Box 84 Typical composition of haemofiltration replacement fluid mmol litre1

• Potassium - Variable and added just before use

passed through the filter under arterial pressure alone - but circuit disconnection could lead to rapid blood loss and patients with low blood pressures often had slow moving circuits with the associated risk of blood clotting. In more common use today is continuous venovenous haemofiltration or CVVH. A large vein is cannulated using a double lumen catheter and a pump controls blood flow. The extracorporeal circuit is anticoagulated in both CAVH and CVVH. Automated systems have a replacement fluid pump that can either balance input and output or allow a programmed rate of fluid loss.

Haemofiltration removes virtually all ions from plasma including calcium and bicarbonate. Replacing these is difficult, since solutions containing enough of these two ions can precipitate. Lactate is commonly used instead of bicarbonate but, although in normal people lactate is converted to bicarbonate, this is not true of patients with lactic acidosis. In these situations bicarbonate infusions must be given separately. Haemofiltration has advantages over haemodialysis in the critical care setting because such patients cannot be fluid restricted and often have a compromised circulation. CVVH avoids the hypotension often seen in dialysis, is continuous so it can remove large volumes of water in patients receiving parenteral nutrition and other infusions, offers better clearance of urea and solutes, may better preserve cerebral perfusion pressure, and also has a role in clearing inflammatory mediators.

The differences between haemodialysis and haemofiltration are shown in Figure 8.4.

Haemodialysis to patient from patient

Concentration gradient

K urea creat phos

BLOOD

Dialysis fluid

"Dialysate"

Haemofiltration

Replacement fluid from patient <-

to patient

urea creat phos Na

Pressure gradient d

BLOOD

"Haemofiltrate"

Figure 8.4 The difference between haemodialysis and haemofiltration. creat, creatinine; K, potassium; Na, sodium; phos, phosphorus

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