Continuous Renal Replacement Therapy

The concept behind continuous renal replacement techniques is to dialyse patients in a more physiologic way, slowly, over 24 hours, just like the kidney. Intensive care patients are particularly suited to these techniques as they are, by definition, bed bound, and, when acutely sick, intolerant of the fluid swings associated with IHD.

The techniques used are:

1. CVVH (click here)– continuous venovenous hemofiltration, a form of convective dialysis. The ultrafiltration rate is high,  and replacement electrolyte solution is required to maintain hemodynamic stability. This mode is also very effective for clearing mid sized molecules, such as inflammatory cytokines. It is hypothesized that removal of such mediators may play a role in improving outcome in sepsis. A very simple version of this is SCUF (click here) - slow continuous ultrafiltration, which is used for volume control in fluid overloaded patients. SCUF does not require the use of replacement fluid, and fluid removal is 300ml to 500ml per hour.

2. CVVHD (click here) -continuous venous venous hemodialysis– which is continuous diffusive dialysis, the dialysate is driven in a direction countercurrent to the blood. This provides reasonably effective solute clearance, although mostly small molecules are removed.

3. CVVHDF (click here) continuous venous venous hemodiafiltration, which is the most popular method of dialysis in ICU, combines convective and diffusive dialysis. Both small and middle molecules are cleared, and both dialysate and replacement fluids are required.

Most of these modes can remove up to 1 litre per hour of fluid. It is rare that this volume of fluid removal is required in intensive care (critically ill patients rarely tolerate any significant fluid removal).

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