Limiting the amount of downtime is the reason, as posted above. There is a smaller, more predictable window (assuming normal liver and kidney function) durign which ACT is not in effect.
There are several reasons why they use Lovenox for bridging. Lovenox is a type of heparin, called a low-molecular-weight heparin. There are a number of different types of LMWH, each somewhat different in the way they're made and their actions, but Lovenox (enoxaparin) is by far the most used at this point, having the most reliable action and the fewest interactions and side effects.
Standard heparin is administered by IV and requires frequent monitoring, as it has a somewhat unpredictable and fluid level of action. Because of this, standard heparin isn't very useful outside of a hospital setting. Lovenox can be given by injection once a day and doesn't need to be monitored, so it can be used more easily by outpatients.
Another beneficial feature is that enoxaparin doesn't tamper with thrombin production as much as regular heparin, and has a much lower incidence of heparin-induced thrombocytopenia. There are two types of thrombocytopenia (which occurs in about 5% of standard heparin users), a relatively harmless one that lowers platelet count temporarily, and Type II, which is a bodily counterreaction to the heparin, overcreating thrombin (a coagulant) and causing platelets to become more sticky and prone to clumping. Type II often causes damaging thromboses (clots) and is fatal in about 1/3 of cases.
However, standard heparin is quickly reversible with protamine sulfate, and enoxaparin doesn't undo chemically as easily, so more time is required for the effects to dissipate.
Best wishes,
