The risk of thromboembolism is high during the first month after mechanical prosthetic heart valve (MPHV) surgery,1 2 particularly during the lag time before oral anticoagulant (OAC) treatment reaches effective levels. Indeed, vitamin K antagonists (VKAs) take at least 5 days theoretically—and about 2 weeks in practice3—to achieve a therapeutic international normalised ratio.

A “bridging” anticoagulant is usually prescribed to cover this period. Traditionally, the bridging agent has been unfractionated heparin (UH),4 but low molecular weight heparin (LMWH) has been used more recently.3 5 6 As there are few studies to validate the efficacy and safety of either UH or LMWH in this setting, the need for heparin remains controversial and practice guidelines are not very clear cut.1 Thus, in some centres, OAC starts with VKA monotreatment, so that bridging does not become necessary unless there is an unusual delay in achieving a therapeutic international normalised ratio.4 7 8 However, a recent survey4 confirms that, after MPHV surgery, UH and LMWH are routinely administered by about 65% and 22% of surgeons, respectively.

In most centres UH is rarely introduced immediately in a therapeutic dose, because of the fear of bleeding,8 and it often takes several days to obtain a therapeutic activated partial thromboplastin time (APTT).

The use of UH raises several other concerns. First, the bioavailability and predictability of intravenous UH anticoagulation are poor (reviewed by Meurin et al5), especially in patients treated subcutaneously. Montalescot et al reported that …