College Answer

Likely mechanisms include pre-renal, renal and post-renal causes.

Pre-renal renal failure includes hypovolaemia (inadequate resuscitation), hypotension (inadequate perfusion pressure compared to his normal BP, ? hypertensive), and impaired cardiac output (myocardial depression, myocardial ischaemia/infarction, arrhythmias).

Renal mechanisms include toxins (circulating, nephrotoxic drugs [eg. aminoglycosides]) and microcirculatory failure (sepsis and inflammatory response) with medullary ischaemia, tubular obstruction and vasoconstriction (acute tubular necrosis).

Post-renal mechanisms include increased intra-abdominal pressure, ureteric obstruction and catheter problems (unrecognised, resulting in obstruction).

Discussion

Again, this is a question where the candidate is expected to demonstrate a systematic approach to the evaluation of renal failure.

In that, the question closely resembles Question 16 from the first paper of 2004, except in 2004 the patient was suffering from pneumonia and the candidate had to evalue their oliguria.

Given the differences between the two questions, the answer from 2004 is appropriately modified and reproduced below.

2) Discriminate between renal success and renal failure.

• Oliguria may be the manifestation of normal volume conservation mechanisms:
  • History will suggest decreased fluid intake
    • Vasopressin use may also be an obvious clue
  • Examination will demonstrate features of dehydration
  • Biochemistry will show an increased urea/creatinine ratio
  • Urinalysis will reveal concentrated urine with low urinary sodium
• Such oliguria will also respond to fluid boluses.
• Oliguria in the presence of normal or increased fluid balance suggests a failure of normal renal excretory function.
• Hypovolemia and dehydration may also be associated with renal failure; a failure to respond to fluid boluses can be an indication that pre-renal failure has developed. Oliguria of normal renal fluid conservation in hpovolemia is a continuum with the oliguria of renal failure due to renal hypoperfusion, and these categories may overlap.

Likely mechanisms of renal failure in a patient with multi-organ system failure and septic shock

• Pre-renal causes
  • Poor renal perfusion due to
    • decreased cardiac output
    • hypovolemia
    • septic shock with uncontrolled hypotension
    • abdominal compartment syndrome
    • renal arterial disease
    • renal venous thrombosis

• Intrarenal causes
  • ATN due to proloned renal hypoperfusion
  • Microvascular thrombotic damage due to DIC of sepsis
  • Nephrotoxic agents eg. gentamicin, IV contrast
  • Rhabdomyolysis
  • Autoimmune glomerulonephritis, eg. post-streptococcal glomerulonephritis or Goodpasture's syndrome (these are less likely)
  • Direct cytokine-associated nephrotoxicity of sepsis

• Post-renal causes
  • Obstruction of the urinary tract:
    • Ureteric obstruction by calculi or infection
    • Ureteric stenosis due to prior radiotherapy
    • Ureteric or bladder injury perioperatively
    • Uretheral obstruction by kinked or malpositioned IDC.