In the hospital, you may see patients who need to be started on coumadin for a variety of reasons. These patients are often put on a “bridging” protocol, which helps ensure that they are anticoagulated even if their INR is subtherapeutic. Bridging protocols differ from hospital to hospital.
In general, patients should receive heparin for the first five days of coumadin therapy, plus 24 hours after the patient reaches a therapeutic INR (often 2.0-3.0).
Why do patients need to be bridged at all? Coumadin acts on vitamin-K dependent factors: 2, 7, 9, 10, and protein C and S. Coumadin first decreases protein C activity, within hours. However, it does not decrease pro-thrombotic factors like 2,7, and 9 until several days later. This means that for the first 3-5 days of coumadin therapy, a patient is, paradoxically, at a higher risk of clotting (demonstrated in extreme cases by warfarin skin necrosis).
A 1992 NEJM study compared patients who received acenocoumarol (like coumadin) + heparin versus acenocoumarol alone. The study had to be stopped early because while the acenocoumarol + heparin patients had a 4/60 risk of developing venous thromboembolism, the patients on acenocoumarol alone had a 12/60 risk. Furthermore, there was greater clot extension in the acenocoumarol-alone group.
Would the bleeding risk be increased if a patient was on both heparin and coumadin? The NEJM study found that there was no significant increased bleeding risk in the combination therapy group.
For more information on when to initiate bridging, see this UT Austin pharmacology rounds lecture.