This is an oral product and it doesn’t work by activating Antithrombin III the same way Heparin does. It works on the clotting factors to stop the cascade. So it works in the earlier part of the cascade. It’s very specific to the Vitamin K dependent clotting factors in the liver. So does this dissolve a clot by itself? No! Just like the other anticoaulants, they will be on anticoagulant therapy for months to a year.
Drug Interaction Warning: When we talked about pharmacokinetics, we talked about protein binding and gave an example involving Warfarin. This drug is 99% protein bound to albumin and 1% is free to float around in the blood. If we accidentally introduce another drug such as a NSAID that kicks off the warfarin from the albumin, then more warfarin will be free. If we now have 5% free instead of 1%, that’s a 500% increase and they bleed. If they must be on that other drug, the dose needs to be lowered dramatically and this is where we run into problems with lots of patients.
Prothrombin Time: We must monitor their Prothrombin Time (International normalized ratio; INR): We are looking for an INR of 2-3. If the result is higher than 3, it means they will be prone to bleeding. If it’s lower than 2, then we must increase the dose.
Onset Time: The dilemma we have is that the effect of each dose is not seen for 3 days. This is because warfarin is stopping the production of clotting factors and there’s already clotting factors floating around, so it actually takes 3 days for this to happen. While heparin works immediately, warfarin takes 3 days to kick in. When a person is put on heparin, they are also given warfarin so that hopefully they are ready to go home by the 3rd day and just take the warfarin orally.
Maintain consistent vitamin K diet: The person must be careful with a Vitamin K diet. Green, leafy vegetables are rich in Vitamin K. Vegetarians have a diet that’s high in Vitamin K. If ones intake of Vitamin K is really high, they must be on a higher dose of warfarin. The problem though, is that the patient must be consistent with the diet because if they stop taking in the vitamin K foods, they may bleed.
Interaction with Antibiotic: The problem is also that Vitamin K is produced in the colon by bacteria. If you put a person on a broad spectrum antibiotic and it kills the bacteria, the person may bleed out because there is less Vitamin K.
Recap
- Decreases production of vitamin K dependent clotting factors in the liver
- High protein binding – leads to many drug interactions
- Monitor Prothrombin Time
- INR 2 to 3
- Effect of each dose is not seen for 3 days
- Maintain consistent vitamin K diet
