Part 6: Warfarin: Mechanism, Uses, and Monitoring

Overview

Lecture covers anticoagulants with focus on warfarin for prevention and treatment of thromboembolic disorders, its mechanism, onset, monitoring, and INR targets.

Thromboembolic Disorders

• Thromboembolism: thrombus forms; embolus is mobile clot blocking distant artery.
• Common embolus sites: coronary arteries, cerebral arteries, pulmonary arteries.
• Conditions producing thrombi: atrial fibrillation, deep vein thrombosis, pulmonary embolism.

Warfarin: Mechanism of Action

• Inhibits vitamin K epoxide reductase, an enzyme for activating clotting factors.
• Affects vitamin K–dependent clotting factors: II, VII, IX, X; factor II also called prothrombin.
• Also affects proteins C and S.
• Prevents activation of clotting factors, slowing coagulation cascade and reducing clot formation.

Therapeutic Effects and Risks

• Benefits: reduces risk of stroke, embolism, and other clot-related complications.
• Risks: significant bleeding due to reduced clot formation, even after minor injuries.

Onset, Duration, and Bridging

• Warfarin has delayed onset; takes several days for full anticoagulant effects.
• Only affects newly synthesized clotting factors; existing factors persist until degraded.
• Patient variability in time to effect; no universal formula for onset.
• Immediate anticoagulation needs bridging therapy with heparin or low molecular weight heparin.

Monitoring and INR

• Regular INR monitoring is essential due to narrow therapeutic range and bleeding risk.
• INR: International Normalized Ratio; unitless ratio standardizing anticoagulation measurement across labs.
• Normal INR (no anticoagulation): under 1.

Therapeutic INR Ranges

• Typical therapeutic INR: 2.0–3.0 for most indications.
• Mechanical heart valve: higher target 2.5–3.5.
• Some high-resistance patients may require even higher ranges.

Key Terms & Definitions

• Thrombus: stationary blood clot formed in a vessel.
• Embolus: clot that becomes mobile and occludes another artery.
• Anticoagulant: medication reducing blood’s ability to clot.
• Warfarin: oral anticoagulant inhibiting vitamin K–dependent clotting factor activation.
• Vitamin K epoxide reductase: enzyme blocked by warfarin; required to activate factors II, VII, IX, X.
• Prothrombin (Factor II): vitamin K–dependent clotting factor; key in coagulation cascade.
• Proteins C and S: vitamin K–dependent proteins influenced by warfarin.
• INR (International Normalized Ratio): standardized, unitless measure of anticoagulation intensity.
• Bridging therapy: temporary anticoagulation (heparin or LMWH) until warfarin becomes therapeutic.

INR Targets Table

Action Items / Next Steps

• Monitor INR regularly to maintain therapeutic range and minimize bleeding risk.
• Educate patients on safe administration and bleeding precautions while on warfarin.
• Use heparin or low molecular weight heparin bridging when immediate anticoagulation is required.