Anticoagulation Therapy
A guide to anticoagulant pharmacology including heparins, warfarin, and direct oral anticoagulants, covering mechanisms, monitoring, and reversal.
## Overview
Anticoagulants prevent pathological clot formation by inhibiting specific coagulation factors. They are essential for treating venous thromboembolism (VTE), preventing stroke in atrial fibrillation, and managing mechanical heart valves. The major challenge is balancing thrombosis prevention against bleeding risk.
## Unfractionated Heparin
UFH binds antithrombin III, accelerating its inhibition of thrombin (factor IIa) and factor Xa by approximately 1,000-fold. Administered IV or subcutaneously, it has a short half-life (1-2 hours IV), making it ideal for situations requiring rapid anticoagulation with easy reversibility. Monitored by aPTT. Reversed by protamine sulfate. Heparin-induced thrombocytopenia (HIT) is a serious immune-mediated complication occurring in 1-5% of patients.
## Low-Molecular-Weight Heparins
Enoxaparin, dalteparin, and tinzaparin preferentially inhibit factor Xa over thrombin due to shorter polysaccharide chains. They offer more predictable pharmacokinetics, allowing weight-based subcutaneous dosing without routine monitoring. Anti-Xa levels are checked in renal impairment, obesity, and pregnancy. Protamine provides only partial reversal (approximately 60%).
## Warfarin
Warfarin inhibits vitamin K epoxide reductase (VKORC1), preventing the gamma-carboxylation of factors II, VII, IX, and X. Full anticoagulant effect requires 5-7 days as existing functional factors are cleared. Monitored by INR with typical targets of 2.0-3.0 (2.5-3.5 for mechanical mitral valves). Warfarin has extensive drug and food interactions via CYP2C9, CYP3A4, and vitamin K intake. Reversed by vitamin K, fresh frozen plasma, or four-factor PCC.
## Direct Oral Anticoagulants
DOACs target single coagulation factors with predictable pharmacokinetics, eliminating routine monitoring.
- **Direct thrombin inhibitor**: Dabigatran binds thrombin's active site. Reversed by idarucizumab.
- **Factor Xa inhibitors**: Rivaroxaban, apixaban, edoxaban block factor Xa. Reversed by andexanet alfa.
DOACs are preferred over warfarin for non-valvular AF and VTE in most patients. They are contraindicated with mechanical heart valves and antiphospholipid syndrome.
## Selecting an Anticoagulant
Choice depends on indication, renal function, drug interactions, cost, and patient preference. Warfarin remains necessary for mechanical valves. DOACs are preferred for AF and VTE when no contraindications exist. Heparins are used for acute inpatient anticoagulation and bridging.
## Key Takeaways
- UFH and LMWH enhance antithrombin activity; DOACs directly inhibit single factors
- Warfarin requires INR monitoring and has extensive interactions
- DOACs offer predictable dosing without routine monitoring
- Specific reversal agents exist for heparin (protamine), warfarin (vitamin K/PCC), and DOACs