harbor bacteria that do not show up on a preoperative urine culture and despite best efforts to sterilize the urine prior to an operation, risks of infectious complications are not fully mitigated simply by giving antibiotics. Omar et al showed that 35% more patients had systemic inflammatory response syndrome (SIRS) if they underwent percutaneous nephrolithotomy at 200 mmHg than 80 mmHg.¹ Thus, it is prudent that irrigation pressure be minimized as much as reasonably possible to minimize infectious related complications. Similar to radiation, one might consider the ALARA (as low as reasonably achievable) principle to apply to intrarenal pressures.

What are some ways this has been accomplished?

In a benchtop cadaveric study, Landman et al in 2002 showed that with both a 10/12F and 12/14F ureteral access sheaths, intrapelvic pressures were reduced with concomitant increase in irrigant flow.³ In a similar study for a ureteroscopy model, even in the presence of increased irrigation pressures renal pelvic pressures remained low (<20 mmHg).⁴ However, moving to smaller ureteral access sheaths (ie 10F) has been shown to negatively impact irrigation flow and lead to increased intrapelvic pressures (>40 mmHg).⁵ In clinical retrospective studies, smaller ureteral access sheaths and increased irrigation pressures have been shown to be associated with increased rates of SIRS. In a prospective study of 5 patients undergoing flexible ureteroscopy, measurements of renal pelvis pressures were measured through a nephrostomy tube. It was shown uniformly that pressures in the absence of ureteral access sheaths were higher in all locations including renal pelvis, proximal ureter and mid ureter.^6,7

Besides ureteral access sheaths, are there any other mechanisms to safeguard against increased intrapelvic pressures and resultant pyelovenous backflow?

Running irrigation fluid solely by gravity may mitigate concerns about increased intrapelvic pressures and pyelovenous backflow.⁸ Manual pump irrigation to improve visualization invariably leads to increased pressures within the collecting system. One potential solution is to combine gravity drainage and the use of a single action hand pump only at times where increased visualization is needed during ureteroscopy. In an in vitro study, multiple irrigation systems were evaluated and, besides gravity irrigation, the single action irrigation pump exerted the least amount of maximal force during use.⁹ The benefit of using a single action irrigation pump can be twofold in that stone migration can be minimized compared to continuous pressurized irrigation.

Effects of irrigation and as a consequence intrapelvic pressures can be magnified in the case of percutaneous nephrolithotomy (PCNL), as some of these patients are at higher baseline risks of sepsis prior to the operation. With the recent addition of mini-PCNL to the endourologist’s armamentarium, the question becomes, “is it safe?” In porcine models, mini-PCNL was associated with higher intrapelvic pressures and higher bacterial seeding compared to standard PCNL.¹⁰ Clinical studies are limited in terms of evaluation of intrapelvic pressures and sepsis risks for mini versus standard PCNL. Investigators from China reported an inverse relationship to percutaneous renal sheath diameter and intrarenal pressure, with higher pressures (>20 mmHg) associated with a higher risk of postoperative fever.¹¹ Head-to-head prospective trials are underway to try to answer this very question regarding both safety and efficacy to each technique. While concerns about sepsis with PCNL exist, irrigation by gravity and adjunct measures to limit pressure increases (ie ureteral access sheaths) can be utilized to lower sepsis risks.

Until pressures can be obtained from the collecting system during endoscopic stone surgery, the recommendation would be to limit operating room time, utilize as much gravity irrigation as possible and maintain optimal outlets for drainage during the case to balance inflow of irrigation with outflow.