Question 5.4

You have decided  to initiate  CVVHDF in a septic patient  with acute  renal failure

After 24 hrs of CVVHDF  there has  been  no  worsening  in  the  patients  clinical  state. Repeat plasma biochemistry is as follows:

Normal Range

On Admission

After 24 hrs of CVVHDF

Na (mmol/L)

135 – 145



K (mmol/L)

3.5 – 4.5



Urea (mmol/L)

3 – 8



Creatinine (umol/L)

50 – 100



Phosphate (mmol/L)

0.7 – 1.4



Lactate (mmol/L)

0.2 - 2



What changes will you make to the CRRT to improve the biochemistry?

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College Answer

  •     Increasing the rate of the dialysate flow
  •     Increasing the rate of blood flow
  •     Change composition of dialysate fluid to increase the concentration gradient
  •    Increasing the surface area of the membrane
  •    Replacement fluid to go post dilution


This is another one of those "how would you increase the efficiency of dialysis" questions. This patient is underdialysed - on the grounds that the urea and creatinine have hardly changed after 24 hours of CVVHDF.

There are many things one can do to improve the rate of solute removal:

  • Increase the blood flow rate
  • Increase the dialysate flow rate
  • Increase the ultrafiltration rate
  • Increase the replacement fluid rate
  • Use some proportion of pre-dilution replacement fluid to elute more urea out of red cells and allow longer circuit lifespans (thus potentially increasing total daily clearance by virtue of having a longer uninterrupted session)
  • Use post-dilution replacement to increase the efficiency of middle molecule clearance (i.e. increase the filtration fraction up to the acceptable maximum, 0.25-0.30)
  • Change the composition of the dialysate to improve the concentration gradient (hardly relevant here because conventionally there is no urea in the dialysate anyway, but yes - one can adjust the concentration of potassium and bicarbonate)
  • Change to a filter with a greater surface area
  • Change to a filter with higher membrane porosity, if middle molecule clearance (eg. endotoxin) is your main concern

The general strategies to increase solute clearance in CRRT are discussed in a summary chapter from the Required Reading section. Some combination of pre and post dilution would be ideal here. In short, you would increase the dialysate flow rate, as well as increasing the pre-dilution volume (to elute urea, if you believe in that sort of thing- it seems to be something inferred (eg. by Brunet et al, 1999) from the different clearance rates of urea, creatinine and urate when comparing pre-dilution to post-dilution.) With the increased pre-dilution volume, you are able to increase your ultrafiltration rate significantly (thus removing lots of middle molecules) up to some safe maximum of filtration fraction. The addition of a large volume of clean post-filter replacement fluid finishes the process by diluting the remaining solutes on the way back to the patient's circulation.   


John, Stefan, and Kai-Uwe Eckardt. "Renal replacement strategies in the ICU."CHEST Journal 132.4 (2007): 1379-1388.

Brunet, Sylvain, et al. "Diffusive and convective solute clearances during continuous renal replacement therapy at various dialysate and ultrafiltration flow rates." American journal of kidney diseases 34.3 (1999): 486-492.