Warfarin blocks vitamin K which is used to make clotting factors 2-8 and hence takes time to work until the circulating factors are depleted -about 3 days .
It prolongs PT and PTT . Vitamin K injections and plasma transfusions reverse its effects.
Warfarin is bound to plasma proteins and hence those drugs that affect protein production eg digoxin interfere with its effects . Its broken down by the liver .
It stops clots from forming but does nothing to those already formed and does not stop platelets from aggregating .
Warfarin is measured by INR a modified form of Prothrombin Time and can be given orally in 1-3 mg tablets or via a peripheral vein slowly over 2 mins - 5mg vials after mixing with water .
Initiation is with a dose of 2-5mg oral once daily , or iv or rarely 10mg for 2-3 days and then INR . Heparin is started with the first dose and then discontinued once INR is satisfactory.
INR 2-3 , oral dose 2-5mg once daily for 6w to 6m
INR 2-3 plus aspirin for 12 weeks
For prophylxis after MI
INR 3-4.5 or aspirin alone
Mitral valve disease , Left ventricular failure , cardiomyopathy , atrial fibrillation
INR 3 or 2-4 . Danger if it goes above 5
Mostly the local hospital will set the guideline and the aim should be to keep with 0.5 of it . For TIAs aspirin alone is preferable
INR is done daily and then at longer intervals till 12weekly.
For a major bleed warfarin is stopped and 5mg vit K is given slowly over 2 mins and also fresh frozen plasma or prothrombin concentrate.
For INR > 8 and minor bleeding , stop warfarin , give oral vitK 5mg
For INR 6-8 and minor or no bleeding , stop warfarin and recheck INR daily and restart when INR is less than 5
INR more than .5 above , stop warfarin and restart when less than 5
INR normal and bleeding , check for cancer
Has a rapid onset and offset of action and so is given subcutaneous or as slow iv infusion. 10,000 units loaded initially and then 15-25u/kg/hr according to PTT levels . It is used for 3 days till warfarin starts to take effect . It is used in arterial blockages , dvt , pumlonary embolism , unstable angina and for prophylaxis in surgery (5k 2hrs pre surgery and then s/c twice daily for seven days or till patient can walk ) .
If bleeding occurs it is withdrawn and sometimes protamine sulphate can be given . If used longer than five days platelet counts need to be done . Low molecular wt heparins have longer duration of action and can be used once daily .
Aspirin stops platelets from aggregating and is used in doses from 75mg to 300mg daily for prophylaxis after transient ischaemic attacks , heart attacks and surgery . It is not used in children under 12 because of the very rare Reye's syndrome . It can cause bleeding from the stomach .