A 62-year-old woman with a history of subarachnoid hemorrhage is brought to the emergency department because of shortness of breath and sharp chest pain that worsens on inspiration. She underwent surgery for a hip fracture 3 weeks ago. Her pulse is 110/min, respirations are 20/min, and blood pressure is 112/74 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 92%. The lungs are clear to auscultation and there is no jugular venous distention. A ventilation and perfusion scan shows a small perfusion defect in the left lower lung. A drug with which of the following mechanisms of action is most appropriate for this patient?

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Eductional Objective: Antithrombin III is a non-vitamin K-dependent protease that cleaves factors IIa, IXa, Xa, thereby inhibiting the coagulation cascade. The activity of this enzyme is potentiated by heparin, which is indicated as initial anticoagulation therapy in patients with pulmonary embolism, thrombosis, unstable angina pectoris, and disseminated intravascular coagulation. Two forms of heparin in clinical use are low molecular weight heparin (LMWH) and unfractionated heparin (UFH). LMWH is preferred as it binds selectively to antithrombin III and has lower risk of side effects.

A 62-Year-Old Woman with a History of Subarachnoid Hemorrhage Is