Haemo(dia)fi ltration (2)

Anticoagulation

The circuit can be anticoagulated with unfractionated heparin (200–2000IU/h), a prostanoid (prostacyclin or PGE1) at 2–10ng/kg/min, or a combination of the two. Other alternatives include regional citrate anticoagulation, the low molecular weight heparinoid danaparoid, or direct thrombin inhibitors (e.g. lepirudin, argatroban). These agents are, as yet, under-evaluated for renal replacement therapy and they risk important complications such as hypocalcaemia (with citrate) and prolonged bleeding with the long half-lives of the thrombin inhibitors. Bivalirudin is a new agent related to hirudin and lepirudin but is reversible with a short half-life of only 25min. No anticoagulant may be needed if the patient is auto-anticoagulated. Premature clotting may be due to mechanical kinking/obstruction of the circuit, insufficient anticoagulation, inadequate blood flow rates, or lack of endogenous anticoagulants such as antithrombin III. Usual filter lifespan should be at least two days, but is often decreased in septic patients due to decreased endogenous anticoagulant levels. In this situation, consider use of fresh frozen plasma, a synthetic protease inhibitor such as aprotinin, or antithrombin III replacement (costly).

Filter blood flow

Flow through the fi lter is usually 100–200mL/min. Too slow a flow rate promotes clotting. Too high a flow rate will increase transmembrane pressures and decrease filter lifespan without significant improvement in clearance of ‘middle molecules’ (e.g. urea).

Complications

• Disconnection leading to haemorrhage.

• Infection risk (sterile technique must be employed).

• Electrolyte, acid-base or fluid imbalance (due to excess input or removal).

• Haemorrhage (vascular access sites, peptic ulcers) related to anticoagulation therapy or consumption coagulopathy. Heparin-induced thrombocytopaenia may rarely occur.

Cautions

• Haemodynamic instability related to hypovolaemia (especially at start).

• Vasoactive drug removal by the filter (e.g. catecholamines).

• Membrane biocompatibility problems (especially with cuprophane).

• Drug dosages may need to be revised (consult pharmacist).

• Amino acid losses through the filter.

• Heat loss leading to hypothermia.

• Masking of pyrexia.