Warfarin Flashcards
1
Q
What is warfarin?
A
- Warfarin is an oral anticoagulant which inhibits epoxide reductase preventing the reduction of vitamin K to its active hydroquinone form, which in turn acts as a cofactor in the carboxylation of clotting factor II, VII, IX and X and protein C.
2
Q
What are the indications for warfarin?
A
- Venous thromboembolism: target INR = 2.5, if recurrent 3.5
- Atrial fibrillation, target INR = 2.5
- Mechanical heart valves, target INR depends on the valve type and location.
- Mitral valves generally require a higher INR than aortic valves
3
Q
What factors may potentiate warfarin?
A
- Liver disease
- P450 enzyme inhibitors, e.g.: amiodarone, ciprofloxacin
- Cranberry juice
- Drugs which displace warfarin from plasma albumin, e.g. NSAIDs
- Inhibit platelet function: NSAIDs
4
Q
What are the side effects of warfarin?
A
- Haemorrhage
- Teratogenic, although can be used in breastfeeding mothers
- Purple toes
- Skin necrosis:
- when warfarin is first started biosynthesis of protein C is reduced
- results in a temporary procoagulant state after initially starting warfarin, normally avoided by concurrent heparin administratio
- Thrombosis may occur in venules leading to skin necrosis
5
Q
Name inducers of the P450 system - INR will decrease
A
- Antiepileptics: phenytoin, carbamazepine
- Barbiturates: phenobarbitone
- Rifampicin
- St John’s Wort
- Chronic alcohol intake
- Griseofulvin
- Smoking (affects CYP1A2, reason why smokers require more aminophylline)
6
Q
Name inhibitors of the P450 system - INR will increase
A
- antibiotics: ciprofloxacin, clarithromycine/erythromycin
- isoniazid
- cimetidine,omeprazole
- amiodarone
- allopurinol
- imidazoles: ketoconazole, fluconazole
- SSRIs: fluoxetine, sertraline
- ritonavir
- sodium valproate
- acute alcohol intake
- quinupristin
7
Q
What do you when there is major bleeding?
A
- Stop warfarin
- Give intravenous vitamin K 5mg
- Prothrombin complex concentrate - if not available then FFP*
*as FFP can take time to defrost prothrombin complex concentrate should be considered in cases of intracranial haemorrhage
8
Q
What do you do if INR > 8 + Minor bleeding ?
A
- Stop warfarin
- Give intravenous vitamin K 1-3mg
- Repeat dose of vitamin K if INR still too high after 24 hours
- Restart warfarin when INR < 5.0
9
Q
What do you if INR > + No bleeding?
A
- Stop warfarin
- Give vitamin K 1-5mg by mouth, using the intravenous preparation orally
- Repeat dose of vitamin K if INR still too high after 24 hours
- Restart when INR < 5.0
10
Q
What do you do if INR 5-8 and minor bleeding?
A
- Stop warfarin
- Give intravenous vitamin K 1-3mg
- Restart when INR < 5.0
11
Q
What do you do if INR 5-8 and there is NO bleeding?
A
- Withhold 1 or 2 doses of warfarin
- Reduce subsequent maintenance dose
