Pyelovenous Backflow During Flexible Ureteroscopy Is Influenced by Renal Pelvis Pressure

Flexible ureteroscopy (URS) is quickly becoming the most common procedure in endourology. Still, a recent meta-analysis of modern literature published in the Journal of Endourology concluded the sepsis rate is 5%.

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It's suspected that the use of irrigation fluid through the ureteroscope to maintain visibility may lead to increases in renal pelvis pressure, which results in pyelovenous backflow (retrograde flow of fluid from the urinary tract into the vascular system). The pressures at which this can occur have not been characterized except in small human case series and ex vivo animal studies.

Christina Kottooran, MD, a clinical fellow in surgery in the Department of Urology at Massachusetts General Hospital, Brian H. Eisner, MD, co-director of the Kidney Stone Program in the Department, and colleagues used a porcine model to conduct a true systematic in vivo evaluation of the variation in fluid absorption at various renal pelvis pressures. They report the results and clinical implications in BJU International.

Methods

The researchers performed flexible URS in 18 female Yorkshire pigs of approximately the same weight, using a standardized anesthesia protocol. Throughout the one-hour procedure, the ureteroscope was maintained in a single location in the renal pelvis.

The target renal pelvis pressures were 37 mmHg (n=5 pigs), 55 mmHg (n=5), 75 mmHg (n=6), and 150 mmHg (n=2). Each procedure started with the blood alcohol level at 0 mg/dL, confirmed via a breathalyzer. Venous blood was sampled every 10 minutes until the breathalyzer detected peak blood alcohol.

The volume of irrigation fluid absorbed for each procedure was estimated using established equations that consider blood ethanol concentration, weight, and irrigation fluid ethanol concentration.

Volume of Fluid Absorption

The average volume of fluid absorbed (magnitude of pyelovenous backflow) during URS was:

• 37 mmHg—7.6 mL; undetectable in 60% of procedures
• 55 mmHg—10.8 mL (P=0.261 vs. 37 mmHg); undetectable in 40% of procedures
• 75 mmHg—26.0 mL (P=0.026); >7 mL in 100% of procedures
• 150 mmHg—56.8 mL (P=0.047); >7 mL in 100% of procedures

Timing of Fluid Absorption

Lower renal pelvis pressures resulted in a longer time for ethanol to reach detectable levels in the bloodstream. On average, the time to detect >7 mL of irrigation fluid absorption was:

• 37 mmHg—50 minutes
• 55 mmHg—30 minutes (P=0.2621 vs. 37 mmHg)
• 75 mmHg—20 minutes (P<0.001)
• 150 mmHg—20 minutes (P<0.001)

Applying the Findings to the Clinic

The key takeaway messages from this study are:

• A surgeon performing URS in a patient who is at high risk of postoperative infection (history of pyelonephritis or urosepsis) should consider making efforts to maintain renal pelvis pressure at lower levels
• There may be a pressure threshold, somewhere between 55 and 75 mmHg, above which pyelovenous backflow will occur during every procedure
• The duration of URS plays a role in the magnitude of pyelovenous backflow and should be limited to about 30 minutes if possible; otherwise, it is particularly important to try to maintain renal pelvis pressure at lower levels

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