Heparin forms the mainstay of anticoagulation therapy for the critically ill as a continuous infusion for prevention of coagulation in the extracorporeal circulation or treatment of thromboembolism. Intermittent subcutaneous heparin is more commonly used for prevention of thromboembolism in the immobile patient or following myocardial infarction.

Often, when used for the extracorporeal circulation, systemic anticoagulation is not required. Alternative methods of giving heparin include the following.

1. Regional anticoagulation: heparin infused into the blood leaving the patient may be neutralized by protamine sulfate on return. It is assumed that 1 mg protamine neutralizes about 100 IU of unfractionated heparin. However, protamine sulfate has a number of side-effects which would be particularly deleterious in the critically ill. These include reduced cardiac output, decreased systemic vascular resistance, increased pulmonary vascular resistance, decreased platelet function, and bronchoconstriction.

2. Covalent bonding of heparin to surfaces containing amine groups: the technique is used to produce a biologically active surface on circuits, catheters, and membranes (Carmeda, Stockholm, Sweden). The surface heparin coating acts as a catalyst, accelerating the active inhibition of thrombin binding to the surface by circulating antithrombin III. The limitations of surface heparin coating are similar to those of the vascular endothelium: blood flow must be maintained since blood will clot if stagnant and a high ratio of surface area to volume is required to clear thrombin adequately.

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