Changing over from Heparin to Warfarin
Abstract: Heparin should be overlapped with warfarin till the INR is within therapeutic range, with a minimum of four days after initiation of warfarin therapy.
It is usual practice to start heparin initially and then switch over to warfarin in many situations like peripheral embolism and pulmonary embolism. Sometimes warfarin is not initiated along with heparin when potential for a surgical intervention is considered in the near future because of the longer wane out period of warfarin effect. When such a possibility does not exist, warfarin can be initiated along with heparin so that duration of heparin therapy can be minimised. It may be noted that longer duration of heparin therapy not only increases the cost and pain associated with injections, but also the chance of the rare heparin induced thrombocytopenia syndrome (HIT). When a longer duration of parenteral anticoagulation is expected, it may be better to initiate low molecular weight heparin (LMWH), which has a more predictable bioavailability.
The peak effect of a given dose of warfarin is reached in only about 96 hours, even if the INR (international normalized ratio of prothrombin time) is within the therapeutic range earlier than that. Hence a minimum heparin overlap period of four days is often considered mandatory while switching over from heparin to warfarin therapy. Once the suggested therapeutic range of INR for the thrombotic condition being treated is achieved, heparin can be discontinued, after a minimum period of four days.
Usual target INR is 2-3 in most cases. INR of 2.5-3.5 is recommended in those with mechanical prosthetic valve at mitral position as well as those who have gone aortic and mitral valve replacement. If it is only aortic mechanical prosthesis, target INR is 2-3.