Question 16

A 76-year-old man is admitted to the ICU following a laparotomy for faecal peritonitis. He has developed multi-organ failure over two days, requiring ventilatory and inotropic support. He is oliguric, increasingly acidotic, uraemic and has a rising serum creatinine.

a) List the likely mechanisms for this patient's renal failure

b) What would be your indications for renal dialysis in this man?

c) Outline the means by which you would maximise urea clearance when using CVVHDF

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College Answer

a) List the likely mechanisms for this patient's renal failure
Likely mechanisms include pre-renal, renal and post-renal causes

Hypovolaemia (inadequate resuscitation)
Hypotension (inadequate perfusion pressure compared to his normal BP despite inotropes)
Impaired cardiac output (septic cardiomyopathy, myocardial ischaemia/infarction, dysrhythmias)

Toxins (eg nephrotoxic drugs – need to specify gentamicin / NSAIDs / contast for CT)
Microcirculatory failure (sepsis and inflammatory response) with medullary ischaemia, tubular
obstruction and vasoconstriction (acute tubular necrosis)

Raised intra-abdominal pressure
Unrecognised catheter problems

b) What would be your indications for renal dialysis in this man?
Uncontrolled electrolyte disturbances (hyperkalaemia, hypernatraemia)
Uncontrolled metabolic acidosis
Uraemia 30-35 mmol/l (optimal timing not known, uncontrolled studies suggest early CRRT better
than late, candidate should have his/her own threshold level)
Fluid overload unresponsive to diuretics
Early intervention to minimise inflammatory response in sepsis may be considered but is unproven

c) Outline the means by which you would maximise urea clearance when using CVVHDF
Urea clearance depends on ultrafiltrate flow rate and dialysate flow rate so clearance enhanced by
increasing blood flow rate and/or dialysate flow rate
Use of filters with larger membrane surface areas
Use of predilution
Changing filter if failing
Maximising time on CRRT by ensuring good vascular access, optimising filter life and limiting/rationalising time out of the ICU for imaging, surgery etc


This dialysis question assesses the candidate's understanding of the mechanims of solute clearance during CVVHDF.

Part a) is reasonably simple, and requires one to regurgitate the standard renal failure triad (pre-renal, renal, post-renal causes). The salient feature of the answer is the college's demand for specific nephrotoxic agents.

A big table of causes of renal failure is available in the Required Reading section.

Part b) is also reasonably simple, and requires one to recall the indications for dialysis. Bellomo (in Oh's Manual) suggests the following criteria:

  • Oliguria (less than 200ml in 12 hours)
  • Anuria (0-50ml in 12 hours)
  • Urea over 35 mmol/L
  • Creatinine over 400mmol/L
  • Potassium over 6.5mmol/L
  • Refractory pulmonary oedema
  • Metabolic acidosis with pH less than 7.10
  • Hypernatremia over 160mmol/L
  • Hyponatremia under 110 mmol/L
  • Temperature over 40°C
  • Complications of uraemia: encephalopathy, pericarditis, myopathy or neuropathy
  • Overdose with a dialysable toxin

The salient feature is the colleges' demand that the candidate have their own threshold for how much urea is too much. Also, cytokine clearance to reduce the severity of sepsis is mentioned, but it would be wise to write that the evidence for it is lacking.

Part c) requires some thinking. Solute clearance with CVVHDF is maximised by increasing the dialysate flow rate, the ultrafiltration rate, and the increase in membrane surface area. Maximising filter life and maximising time on CRRT by optimising access are practical considerations worth mentioning.

General strategies to improve solute clearance are listed in the Required Reading section.

A summary box available in that chapter is reproduced below, to simplify revision.

  • Increase filter lifespan with anticoagulation and predilution
  • Rationalise planned interruptions to CRRT (eg. scans and procedures)
  • Improve vascular access to minimise interruptions to CRRT
  • Increase the blood flow rate
  • Increase the dose of dialysis
    • Increase the dialysate flow rate
    • Increase the ultrafiltration rate
    • Increase the replacement fluid rate
  • Use of pre-dilution
  • Adjustment of dialysate to modify concentration gradients
  • Increasing the surface are of the filter


For a definitive treatment of all of this, you ought to pay homage to the gigantic and all-encompassing "Critical Care Nephrology" by Ronco Bellomo and Kellum (2009).

There is also extra stuff is from the Ronco et al article "The haemodialysis system: basic mechanisms of water and solute transport in extracorporeal renal replacement therapies" in Nephrol Dial Transplant ( 1998) 13 [Suppl 6 ]: 3–9.

I have tried to explore the argument of pre vs post dilution in my notes.

Finally, the Gambro and Fresenius websites have been an excellent source of information.