How should the anticoagulation of a patient with a mechanical heart valve be managed for elective surgery?

Mechanical heart valves are associated with an annual risk of arterial thromboembolism of <8%. Although warfarin greatly reduces the risk, it is at the expense of an INR-related risk of serious haemorrhage. This constitutes an unacceptable risk for patients undergoing major surgery, and it is necessary to temporarily institute alternative anticoagulant measures.

The anticoagulant effect of oral warfarin is prolonged (half life 36 hours) and it can take 3–5 days for a therapeutic INR to fall to less than 1.5. It is also dependent on the half life of the vitamin K dependent clotting factors (particularly factors X and II, with half lives of 36 and 72 hours respectively). The surgical procedure must therefore be planned with this in mind. A “safe” INR depends on the surgery being undertaken. An INR <1.5 is usually suitable, although this should be <1.2 for neurosurgical and
ophthalmic procedures.

Four days prior to surgery warfarin should be stopped. Once the INR falls below a therapeutic level heparin should be started. Unfractionated heparin (UFH) should be administered as an intravenous infusion. It has a short lasting effect (half life 2 to 4 hours) and is monitored using daily measurements of the APTT ratio (aim for APTT 1.5–2.5 times greater than control APTT). Alternatively, a weight-adjusted dose of low molecular weight heparin (LMWH) is given subcutaneously once daily with
predictable anticoagulant effect, although data are limited. The night prior to surgery the INR should be checked and if it is inappropriately high then surgery should be delayed. If surgery cannot be delayed, the effect of warfarin can be reversed by fresh frozen plasma (2–4 units) or a small dose of intravenous vitamin K (0.5–2mg). Six hours prior to surgery heparin should be stopped to allow the APTT to fall to normal.

Recommencing intravenous heparin in the immediate postoperative period may increase the risk of haemorrhage to greater levels than the risk of thromboembolism with no anticoagulation. Heparin is usually restarted 12–24 hours after surgery, depending on the type of surgery and the cardiac reason for warfarin. Each case must be considered individually. Warfarin should be restarted as soon as the patient is able to tolerate oral medication. Prophylactic heparin should be stopped once an INR greater than 2.0 is established.