A 62-year-old woman is admitted to the medical intensive care unit following acute-onset dyspnea and tachycardia. A computed tomography pulmonary angiogram confirms the presence of a massive pulmonary embolism in the right pulmonary artery. The critical care team immediately initiates a continuous intravenous infusion of unfractionated heparin. To ensure the dosage remains within the therapeutic window, the nursing staff routinely monitors the patient's activated partial thromboplastin time (aPTT). Which of the following best describes the primary mechanism by which this medication exerts its therapeutic effect?
- Enhances antithrombin activity to neutralize thrombin and factor XaAnswer
- BInhibits the hepatic synthesis of vitamin K-dependent clotting factors
- CDirectly and reversibly binds to the active site of thrombin
- DSelectively inhibits factor Xa without requiring antithrombin
- EPromotes the conversion of plasminogen to plasmin to degrade fibrin
Answer
Enhances antithrombin activity to neutralize thrombin and factor Xa
Unfractionated heparin functions as an indirect anticoagulant. It binds to antithrombin (previously known as antithrombin III), inducing a conformational change that increases the activity of antithrombin by approximately 1,000-fold. The activated antithrombin complex then rapidly and efficiently inactivates several key coagulation factors, primarily thrombin (factor IIa) and factor Xa. This disruption prevents the conversion of fibrinogen to fibrin, halting the propagation of the thrombus.
Step-by-Step Solution
Key Concept
Unfractionated heparin exerts its anticoagulant effect indirectly by binding to and activating antithrombin, which subsequently inhibits thrombin (factor IIa) and factor Xa.
Estimated Time:45s