Understanding Renal Hydrodynamics

Author: Mr Sunil Kumar, Consultant Urologist at the Royal Berkshire Hospital, UK

Irrigation is a mandatory requirement during any endoscopic procedure in the ureter or kidney. Irrigation is required for access into the ureter, and maintenance of visibility throughout the procedure, particularly during stone fragmentation to remove the vision obscuring dust. Good visibility should not come at the cost of a vastly increased intrarenal pressure and associated risk of pyelotubular and pyelovenous backflow, which in turn may lead to Systemic Inflammatory Response Syndrome (SIRS) or full-blown sepsis.

Helene Jung and Palle Osther [1] have shown that the baseline renal pelvis pressure is 10 (+/- 4.0) mm Hg. They showed that simple diagnostic ureterorenoscopy with forced flow continuous irrigation as low as 8ml/min will increase average renal pelvic pressure to 35 (+/- 10) mm Hg. During stone management, however, the average renal pelvic pressure can rise to 54 (+/-18) mm Hg. The same authors also showed that forced irrigation with a 20ml syringe is capable of producing peak pressures as high as 328 mmHg. This is very concerning, taking into account the threshold for pyelovenous backflow is reached at about 35 mmHg.

Wen Zhong studied the incidence of SIRS after flexible ureteroscopic lithotripsy [2] and found a strong statistical correlation (p<0.001) between the rate of SIRS and both total volume of fluid used for irrigation and flow rate.

Evidence suggests that we need to look for ways of reducing renal pelvic pressures during ureterorenoscopy, examining the role of forced continuous flow, overall flow rate, total volume of fluid used, maximal volume of bolus delivered, its timing, and the drainage capacity of the renal pelvis

The main determinant of intrarenal pressure during ureterorenoscopy is the balance between inflow and outflow. In order to prevent the build-up of dangerously high intrarenal pressures the inflow should match the outflow. Drainage of fluid from the renal pelvis is a limiting factor, even in the presence of UAS.

Pre-existing distention in the presence of limited outflow is dependent on the magnitude of continuous forced irrigation (inflow). Total volume of fluid used for irrigation is also an independent risk factor, as it is related to the same limiting factor of drainage from the renal pelvis.

The bolus size used in irrigation in comparison to renal pelvis capacity deserves careful consideration.

Delivering a bolus size many times larger than the total capacity of the renal pelvis (8-10ml) is bound to create a significant increase in pressure. This is important as most irrigation devices are capable of delivering a bolus with a volume lying between 150% and 300% of the total capacity of the renal pelvis.

Bolus Size in relation to renal pelvis capacity

The human kidney is poorly compliant, and a small amount of liquid injected into it will result in a significant increase in pressure. The increase in the intrarenal pressure of a noncompliant body is exponential in nature. Initiating bolus delivery to a partially drained pelvis will initially cause little increase in renal pressure, but when the renal pelvis becomes full even small volumes will result in a marked pressure increase.

The most physiologically sound irrigation would call for elimination of forced continuous flow to keep the renal pelvis partially drained. The bolus would need to be much smaller than renal pelvis capacity, thus preventing build up of excessive intrapelvic pressures.

Peditrol is safe and effective device that offers limited bolus size. Lack of the continuous forced flow, together with limitation of the bolus size results in a significant reduction in total fluid used; up to 50% compared to forced irrigation devices [3].

In our institution Peditrol has been successfully used on hundreds of ureteroscopic procedures, both rigid and flexible, over the last 15 years. It represents the departure from continuous forced irrigation to a small intermitten bolus method, as a safer and more physiologically sound alternative.

References

1. Jung and Osther. SpringerPlus (2015) 4:373)

2. Journal of Endourology, ”Systemic Inflammatory Response Syndrome after Flexible Ureteroscopic Lithotripsy , Study of Risk Factors” doi:10.1089/ end2014.0409

3. Uropump versus Peditrol – a Prospective Comparative Study, K.Dattatray Wani. Mahesh Desai – poster presentation