Anticoagulants: Understanding Their Uses and Different Types

Anticoagulants are medications that prevent blood from clotting. These can breakdown the already present clots or prevent new clots from forming.

Uses of Anticoagulants

• Deep Vein Thrombosis: The reduced blood flow and movement of the legs (Immobility) causes fibrin deposition and forms fibrin thrombus in the legs leading to deep vein thrombosis. The anticoagulants have a clinical role in the treatment of deep vein thrombosis.
• Pulmonary Embolization: An embolus can cause blockage in the blood vessels of the lungs causing this condition. Anticoagulants can dissolve this embolus.
• ICU patients or patients after knee or hip surgery or immobile patients are at risk of development of thrombosis. So in these patients, anticoagulants can be prescribed prophylactically to avoid any thrombus formation.
• Transient ischemic attack or ischemic stroke patients are treated by anticoagulant drugs.
• Disseminated Intravascular Coagulation: In patients of DIC they have over-activation of the clotting pathway that leads to disseminated intravascular coagulation. DIC can occur due to infection, sepsis, amniotic fluid embolism, cancer, catheter-induced thrombosis, and mechanical artificial heart valves.

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There are two types of anticoagulants. These are divided on the basis of their route.

1. Oral Anticoagulants:
   • Vitamin K antagonists
   • Direct factor Xa inhibitors
   • Direct thrombin inhibitors
2. Parenteral anticoagulants:
   • Indirect thrombin inhibitors
   • Direct thrombin inhibitors

1. Oral Anticoagulants
   • Vitamin K antagonists
   • These are also called coumarins. The drugs included in the vitamin K antagonist category are warfarin and dicumarol.
   • These act by reducing clotting factors II, VII, IX, X, Protein C, and Protein S. Vitamin K has a crucial role in the synthesis of these clotting factors therefore the antagonists act by opposing the vitamin K.
   • There is a high risk of bleeding with warfarin therefore it is very important to regularly monitor the prothrombin time and INR.
   • Warfarin is a drug of choice for atrial fibrillation, severe mitral stenosis, and mechanical heart valves.
   • Important points about Warfarin:
     • The synthesis of clotting factors occurs in the liver and it requires vitamin K for the synthesis.
     • Warfarin inhibits the vitamin K epoxide reductase and hence inhibits the activation of vitamin K.
     • It is a competitive inhibitor of the epoxide reductase enzyme in the liver. 
     • Vitamin K along with the synthesis of clotting factors, it also produces anti-clotting factors like protein C and protein S.
     • Factor VII is the first one to be reduced by warfarin and it occurs within six hours but the protein C which is an anti-clotting factor is reduced by warfarin within 8 hours.
     • Warfarin cannot be used in emergencies because the complete anticoagulant effect of warfarin occurs after 5 days. Warfarin requires heparin injection in the initial 5 days and it is known as heparin bridge therapy.
     • There is a major side effect of warfarin that occurs in the initial 5 days. Warfarin increases coagulation due to a deficiency of protein C and protein S and it is known as Dermo-vascular necrosis.
     • Warfarin has a narrow therapeutic index therefore it is known to have a low safety margin.
     • It does not require therapeutic drug monitoring.
     • Monitoring is accomplished by prothrombin time (PT) and an international normalized ratio (INR).
     • It is very important to notice any kind of drug interaction in warfarin use.
     • Warfarin toxicity is managed by regularly checking the international normalized ratio and the antidote is vitamin K in case the patient is asymptomatic.
     • If the patient is symptomatic with symptoms like gum bleeding, hematuria, and intracranial hemorrhage then fresh frozen plasma (FFP) is the treatment of choice.
2. Direct factor Xa inhibitors
   • It includes apixaban, rivaroxaban, edoxaban, and Betrixaban.
   • It inhibits the clotting factor Xa.
   • It has a low risk of bleeding and hence no monitoring is required.
   • These are called novel oral anticoagulants and these are preferred over warfarin in DVT, pulmonary embolism, transient ischemic attack, and stroke.
   • The antidote of factor Xa inhibitor is Andexanet alpha which is a recombinant form of factor Xa.
3. Direct Thrombin inhibitors
   • The drug included in this category is Dabigatran.
   • It inhibits clotting factor II.
   • There is a lower risk of bleeding and hence no monitoring is required.
   • Antidote is Idarucizumab.
   • It is preferred over warfarin in conditions like DVT and pulmonary embolism

2. Parenteral Anticoagulants

1. Indirect thrombin inhibitors:

• Heparin is the most important drug that comes under this class.
• Mechanism of action- In the blood vessel, the heparin binds the antithrombin III protein. This changes the structure of antithrombin III. The conformational changes inhibit factor X and factor II.
• There are three types of Heparins:
  • High molecular weight heparin
  • Low molecular weight heparin
  • Ultra-low molecular weight heparin
• High molecular weight heparin
  • It is the safest in renal failure patients.
  • It is also called unfractionated heparin
  • It has the shortest half-life
  • The route of administration is intravenous.
  • It is given in emergency or acute illness conditions like DIC.
• Low molecular weight heparin
  • It consists of enoxaparin, dalteparin, and Tinzaparin.
  • The other name for low molecular weight heparin is fractionated heparin.
  • It is unsafe in renal failure.
  • The route of administration is subcutaneous.
  • It is given prophylactically in DVT and pulmonary embolism patients.
• Ultra-low molecular weight heparin
  • It consists of fondaparinux and Idraparinux.
  • It is also called synthetic heparin.
  • It is unsafe in renal failure patients and it has the longest half-life.
  • The route of administration is subcutaneous.
  • It is given for prophylaxis of DVT and pulmonary embolism.
  • Protamine sulfate is the antidote for heparin.
• Adverse effects of Heparin
  • Alopecia
  • Hyperkalemia
  • Osteoporosis
  • Bleeding
  • Heparin induced thrombocytopenia
  • It reduces blood lipid and triglyceride levels.

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