Warfarin Flashcards
What are the indications for warfarin?
Atrial fibrillation with high CHADS2 risk
Mechanical heart valves
Venous thromboembolism - usually 3 months but can be long term if recurrent
Antiphospholipid syndrome
Contraindications for warfarin
Haemorrhagic stroke
Uncorrected bleeding or bleeding disorder
Potential bleeding lesions - varices, active ulcer, 48h postpartum, recent trauma or biopsy
Pregnancy - teratogenic
Cautions:
Protein C deficiency - skin necrosis
Unreliable/uncooperative patient
Prone to falls
Side effects of warfarin
Bleeding gums Menorrhagia Bruising after injury/injections Haematuria, melaena, dizziness Skin necrosis esp in protein c deficiency
How is warfarin metabolised?
CYP450 system, none excreted unchanged so must all be inactivated in liver
Name drugs which increase the effect of warfarin
Antiplatelets, aspirin SSRIs and venlafaxine - antiplatelet effect Cranberry juice Azoles Antibiotics - erythro, clarithromycin Amiodarone Corticosteroids Fibrates Glucosamine Metronidazole NSAIDs - GI bleed Thyroxine
Name some drugs which decrease the effect of warfarin
TCAs St John's wort Carbemazapine Phenobarbital, phenytoin Rifampicin Vitamin K - from enteral feeds, diet or supplements
What else can affect warfarin metabolism?
Stopping smoking
What about warfarin in hepatic/renal impairment?
Hepatic- beware coagulopathy increased bleeding risk, and varices
Renal - no dose adjustments but be aware active metabolites may accumulate
What is the test used to monitor warfarin and how is it measured?
International normalised ratio - prothrombin time of patient divided by reference prothrombin time
What is a reasonable target INR
2.5- may need higher in prosthetic valves
What is a suitable loading dose of warfarin?
5mg for 2-3 days, then adjust dose using INR. Most patients are controlled with dose between 2-10mg.
How does warfarin work?
Inhibits VKORC1, stopping synthesis of vitamin K dependent clotting factors 2, 7, 9, 10 and proteins C, S and Z.
