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Anticoagulants (Heparin (Drugs:
Unfractionated Heparin
Low-Molecular…
Anticoagulants
Heparin
Drugs:
Unfractionated Heparin
Low-Molecular-Weight Heparin (Dalteparin, Enoxaparin)
Mechanism of action:
- Increases the effect of antithrombin III which inactivates certain factors
- Unfractionated heparin increases the effect on factors IIa, IXa, Xa, XIa, XIIa
- LMWH increases the effect on factor Xa only
Clinical effects:
- Prolonged coagulation time
Clinical use:
- Prevention of venous thrombo-embolism
- Treatment of acute coronary syndrome
Adverse effects:
- Bleeding
- Heparin-induced thrombocytopenia & thrombosis (HITT) due to antibody-mediated activation of platelets via heparin-antibody complexes
Reversal:
- Unfractionated heparin is reversible with protamine sulphate
Half-lives:
Unfractionated heparin - 60 minutes
LMWH - 6 hours
Therefore LMWH can be given less frequently but is more expensive
Warfarin
Mechanism of action:
- Inhibits vitamin-K reductase, therefore inhibiting the production of vitamin-K dependent clotting factors (II, VII, IX, X)
- This process is competitive with vitamin K
Clinical effect:
- Prolonged coagulation time
Onset of action:
- Does not affect already formed clotting factors and therefore the effects manifest as the pre-formed factors are cleared from the body
- Takes 2-3 days for the anticoagulant effects to take place
- May cause hypercoagulability for the initial 1-2 days as warfarin inhibits proteins C and S which are vitamin-K dependent anticoagulants
Clinical use:
- Prevention & treatment of venous thrombo-embolism
- Prevention of stroke in patients post-MI & increased embolic risk
- Prevention of thrombus formation in AF
Monitoring:
- Aim for an INR of 2-3
- A healthy person not on warfarin should have an INR of 1.0
Drug variability:
Many factors can vary the effect of warfarin including:
- Drugs that inhibit or induce CYP450 enzymes that metabolise warfarin
- Vitamin-K levels through diet, antibiotics (can reduce vitamin-K via bacteria), other drug preparations containing vitamin-K
- Liver disease can reduce warfarin metabolism or production of clotting factors thereby increasing or decreasing the effects of warfarin
Reversal:
- Reversible via vitamin-K infusion
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Factor Xa inhibitors
Mechanism of action:
- Selectively inhibits factor Xa, thereby inhibiting thrombin formation
- Direct factor Xa inhibitors directly inhibit factor Xa
- Indirect factor Xa inhibitors inhibit factor Xa via antithrombin III
Drugs:
Direct factor Xa inhibitors: Apixaban (oral), Rivaroxaban (oral)
Indirect factor Xa inhibitors: Fondaparinux
Clinical effects:
- Prolonged coagulation time
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