Warfarin is an oral anticoagulant drug that is used to prevent blood clots. It works by inhibiting vitamin K, which is necessary for blood clotting. Warfarin is indicated to prevent blood clots associated with conditions like atrial fibrillation, heart attacks, and strokes. It is administered orally and dosage is adjusted based on prothrombin time and INR levels to maintain an appropriate level of anticoagulation. Adverse effects include bleeding and interactions can occur with foods and other drugs that affect vitamin K levels or clotting. Vitamin K can reverse the effects of an overdose.
2. What is Warfarin?
Warfarin is an oral coumarin anticoagulant (blood
thinner) widely used to control and prevent
formation of blood clot/thromboembolic disorders
(heart attacks, strokes, and blood clots in veins and
arteries.
3. Mechanism of Action
Warfarin acts by antagonizing the antihemorrhagic
effect of vitamin K.
It inhibits hepatic synthesis of vitamin K dependent
coagulation factors II, VII, IX, and X by inhibiting
vitamin K1 -2,3 epoxide reductase, preventing
vitamin K from being reduced to its active form.
4. Indications
1) prophylaxis and/or treatment of venous thrombosis
and its extension, and pulmonary embolism.
2) prophylaxis and/or treatment of the
thromboembolic complications associated with
atrial fibrillation and/or cardiac valve replacement.
3) to reduce the risk of death, recurrent myocardial
infarction, and thromboembolic events such as
stroke or systemic embolization after myocardial
infarction.
5. Dosage and Administration
Adults: PO 2-5 mg/day initially; adjust daily dose
according to PT or INR determinations. Usual
maintenance dose is 2-10 mg/day.
The IV dosages would be the same as those would
be used orally. Administer as a slow bolus injection
over 1 to 2 min in a peripheral vein.
6. Warfarin Monitoring
Prothrombin time (PT) — The most commonly used
test to measure the effect of warfarin. It measures
the time it takes for the clotting mechanism to
progress. Normal range (12–15 seconds).
International Normalized Ratio (INR) — The INR is a way
of expressing the PT in a standardized way; this ensures that results obtained
by different laboratories can be reliably compared.
• The longer it takes the blood to clot, the higher the
PT and INR. In most cases the target INR range
will be 2 to 3, although other ranges may be chosen
if there are special circumstances.
9. Drug Interactions
Divided into:
1. Pharmacokinetic mechanisms:
1) enzyme induction 2) enzyme inhibition
3) reduced plasma protein binding.
2. Pharmacodynamic mechanisms:
1) synergism 2) competitive antagonism (vit K)
3) an altered physiologic control for vitamin K
(hereditary resistance to oral anticoagulants).
10.
11. Overdose Management
Without bleeding: cessation of the drug may be
enough.
With minor bleeding: by stopping the drug and
administering vitamin K1 (Phytonadione) 5-20 mg
PO or 10 mg IV.
In emergency situations of severe hemorrhage
1. activated charcoal 1 g/kg PO.
2. vitamin K1 5-20 mg PO or 10 mg IV.
3. administering 200-500 mL of fresh whole blood or
15 mL/kg fresh frozen plasma.
12. *Vitamin K reverses the signs of
warfarin
•Foods with High Vitamin K
•- dark, green leafy vegetables (spinach,
broccoli, Lettuce, cabbage)
•-Fruits: Kiwi, Avocado, Dried prunes,
blueberries, blackberries
•-Green Tea
•** Cranberry juice and alcohol increase
effects of Warfarin
