Notes:
The blood coagulation process can
be activated by one of two pathways, the tissue Factor pathway
(formerly known as the extrinsic pathway) and the contact
activation pathway (known as the intrinsic pathway).
Tissue Factor binds to and activates
Factor VII and the Tissue Factor/VIIa complex then activates
Factor X and Factor IX to Xa and Ixa respectively. Factor
X can also be converted to Xa by Ixa (in the presence of Factor
VIII)
The intrinsic pathway is activated
when Factor XII comes in contact with a foreign surface. The
resulting Factor XIIa then activates Factor XI, which in turn
activates Factor IX. Factor Ixa then activates Factor X.
Thus Factor Xa can be generated
by activation of the tissue factor or contact activation pathways.
Factor Xa then cleves prothrombin and the resulting thrombin
converts fibrinogen to fibrin.Four of these clotting factors
(Factors IX, VII, X and prothrombin) are Vitamin K dependent
and therefore their activity is decreased by the Vitamin K
antagonist, warfarin. The half-lives of these four Vitamin
K dependent clotting factors are shown on this slide.
Factor VII has the shortest half
life of the Vitamin K dependent coagulation factors. However,
for adequate anticoagulation one needs to reduce the other
coagulation factors appropriately, including Factor II (prothrombin)
which has a 60 hour half life. It takes several days after
initiation of warfarin therapy to reduce Factor II and thus
warfarin and heparin need to overlap for approximately 4–5
days when starting therapy.
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Stroke Education
