College Answer

Dialytic techniques in the critically ill are becoming more widely used. Traditional indications used for acute renal failure, are concerns about fluid overload (actual or to facilitate nutritional support), hyperkalaemia or other uncontrolled electrolyte disorders, metabolic acidosis, hyponatraemia, uraemic symptoms or elevated urea (e.g. 30 mmol/L). As complications associated with techniques have been minimised, dialysis is often initiated earlier (anticipatory, oliguria, lower urea), and even for non-renal indications (including sepsis or septic shock). Dialysis or haemofiltration (e.g. with charcoal filter) can be used to increase the clearance of toxic products from the circulation (e.g. lithium, theophylline, myoglobin). Newer related extracorporeal techniques have also been developed to support liver dysfunction.

Discussion

The question really asks "what are the indications for dialysis"?

A good resource for information about this topic is adqi.org, home of the Acute Dialysis Quality Initiative. Particularly, their "reports" section contains a series of recommendations for commencement of dialysis in AKI. Most of these recommendations are based on expert opinion rather than strong evidence.

In summary, the indications for dialysis are as follows:

Renal Indications

• Oliguria with volume overload
• Oliguria is relative; urine output may be high and still inadequate in clearing the fluid.
• Uremia with symptoms
• Hyperkalemia ( K+ over 6.0)
• Metabolic acidosis due to renal failure or lactate (pH < 7.2)

Non-renal Indications

• Removal of dialysable toxins, i.e. ones which aren’t very lipophilic or protein-bound
  • Lithium
  • Methanol
  • Ethylene glycol
  • Salicylates
  • Theophylline
  • Valproate
  • Pretty much any drug with a volume of distribution less than 0.5L/kg
  • If a toxin is equally well cleared by hemodialysis and hemoperfusion, then hemodialysis is preferred, because it will also correct any underlying acid-base disturbance.
• ​Removal of contrast agent
  • More relevant with old-school high-osmolar contrast
• Clearance of cytokines to decrease severity of sepsis
  • Still controversial. May be of use in patients with renal failure and sepsis.
  • No evidence that it helps in patients with sepsis who don’t have renal failure.
• Control of body temperature
  • An extracorporeal circuit can help control hypo or hyperthermia which is resistant to other methods of control.
• Control of otherwise uncontrollable electrolytes
  • Hypercalcemia refractory to bishosphonates

Timing of dialysis

Guided by expert opinion and the IDEAL trial of 2010 (no mortality difference between early and late dialysis groups)

The current (2011) guidelines suggest the following:

• Start when the GFR has fallen to below 15ml/min
• Definitely start before the GFR falls to below 6ml/min
• Symptomatic uremia
• Inability to control fluid balance
• Inability to control blood pressure
• Progressive deterioration in nutritional status
• Diabetics may benefit from an earlier start