Question 3

Outline the major clotting factors and steps in the haemostasis pathway  (70% marks). Outline the mechanism of action of thrombolytics (30%  marks).

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College Answer

This question was also best answered using a structured response and illustrations. Discussion and/or diagrams of the process of formation of temporary platelet plug and conversion to a definitive haemostatic plug after injury to the vessel wall, showing the Intrinsic, Extrinsic and Common Pathways with note of essential cofactors (tissue thromboplastin, Ca++) and fibrinolysis and clot resolution, inhibitors and controlers that prevent excessive coagulation. The latter would lead into outlining the mechanism of thrombolytics. It was expected candidates would mention such mechanisms as catalysing the formation of plasmin from plasmingoen, activation of endogenous plasminogen and direct conversion of plasminogen to plasmin.

Syllabus: J2. 1, J2. 2e Reference: Pharmacology and Physiology in Anesthetic Practice, Stoelting pgs 510 - 511, Basic and Clinical Pharmacology, Katzung pg 380 - 383

Discussion

Haemostatic events:

  • simultaneous activation of clotting factors and platelets to prevent blood loss.
  • Initiation:
    • tissue factor (TF)  is exposed
    • TF and Factor VIIa act to produce a small amount of thrombin (the extrinsic pathway)
    • Thrombin then activates platelets
  • Platelet plug formation occurs when platelets are exposed to TF and collagen:
    • Adhesion to the denuded surface collagen via VWF, as well as directly
    • Aggregation (platelet to platelet) mediated by fibrin and VWF
    • Activation (degranulation, shape change, phosphatidylserine exposure on the platelet surface)
  • Amplification
    • Intrinsic pathway activation by the available thrombin and other platelet granule content leads to the increase in available clotting factors in the region of the platelet plug
    • The available thrombin activates factor XI and leads to the activation of FXI
    • Activate platelet surfaces act as sites of attachment for FVIIIa and FVa
  • Propagation
    • Platelet-bound Factors FVIIIa  FVa and FX activate thrombin (the "thrombin burst")
    • The large amount of thrombin made available allows the generation of a large amount of fibrin from fibrinogen
  • Ionised calcium is a cofactor in all three pathways and is essential for platelet activation
  • Contraction of platelets occurs in later stages of clot maturation

Mechanism of thrombolytic agents

  • The  available thrombolytic agents are recombinant analogs of the tissue plasminogen activator (tPa)
  • This is a protease that has a specific affinity for plasminogen
  • Specifically, its affinity is massively increased by the presence of fibrin (otherwise, the affinity for plasminogen alone would be rather low)
  • Plasminogen binds to fibrin by attaching to its lysine residues 
  • When tPa binds to the plasminogen-fibrin complex, it activates plasminogen into plasmin
  • Plasmin is a protease that then cleaves fibrin into soluble peptide fragments
  • This degrades clot stability and ultimately causes its disintegration