Tips to Control Intrarenal Pressure during Ureteroscopy
Irrigation is a mandatory requirement during ureterorenoscopy. Yet it is poorly understood while being the leading cause for high intrarenal pressure (IRP) which in turn is associated with complications such as infection, bleeding, extravasation and urosepsis.
The paradox of irrigation lies in the two opposing requirements that seemingly cannot be reconciled:
Good visibility requires intense irrigation. The more fluid the better
Safety requires minimal irrigation. The less fluid the better.
Effects of excessive irrigation:
Build-up of high IRP
Bacteria released from the stones during laser fragmentation
Urosepsis
Intra-op bleeding as high IRP tears the capillaries
Fornix rupture and extravasation
Effects of insufficient irrigation:
Poor visibility - can’t see, can’t work
Insufficient cooling of the heat generated by the laser – thermal injuries to the kidney
Poor removal of sand and small fragments
We have made it our mission to understand renal hydrodynamics. Here is what we have learned about controlling IRP during ureterorenoscopy, based on years of research.
Inflow & Outflow
The main determinant of IRP is the balance between inflow and outflow. Ideally the two should match, but they rarely do, outflow is always less.
The driving force that creates inflow is determined by the height of the bag of liquid above the kidney (also called hydrostatic pressure) and how long this pressure is sustained.
The higher the hydrostatic pressure the more distended the kidney is. The key is to reduce the baseline pressure of the kidney by lowering the bag of liquid to 30cm above the patient’s level. This results in reduced hydrostatic pressure.
The driving force that creates outflow is determined by the pressure inside the kidney. On outflow, irrigation fluid needs to travels against gravity, so the force that causes the fluid to drain from the renal pelvis is the pressure inside the kidney.
So good outflow, doesn’t necessarily mean that IRP is controlled. Observing high volumes of liquid draining through the ureteric access sheath (UAS) might be a warning sign of high IRP.
Several factors hamper outflow
1 . The height of the UAS outlet compared to its tip. The longer this length the more IRP is required to achieve drainage, because the fluid needs to travel against gravity to exit the kidney. Taken to extremes if the UAS outlet is at the level of bag of liquid, there will be no flow and distending pressure inside the kidney will be equal to the hydrostatic pressure of the bag.
2 . Too small diameter of UAS in combination with a large diameter of the ureterorenoscope. A small reduction in the diameter of the drainage channel results in a large reduction in flow. This is because flow through a channel is directly proportional to the radius of the channel to the power 4. If the radius is reduced by half, the flow will be reduced 16 times.
For example, UAS with inner lumen Fr11 used in conjunction with a flexible scope with shaft Fr7.5 and the bag of liquid suspended 30cm above the level of the kidney will drain more liquid per minute than UAS with inner lumen Fr10 used in conjunction with a flexible scope with shaft Fr8.5 and the bag of liquid suspended at 100cm, because there is much more space for the liquid to flow freely.
3 . The Trendelenburg position has the same effect as increasing the height of the UAS outlet
Compliance of the human kidney and IRP
There is a difference in response to liquid injection of a complaint and a non-complaint body.
The bladder is a compliant organ and acts like a balloon. It will mainly increase its volume in response to a bolus injection of liquid. There will be little increase in pressure.
The kidney, on the other hand is a non-compliant organ and acts like a soccer ball. It will not increase its volume, but it will mainly increase its pressure in response to the same bolus injection of liquid.
The pressure/volume relationship of a non-complaint organ is an exponential incline. The aim is to achieve inflow/outflow balance, this happens when the kidney is not fully distended.
When the volume inside the kidney approaches the kidney’s total capacity, any additional volume injection will result in a sharp increase in IRP. The bolus size can therefore impact IRP.
Maximal volume of bolus delivery and its timing
The total capacity of the kidney is only 10ml, but most irrigation devices can deliver bolus volume of 10ml to 30ml (between 150% to 300% of the kidney’s capacity). Delivering a bolus size many times larger than the renal pelvic capacity is bound to create significant increase in pressure.
Bolus should be no more than 20% - 30% of total renal pelvic capacity which is equivalent to 2 - 3ml. Allowing time between bolus delivery, allows drainage from the kidney to occur, it takes about 3 seconds for IRP to return to baseline after a single bolus delivery.
Some modern irrigation devices like Peditrol and Mini Pillow limit the bolus size to below 3ml.