Question 2d

A seventy-six (76)  year  old  man  is  admitted  to  the  ICU  following  a  laparotomy  for  faecal peritonitis.  He has developed Multiple System Organ Failure over two days, requiring ventilatory and inotropic support.   He is oliguric, increasingly acidotic, uraemic and has a rising serum creatinine.

(d)       Illustrate   and   label   a   dialysis  circuit   that   depicts  veno-venous  haemodiafiltration (CVVHDF).

(e)       Outline the means by which you would maximise urea clearance and filter life when using CVVHDF.

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College Answer

(e)       Outline the means by which you would maximise urea clearance and filter life when using CVVHDF.

Urea clearance depends on ultrafiltrate flow rate (clearance by convection proportional to flow rate) and dialysate (countercurrent) flow rate (clearance proportional to flow rate). Increasing clearance would therefore be obtained by increasing either flow rates. Other factors include the use of filters with larger membrane surface areas, the use of predilution (ie. prefilter position for replacement fluid) if ultrafiltration rate is significant, and changing filter if it is failing. Independent factors that may prevent filter fibre loss (ie. prolong filter life) include adequate anticoagulation, priming with albumin, filter coating with anticoagulants, the use of predilution (decreasing oncotic pressure and haematocrit), and avoidance of large negative transmembrane pressures.


I can proudly say that my diagram of the CVVHDF circuit is better than the college diagram.

CVVHDF diagram

Question (e) is unfairly broad.

There are numerous methods for prolonging circuit lifespan. The answer to Question 4 from the second paper of 2010 contains a massive table titled "Methods of Prolonging the CVVHDF Filter Lifespan", and I will not repeat it here.

The clearance of urea however is a unique question. Judging by the college answer, they were really asking "what strategies exist to increase the removal of solutes by dialysis?"

  • Diffusive clearance depends on the following factors:
    • Membrane surface area
    • Concentration gradient of the specific solute
    • Blood flow rate (up to a point)
    • Countercurrent dialysate flow rate
    • Size of the molecule (smaller molecules like urea diffuse more easily)
    • Protein and RBC binding of the solute (urea also needs to diffuse out of RBC intracellular fluid)
  • Convective clearance depends on the following factors:
    • Ultrafiltrate flow rate, which in turn is dependent upon transmembrane pressure
    • Porosity of the membrate (more relevant for larger molecules)

Urea is a small highly water soluble molecule, and thus is is easily removed by diffusion as well as convection. Thus, in order to improve the clearance of urea, one may perform the following manoeuvres:

  • Use a filter with a larger surface area
  • Increase the blood flow rate
  • Increase the dialsyate flow rate
  • Increase the rate of ultrafiltration
  • Change the replacement fluid to pre-dilution, in order to improve the elution of urea from the RBC intracellular fluid
  • Ensure that the filter lifespan is maximised, to increase the total duration of the dialysis session.


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.

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