True statement about pulmonary embolism

Unfractionated Heparin UFH anticoagulates by binding to and accelerating the activity of antithrombin, thus preventing additional thrombus formation. UFH is dosed to achieve a target activated paial thromboplastin time (aPTT) of 60-80 s. The two principal indications for inseion of an IVC filter are (1) active bleeding that precludes anticoagulation and (2) recurrent venous thrombosis despite intensive anticoagulation. Ref Davidson edition23rd pg 620