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.
(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.
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?"
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:
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.