Warfarin Flashcards
What are indications for the use of warfarin?
- DVT
- PE
- AF
- Valve replacement
Is warfarin used to treat arterial thrombosis?
No - antiplatelet agents such as aspirin and clopidogrel are used for this
What is the mechanism of action of warfarin?
Inhibits hepatic production of vitamin K-dependent coagulation factors and cofactors. Vitamin K must be in its reduced form for synthesis of coagulation factors. It is then oxidised during the synthetic process.
An enzyme called vitamin K epoxide reductase reactivates oxidised vitamin K. Warfarin inhibits vitamin K epoxide reductase, preventing reactivation of vitamin K and coagulation factor synthesis
What are the main adverse reactions that can occur with warfarin use?
Bleeding
When is warfarin contra-indicated or use?
- Severe bleeding risk
- Pregnancy - First trimester
Why should warfarin not be used in the first trimester of pregnancy?
Can cause foetal growth abnormalities
What are important interactions that you should be aware of when giving someone warfarin?
Cytochrome P450 inhibitors - decrease metabolism, increasing bleeding
- Macrolides
- Fluclonazole
- Protease inhibitors
Cytochrome P450 inducers - increase metabolism, decreasing bleeding
- Phenytoin
- Rifampicin
- Carbemazepine
Antibiotics - kill gut flora which synthesise vitamin K
What should you consider doing when starting someone on warfarin?
Start a LMWH in tandem to cover increase in clotting risk
How long should LMWH be administered for when commencing someone on warfarin therapy?
5 days + adequate INR is reached for 24 hours (i.e. 2 INR’s 24 hours apart)
What does warfarin increase clotting risk initially?
Warfarin inhibits the natural anticoagulant activity of proteins C and S, and it does this before inhibiting the other clotting factors
What should you do before commencing someone on warfarin therapy?
- Obtain pre-treatment INR
- Determine and record target INR
- Record therapeutic indication
What dose of warfarin would you commence someone on?
2mg daily
When after commencing warfarin therapy would you check INR?
Day 3 and 7
If INR on warfarin therapy was between 3.0-4.0 7 days after initiation of treatment, what would you do?
Reduce from 2 mg to 1.5 mg, and check INR after 3 days
IF INR on warfarin therapy is > 4.0 after intiation of treatment, what might you consider doing?
