Why can't warfarin and dicoumarol prevent

[MudPhud20XX]

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coagulation of blood in vitro, but can prevent coagulation in vivo?

Also, Kaplan says that when warfarin is given, heparin is given to prevent thrombosis. If so, why does warfarin increase a chance of having thrombosis? I mean isn't warfarin anticoagulant that would prevent thrombosis?

Many thanks in advance.

 

[notbobtrustme]

Warfarin doesn't affect the coagulation factors directly, they just inhibit synthesis of them in the liver; remember that warfarin inhibits gamma-carboxylation of I, IX, VII, II (X?) and proteins C and S. So having warfarin in a test tube won't do anything because that's not the level warfarin acts at.

The reason you give heparin at the same time as warfarin is because the half-lives of the various factors are different. Proteins C and S, which terminate the coagulation pathway, have the shortest halflife and thus will be depleted before the pro-coagulation factors will be. With that in mind, with less C/S around, the patient is in a hypercoaguable state, necessitating heparin administration simultaneously until the other factors decay to the point where the patient's blood has been "thinned".

 

[Transposony]

It's like driving a car with breaks disabled (Protein C&S) which won't stop till the gas (factors II,VII, IX, and X) runs out !😉

P.S. The way I remember vitamin K-dependent factors is: 2+7=9 and 10 comes after 9.
Hope this helps someone 🙂