Coagulation is the body’s natural process of stopping bleeding through clot formation. Drugs can either promote clotting (coagulants) or prevent clot formation (anticoagulants). These agents are widely used in surgery, cardiovascular diseases, and bleeding disorders.
Blood Coagulation – Overview
When blood vessels are injured, platelets and clotting factors work together to form a stable clot. This process involves:
- Intrinsic pathway
- Extrinsic pathway
- Common pathway (formation of fibrin)
COAGULANTS
Coagulants are drugs that help increase clot formation. They are used in conditions where bleeding risk is high or clotting factors are deficient.
1. Vitamin K and Analogues
Vitamin K is essential for the synthesis of clotting factors II, VII, IX, X.
- Phytomenadione (Vitamin K₁) – used in newborn bleeding and warfarin overdose.
- Menadione (Vitamin K₃) – synthetic, water-soluble form.
2. Fibrinogen and Fibrin Foam
Promote clot formation when applied directly to wounds.
3. Thrombin
Converts fibrinogen to fibrin; applied topically to control bleeding in surgeries.
ANTICOAGULANTS
Anticoagulants prevent clot formation or stop existing clots from growing. They are used in:
- Deep vein thrombosis (DVT)
- Pulmonary embolism
- Heart valve replacement
- Atrial fibrillation
Classification of Anticoagulants
1. Parenteral Anticoagulants
a) Heparin
Heparin is a natural anticoagulant stored in mast cells.
Mechanism: Enhances the action of antithrombin III, which inactivates clotting factors IIa (thrombin), Xa, IXa, XIa.
Uses: DVT, pulmonary embolism, MI, extracorporeal circulation.
Side Effects: Bleeding, thrombocytopenia.
b) Low Molecular Weight Heparins (LMWH)
- Enoxaparin
- Dalteparin
- Tinzaparin
Preferentially inhibit factor Xa, have longer duration, and lower bleeding risk.
c) Hirudin Analogs
- Lepirudin
Direct thrombin inhibitors; useful in heparin-induced thrombocytopenia.
2. Oral Anticoagulants
a) Warfarin and Coumarin Derivatives
Warfarin blocks synthesis of Vitamin K–dependent clotting factors.
Mechanism of Action
- Inhibits Vitamin K epoxide reductase
- Prevents formation of factors II, VII, IX, X
Uses
- Long-term anticoagulation
- Atrial fibrillation
- Prosthetic heart valves
Side Effects
- Bleeding
- Skin necrosis (rare)
Drug Interactions
- Increased bleeding with aspirin, sulfonamides
- Reduced effect with barbiturates, rifampicin
Anticoagulant Antagonists
1. Protamine Sulphate
Neutralizes heparin by forming a stable inactive complex.
2. Vitamin K
Reverses the action of warfarin; restores clotting factor synthesis.
Tests Used to Monitor Therapy
- APTT → monitors heparin
- PT/INR → monitors warfarin
Fibrinolytics (Brief Note)
Although not full anticoagulants, these drugs dissolve existing clots.
- Streptokinase
- Urokinase
- tPA (alteplase)
Detailed Notes:
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