aspirin daily heparin prophylactic anticoagulation molecular weight mg asa low heparin fondaparinux lmwh warfarin molecular weight derived coumarin low prophylaxis warfarin low heparin thrombin lmwh inhibitors xa dose therapeutic molecular heparin low molecular weight warfarin anticoagulation therapeutic aspirin dose anticoagulants warfarin anticoagulation inr pt receiving patients therapeutic prophylactic dose therapy bleeding dose heparin molecular weight inr stable carries warfarin low heparin low flush acetylsalicylic dwelling dose anticoagulation acid needed maintenance anticoagulation therapeutic receiving patients therapy anticoagulant aspirin require anticoagulants stable