Post-renal acute kidney injury: obstruction and back pressure
Post-renal acute kidney injury (a sudden fall in kidney function) is caused by a blockage downstream of the kidneys that stops urine leaving, so back pressure reduces filtration.
Remember
Post-renal acute kidney injury is a drainage problem: a blockage makes pressure build up behind it, which opposes filtration, and the obstruction usually has to affect both kidneys, or the only working kidney.
Memory aid
A blocked sink: the tap (blood supply) and the basin (kidney) are both fine, but the water cannot leave.
Use the picture to remember that in post-renal AKI the blood supply and the kidney start out normal but urine cannot leave.
Acute kidney injury (AKI, a sudden fall in kidney function) is classed as pre-renal (too little blood arriving), intrinsic (damage within the kidney) or post-renal (urine cannot get out). In post-renal AKI the problem lies after the kidney, in the ureters (the tubes carrying urine from each kidney to the bladder), the bladder outlet or the urethra (the tube carrying urine from the bladder to the outside).
The kidneys keep making urine, but it cannot drain away. Urine backs up, so pressure rises in the urinary tract and is passed back to the glomeruli (the kidney's tiny filters). This back pressure opposes the pressure that drives filtration, so the glomerular filtration rate (GFR, the volume of plasma filtered each minute) falls, and waste products and excess water are retained.
One healthy kidney can compensate, so obstruction typically causes AKI only if it blocks both kidneys, or one kidney in a person who has only one working kidney. A blocked bladder outlet, for example from an enlarged prostate, affects both kidneys at once. The blockage might be a stone, a tumour or a blocked catheter.
An ultrasound scan is the usual first test, looking for hydronephrosis (a kidney swollen with trapped urine). The blood supply and the kidney tissue start out normal, so relieving the blockage, for example with a catheter, can restore function. Recovery is more likely the shorter the obstruction.
Key facts
- Mechanism
- a blockage downstream of the kidneys raises pressure in the urinary tract, and this back pressure reduces filtration.
- Site of the blockage
- the ureters, the bladder or the urethra.
- Which kidneys
- obstruction typically causes AKI only if it blocks both kidneys, or the only working kidney.
- Blood tests
- creatinine and urea rise as the GFR falls.
- Ultrasound
- the usual first test, looking for hydronephrosis or dilated ureters.
- Outlook
- recovery after relief of the blockage is more likely the shorter the obstruction, and prolonged obstruction may cause permanent damage.
Exam traps
- A stone blocking one ureter does not usually cause AKI when the other kidney is working, because that kidney keeps draining and can clear waste for both.
- A blocked bladder outlet does not need the ureters to be blocked as well, because it already stops urine leaving both kidneys.
- Hydronephrosis in one kidney, with a healthy partner kidney, does not by itself mean AKI, because a healthy partner kidney can usually compensate.
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