Monday, 8 November 2010

MRCP revision battle 43.7: Acute renal failure

Acute renal failure is one of those topics which is barely mentioned at medical school then suddenly becomes almost central to your day-to-day life as a medical houseofficer.  It also then pops up not infrequently in MRCP questions....

Acute renal failure is defined as a significant detioration in renal function occuring over hours to days.  

It is usually oliguric (<500mls/24 hrs)
However, it is non-oliguric in 10% of cases, including:
  • gentamycin/amphotericin toxicity
  • radio-contrast nephropathy
  • interstitial nephritis

When approaching renal failure everyone repeats the mantra 'is it pre-renal, renal or post-renal'.  A recap of a few of the causes follows:

Pre-renal causes:
  • sepsis
  • hypovolaemia
  • cardiac failure
  • liver failure
  • NSAIDS/ACE-i (interfere with renal blood flow)

Renal causes (generally acute tubular necrosis):
  • nephrotoxic drugs
    • gentamycin
    • amphotericin
    • tetracyclines
    • contrast agents
  • myeloma
  • myoglobin (rhabdomyolysis)
  • vasculitis
  • glomerulonephritis

Post renal:
  • any urinary tract obstruction

Important points to enable one to distinguish ATN from pre-renal uraemia are shown in the table below:

Essentially in pre-renal failure the kidneys are still working so are trying to concentrate the urine, whereas in ATN they aren't.

Going through the management of acute renal failure is a bit beyond the scope of this battle (and if you're taking MRCP you are probably already very familiar with it) so I'll finish by just briefly recapping indications for dialysis:

  • refractory pulmonary oedema
  • persistent hyperkalaemia (>7mmol/l)
  • severe metabolic acidosis (pH<7.2 or BE <10)
  • uraemic encephalopathy
  • uraemic pericarditis

Thats all the battles for today.  Tomorrow we're back for a mixed bag before embarking on another renal day....