Reducing the risk of thromboembolism

The risk of thromboembolism and associated morbidity depends on the indication for anti-coagulation: if and how long before patients have had previous episodes of thromboembolism and whether or not surgery increases the risk of thromboembolism. The risk of preoperative bleeding is generally low, but is high following major surgery. As the risk of thromboembolism and bleeding are often influenced by the surgical procedure, anti-coagulant management needs to be considered separately for the pre- and postoperative periods.

Preoperative Indications for warfarin

To assess the risks associated with temporarily stopping anticoagulants, the consequences, as well as the absolute risk, of a throm-boembolic event need to be considered. Arterial thromboembolism (ATE) often results in death (~ 40%) or major disability (~ 20%) whereas venous thromboembolism (VTE) rarely presents as sudden death (~ 5 to 10%), and major disability is also unusual (< 5%) in patients with treated VTE.183

(a) Arterial indications for anti-coagulation

Primary prophylaxis of ATE is most commonly undertaken in patients with atrial fibrillation (AF), valvular heart disease, and recent myocardial infarct. Secondary prophylaxis is undertaken after patients (with or without the above conditions), have had an ATE event (usually stroke). Previous thromboembolism is a major risk factor for 183

recurrence.183

Previous ATE

These patients have a higher risk of embolism than patients without previous episodes. Therefore, the period of sub-therapeutic anticoagulation should be kept to a minimum. In patients whose INR is 2.0-3.0, it takes 4 days for it to spontaneously fall to < 1.5, an intensity of anti-coagulation at which increased intra-operative bleeding is not expected after anti-coagulation is stopped. Therefore, 4 daily doses of warfarin should be withheld preoperatively, and the INR should be measured the day before surgery to determine if a small dose of vitamin K is needed to accelerate the reversal of anti-coagulation:

(1) In general, give 1 mg of vitamin K by slow i.v. injection if the INR is > 1.7 the day before the surgery, repeat the INR the morning of surgery.

(2) If necessary, fresh frozen plasma (FFP) can be given prior to surgery if the INR is still not acceptable (i.e., INR 1.3-1.7, 1 FFP unit, 1.7-2.0, 2 FFP units). Administration of blood products should generally be avoided for elective surgery.

No previous ATE

The risk after discontinuing anti-coagulation is lower in patients who have not had a previous episode. Warfarin can be withheld for 5 doses to ensure that coagulation has returned to normal prior to surgery; however inpatients with prosthetic heart valves have a higher risk of thromboembolism. In these patients, i.v. unfractionated heparin should be substituted while warfarin (alternatively LMWH therapeutic schedules may be used) is withheld till INR < 1.3, then heparin is withheld 1 to 2 hours (short half-life) prior to surgery. As the usual intensity of anti-coagulation is higher in such patients, a small dose of vitamin K is required more often in these patients on the day before

surgery.183

Last episode of ATE within 1 month

The risk of recurrent ATE is highest within a month of an acute event (about 0.5% per day). To minimise the possibility of preoperative embolism, i.v. heparin should be administered when INR drops to < 2.0. Stopping i.v. heparin 6 hours prior to surgery should be adequate for the aPTT to return to normal before surgery.183

(b) Venous indications for anti-coagulation

The main indication for anti-coagulation is prevention of recurrent VTE. An exception occurs in selected patients with patients with thrombophilia (e.g., anti-thrombin, protein C, protein S deficiencies, FV Leiden abnormalities, and strong family history of thromboembolism).183

Last VTE event within 1 month

The risk of recurrent VTE declines rapidly with duration of anticoagulation. There is a very high risk of recurrent VTE if anticoagulants were stopped within 1 month of VTE. Therefore, if feasible, surgery should be deferred until patients have received 1 to 3 months of anti-coagulation. If this is not feasible, preoperative thromboembolic risk should be minimised by administering i.v. heparin when INR is less than 2.0.

Last VTE between 1 to 3 months

These patients have a moderately high risk of recurrent VTE if anticoagulants are stopped. Warfarin should only be withheld for 4 doses to minimise this period of high risk.

Last VTE > 3 months

These patients have a much lower risk of recurrent VTE than those who have been treated for < 3 months.

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