Warfarin Flashcards
What is indication?
- VTE treatment and prevention
- AF (first line option for AF associated with mechanical heart valves, biprosthetic valves and rheumatic mitral stenosis).
- Prevent arterial embolism from mechanical heart valves.
What is the mechanism of action?
-Produces clotting factors II, VII, IX and X requires vitamin K in its reduced form to act as co-factor.
- Oxidised vitamin K is generated then recycled to its reduced form by enzyme Vet K epoxide reductase which is inhibited by warfarin.
-Reduces production of Vitamin K dependent clotting factors (Protein C and S) which over several days, produces an anticoagulant effect
What are important side effects?
Bleeding main adverse effect
Severe over-warfarinisationcan trigger warfarinisation can trigger spontaneous bleeding e.g
bleeding from nose
bleeding in areas of GI tract
What treatment is used to reverse the effects of warfarin/
Phytomenadione (Vit K1
Or
Dried prothrombin complex
Who would you prescribe with caution/avoid >
Contraindicated in active bleeding
liver disease impairs both metabolism and clotting factor synthesis.
- First trimester pregnancy (teratogenicity could cause cardiac and crania abnormalities)
-Near term due to risk of permpartum haemorrhage. (Heparins are preferred)
What are important interactions?
Warfarin has a low therapeutic index
The conc needed to prevent clotting is also close to the concentration needed to cause bleeding
Small changes in warfarin metabolism by CYP inhibitors can cause clinically significant changes in anticoagulation.
CYP inducers increase warfarin metabolism and risk of clots
CYP inhibitors decrease warfarin metabolism and increase risk of bleeding
What are dosages for initial administration of warfarin?
Loading dose:10mg orally daily often in day 1-3
Or
5mg daily in older people and those with low body weight or higher bleeding risk.
What are dosages for subsequent doses and how are they calculated?
Doses are guided by INR.
5mg daily initially
‘3-9mg at same time each day’
Why may you want to prescribe a heparin concurrently when initiating warfarin?
heparin may be started concurrently then stopped once the target INR is reached.
Because it takes several days for anticoagulation to be reached.
How long is duration of treatment for each indication?
isolated calf DVT= 6 weeks
Provoked VTE (COC, pregnancy, leg plaster) = 3 months
Unprovoked VTE(AF) =
At least 3 months/long term
What is target INR?
2.5=VTE
3.5=Recurrent VTE
What are patient counselling points
Carry yellow book and anticoagulant card
stop and seek immediate medical attention. If any signs of bleeding e.g nose bleeds or blood in urine.
Report any painful skin rash. consider stopping if calciphylaxis is diagnosed.
What would you do if the INR goes above ideal range but NO bleeding is detected?
INR 5.0-8.0
withhold 1-2 dose
reduce maintenance dose
Measure INR after 2-3 days
INR>8.0
Omit warfarin
oral phytomenadione
Repeat if INR still high after 24 hours
Restart warfarin when > 5.0 INR
What would you do if the INR goes above ideal range but bleeding is detected?
-Omit warfarin
-IV phytomenadione
-Repeat if INR still high after 24 hours
-Restart warfarin when > 5.0 INR
How would you stop warfarin before different surgery types?
Elective:
5 days before
give oral phytomenadione one day if INR >1.5
Restart warfarin next day.
Emergency
Delay 6-12 hours
or No delay give phyotmenadione and dried prothrombin complex
