michael s
(Michael S)
#1
95 How should the anticoagulation of a patient
with a mechanical heart valve be managed for
elective surgery?
Matthew Streetly
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 in-
appropriately 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 post-
operative 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
