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Heparin clearance during continuous veno-venous haemofiltration
M Singer1, T McNally, G Screaton
1Bloomsbury Institute of Intensive Care Medicine, Department of Medicine, UCL Medical School, London, UK.
Objective:
To determine whether premature clotting of haemofiltration circuits could be related to heparin removal across the filter membrane into the ultrafiltrate.
Design:
Randomised study using either unfractionated (n = 8) or low molecular weight (n = 7) heparin for anticoagulation of the haemofiltration circuit at 1000 and 600 U/h respectively. Samples were drawn at 1 and 2 h from arterial and venous limbs of the haemofilter circuit for measurement of plasma heparin (as anti-Factor Xa activity), antithrombin III and haematocrit. Ultrafiltrate samples were collected at the same time for measurement of anti-Xa activity.
Setting:
Intensive care unit.
Patients:
Patients in acute renal failure requiring haemofiltration.
Results:
Both unfractionated and low molecular weight heparin plasma levels were within the range required for therapeutic anticoagulation in all but one patient at 2 h. Ultrafiltrate anti-Xa levels were insignificant. Antithrombin III levels in these critically ill patients were subnormal in 11 of the 15 studies.
Conclusions:
Despite their small sizes, neither unfractionated nor low molecular weight heparins cross the haemofilter membrane into the ultrafiltrate in any measurable quantity. Both heparins were present in plasma at a level suitable for therapeutic anticoagulation. Subnormal levels of antithrombin III may be an important factor in determining filter longevity.