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Haemostasis, Coagulation Pathways & Anticoagulants

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STAGES OF HAEMOSTASIS

Stage 1 - Vascular Spasm (Vasoconstriction)

• The immediate response to vessel injury is reflex vasoconstriction of the damaged blood vessel.
• Mediated by local myogenic reflexes, nervous reflexes, and vasoactive amines (serotonin, thromboxane A2) released from activated platelets.
• Reduces blood flow to the injured site and buys time for platelet plug formation.

Stage 2 - Primary Haemostasis (Platelet Plug Formation)

1. Platelet Adhesion: Disruption of the vascular endothelium exposes subendothelial collagen and von Willebrand factor (vWF). Platelets adhere via glycoprotein GPIb (binds vWF) and GPIa/IIa / GPVI (binds collagen directly).
2. Platelet Activation: Adherent platelets become activated - they change shape (from disc to spiky spheres) and release granule contents:
   • Alpha (α) granules: fibrinogen, vWF, platelet factor 4, P-selectin, growth factors (PDGF, TGF-β, VEGF, EGF, FGF).
   • Dense (δ) granules: ADP, ATP, serotonin, Ca²⁺.
   • ADP and thromboxane A₂ recruit and activate further platelets (positive feedback).
3. Platelet Aggregation: Activated platelets expose GPIIb/IIIa receptors on their surface, which bind fibrinogen, cross-linking platelets into a platelet plug. This is the primary (temporary) haemostatic plug.

Stage 3 - Secondary Haemostasis (Coagulation Cascade)

• Converts fibrinogen to fibrin monomers.
• Activates factor XIII → factor XIIIa cross-links fibrin into a stable, insoluble meshwork.
• Stabilises and reinforces the platelet plug.
• Amplifies the process by further activating factors V, VIII, and XI.

Stage 4 - Fibrinolysis (Clot Resolution)

• Once healing begins, the clot must be dissolved.
• Tissue plasminogen activator (tPA) converts plasminogen → plasmin.
• Plasmin digests fibrin into fibrin degradation products (FDPs) including D-dimers.
• Regulated by α₂-antiplasmin and plasminogen activator inhibitor-1 (PAI-1) to prevent excessive clot lysis.

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THE COAGULATION CASCADE

Extrinsic Pathway (Tissue Factor Pathway)

• Trigger: Vascular injury exposing tissue factor (TF / thromboplastin), a subendothelial transmembrane glycoprotein expressed on fibroblasts and vascular smooth muscle cells.
• TF binds and activates circulating factor VII → TF:FVIIa complex (extrinsic tenase complex).
• TF:FVIIa activates factor X → Xa and factor IX → IXa.
• Measured by: Prothrombin Time (PT / INR).

Intrinsic Pathway (Contact Activation Pathway)

• Trigger: Blood contact with a negatively charged surface (e.g., exposed subendothelial collagen, artificial surfaces, glass in vitro).
• Factor XII (Hageman factor) is activated → XIIa by HMWK + collagen.
• XIIa activates prekallikrein ↔ kallikrein feedback (amplification), and activates factor XI → XIa.
• XIa activates factor IX → IXa.
• IXa + factor VIIIa + Ca²⁺ + phospholipid surface = intrinsic tenase complex → activates factor X → Xa.
• Measured by: Activated Partial Thromboplastin Time (aPTT).

Common Pathway

1. Xa + Va + Ca²⁺ + phospholipid = prothrombinase complex
2. Prothrombinase converts prothrombin (II) → thrombin (IIa)
3. Thrombin cleaves fibrinogen (I) → fibrin monomers (Ia)
4. Thrombin activates factor XIII → XIIIa
5. XIIIa cross-links fibrin monomers into a stable, cross-linked fibrin clot

Cell-Based Model of Coagulation (Modern View)

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KEY COAGULATION FACTORS TABLE

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ANTICOAGULANTS

Classification

Anticoagulants
├── Parenteral
│   ├── Unfractionated Heparin (UFH)
│   ├── Low-Molecular-Weight Heparins (LMWH)
│   ├── Fondaparinux
│   └── Direct Thrombin Inhibitors (DTI): Argatroban, Bivalirudin
└── Oral
    ├── Vitamin K Antagonist: Warfarin
    └── DOACs (Direct Oral Anticoagulants)
        ├── Factor Xa inhibitors: Rivaroxaban, Apixaban, Edoxaban
        └── Direct Thrombin Inhibitor: Dabigatran

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1. Unfractionated Heparin (UFH)

• Route: IV or SC injection.
• Mechanism: Binds antithrombin III (AT-III), causing a conformational change that accelerates AT-III inhibition of thrombin (IIa) and factor Xa by ~1000-fold. Also inhibits IXa, XIa, XIIa.
• Monitoring: aPTT (target 60-100 sec, or 1.5-2.5× normal).
• Reversal: Protamine sulfate (1 mg neutralises ~100 units heparin).
• Adverse effects:
  • Bleeding (most common).
  • Heparin-Induced Thrombocytopenia (HIT): immune-mediated, IgG antibodies against heparin-PF4 complex → paradoxical thrombosis. Stop heparin immediately; replace with argatroban.
  • Osteoporosis (long-term use).
• Advantages: Rapid onset, reversible, can be used in pregnancy (does not cross placenta).

2. Low-Molecular-Weight Heparins (LMWH) - Enoxaparin, Dalteparin

• Mechanism: Complex with AT-III → primarily inhibit factor Xa (anti-Xa activity); less effect on thrombin vs. UFH.
• Advantages over UFH: More predictable pharmacokinetics, SC administration, no routine monitoring needed, lower risk of HIT.
• Monitoring: Anti-Xa levels (in obesity, renal failure, pregnancy).
• Reversal: Protamine sulfate (incomplete - ~60-75% reversal of anti-Xa activity).
• Use in renal impairment: Use cautiously; accumulate in renal failure → prefer UFH.

3. Fondaparinux

• Mechanism: Synthetic pentasaccharide that selectively binds AT-III → selective factor Xa inhibition (no direct anti-IIa effect).
• 300-1000-fold potentiation of AT-III's Xa inhibition.
• No reversal agent (no protamine effect; andexanet alfa may be considered).
• No HIT risk (does not bind PF4).

4. Warfarin (Vitamin K Antagonist)

• Route: Oral.
• Mechanism: Inhibits vitamin K epoxide reductase (VKOR), preventing regeneration of reduced (active) Vitamin K. Without active Vit K, the hepatic γ-carboxylation of factors II, VII, IX, X, Protein C, Protein S is impaired → non-functional coagulation factors.
• Onset: Delayed - 3-5 days (circulating factors must be cleared; shortest half-life is factor VII, so PT prolongs first, then full effect takes 5+ days).
• Monitoring: INR (International Normalised Ratio). Therapeutic INR: 2.0-3.0 for most indications; 2.5-3.5 for mechanical heart valves.
• Reversal:
  • Mild/no bleeding: Withhold warfarin ± oral Vit K.
  • Serious bleeding: IV Vitamin K + 4-factor Prothrombin Complex Concentrate (4F-PCC) or Fresh Frozen Plasma (FFP).
  • Life-threatening: 4F-PCC + IV Vitamin K.
• Drug interactions: Extensive (inhibits/induces CYP2C9). Narrow therapeutic window.
• Contraindicated in pregnancy (crosses placenta - warfarin embryopathy in 1st trimester).
• Bridging: During initiation, overlap with heparin/LMWH for 5 days until INR therapeutic (because Protein C and S - anticoagulants - have short half-lives and may drop before coagulation factors, creating a transient hypercoagulable state).

5. Direct Thrombin Inhibitors (DTI) - Argatroban, Bivalirudin

• Mechanism: Directly and reversibly inhibit thrombin (IIa) - both free and clot-bound thrombin - without requiring AT-III.
• Argatroban: Derived from L-arginine; hepatically metabolised. Used in HIT.
• Bivalirudin: Hirudin analogue; renally excreted (t½ 25 min). Used in PCI in HIT patients.
• Monitoring: aPTT or ACT.

6. Direct Oral Anticoagulants (DOACs)

• Advantages: Fixed dosing, no routine monitoring, fewer food/drug interactions vs. warfarin, predictable pharmacokinetics.
• Disadvantages: No routine monitoring, renally cleared (use cautiously in CKD), contraindicated in pregnancy, expensive.

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NATURAL ANTICOAGULANT MECHANISMS

1. Antithrombin III (AT-III): Serine protease inhibitor - slowly inactivates thrombin, Xa, IXa, XIa, XIIa. Accelerated 1000× by heparin.
2. Protein C + Protein S: Vitamin K-dependent. Activated Protein C (APC) + Protein S inactivate factors Va and VIIIa, limiting coagulation. Thrombomodulin on endothelium activates Protein C.
3. Tissue Factor Pathway Inhibitor (TFPI): Inhibits the TF:VIIa:Xa complex, limiting the extrinsic pathway after initial activation.
4. Prostacyclin (PGI₂) and NO: Released by intact endothelium; inhibit platelet activation and cause vasodilation.

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CLINICAL CORRELATION - COAGULATION TESTS

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SUMMARY DIAGRAM - SITES OF ANTICOAGULANT ACTION

Intrinsic: XII→XIIa→XI→XIa→IX→IXa
                                  ↘
                              X → Xa ← TF:VIIa (Extrinsic)
                                  ↓
Heparin/LMWH → [AT-III] blocks Xa ← FONDAPARINUX blocks Xa
                                  ↓
                           Prothrombin (II) → Thrombin (IIa)
                                                ↑
         WARFARIN blocks synthesis of II, VII, IX, X
         ARGATROBAN/BIVALIRUDIN/DABIGATRAN block IIa
                                  ↓
                      Fibrinogen → Fibrin → Cross-linked Fibrin

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