Warfarin Flashcards
What are the main indications for warfarin? (2)
1) VTE: can be used as treatment or prophylaxis. Need heparin first to treat and then initiate warfarin with INR monitoring
2) Prevent embolism in AF or prosthetic heart valve
Mechanism of action?
> It inhibits hepatic production of vitamin-K dependent coagulation factors (II, VII, IX and X)
Does this by inhibiting vitamin K epoxide reductase (enzyme that restores vit K to its reduced form which is a co-factor in synthesis of these clotting factors)
What are the main adverse effects?
> BLEEDING
Can be from minor trauma or existing conditions e.g. peptic ulcers
In severe cases can trigger spontaneous bleeding (epistaxis or PR haem)
Can be reversed using phytomenadione
Warnings assoc with warfarin?
> Risk vs benefits should be considered before starting (fine line between thrombosis and haemorrhage)
C/I in pt w/ immediate haemorrhage risk e.g. surgery
Avoid in pt with liver disease- less able to metabolise drug
Avoid in pregnancy (teratogenic in 1st trimester and peripartum bleed risk later)
What are some important interactions?
> Warfarin has small therapeutic index so extreme care needed
CYP inducers (phenytoin, carbamazepine, rifampicin) increase warfarin metabolism
CYP inhibitors (fluconazole, macrolides) decrease warfarin metabolism
Points to consider when prescribing?
> Once daily dose
Dose 5-10mg on day 1 and then changed based on INR
If immediate anticoag needed then heparin also started (can be stopped when INR stable)
If one VTE episode–> warfarin 3-6 months
Recurrent VTE or AF= lifelong warfarin
When should it be taken?
> In evening (around 6pm) to give time to have effect on INR next morning
Also means if change made in clinic, can start straight away that evening
Counselling points?
> Importance of having consistent diet > Avoiding alcohol or at least binging > Explain INR test and importance > Avoid NSAIDs > Keep alert card with them everywhere and tell dentist/pharmacist they are on warfarin
