Haemo(dia)fi ltration (1)

Conventional haemodialysis requires a pressurised, purifi ed water supply, and a greater risk of haemodynamic instability due to rapid fluid and osmotic shifts. Haemo(dia)filtration can be arterio-venous, using the patient’s BP to drive blood through the haemofilter, or pumped venovenous. The latter is preferable as it does not depend on the patient’s BP, and the pump system incorporates alarms and safety features. Continuous veno-venous haemo(dia)fi ltration (CVVH or CVVHD) is increasingly the technique of choice. Blood is usually drawn and returned via a 10–12Fr double-lumen, central venous catheter (see figure 4.1).

Indications

• Azotaemia (uraemia).

• Hyperkalaemia.

• Anuria/oliguria; to make space for nutrition.

• Severe metabolic acidosis of non-tissue hypoperfusion origin.

• Fluid overload.

• Drug removal.

• Hypothermia/hyperthermia.

Techniques

CVVH relies on convection (bulk transfer of solute and water) to clear solute. In CVVHD, dialysate flows countercurrent to the blood, allowing small molecules to diffuse according to their concentration gradients.

Membranes are usually hollow fi bre polyacrylonitrile, polyamide, or polysulphone with a surface area of 0.6–1m2.

Both CVVH and CVVHD are effective for small molecule clearance (e.g. urea). CVVH is better at larger molecule clearance and can remove substances up to the membrane pore size cut-off (usually 30–35kD). Filtrate is usually removed at 20–35mL/kg/h; fluid balance is adjusted by varying the rate of fluid replacement. High volume haemofi ltration involves much higher ultrafiltration

rates (e.g. 50–100mL/kg/h, usually for short periods, e.g. 4h) in an effort to remove inflammatory mediators. Variable outcomes are reported in studies.

Creatinine and K+ clearances are higher with CVVHD, but filtration alone is usually sufficient if ultrafiltrate volume is adequate. (1000mL/h approximates to a creatinine clearance of 16mL/min). CVVHD is preferred for pharmacologically-resistant hyperkalaemia.

Replacement fluid

A balanced electrolyte solution buffers acidaemia and is titrated to desired fluid and electrolyte balance. Buffers include lactate (liver metabolised to bicarbonate) and bicarbonate. Acetate (metabolised by muscle) causes most haemodynamic instability and is now rarely used. Bicarbonate may be more efficient than lactate at reversing severe acidosis, but no outcome benefit has been shown. Care is needed when giving Ca2+ since calcium bicarbonate may crystallise. In hypoperfused liver, lactate may be inadequately

metabolised.

 Increasing metabolic alkalosis may be due to excessive buffer so use low buffer (30mmol/L lactate) replacement fluid. K+ can be added to maintain normokalaemia. 20mmol KCl in a 4.5L bag provides a concentration of 4.44mmol/L. K+ clearance is increased by reducing the concentration within replacement fluid or dialysate.

 

 Fig. 4.1 Circuit arrangement for haemo(dia)filtration.