Coagulation

Fibrinolytic System

• Fibrinolysis is the breakdown of cross-linked fibrin to prevent excessive thrombosis.
• Key proteins involved:
  • Plasmin degrades fibrinogen or fibrin
• Inhibitors of fibrinolysis:
  • Plasminogen activator inhibitor 1 (PAI-1)
  • α2-Antiplasmin (α2-AP)
  • Thrombin activatable fibrinolysis inhibitor (TAFI)

Platelet Function Tests

• Bleeding Time
  • Measures time taken for blood vessel constriction and platelet plug formation.
  • Detects defective platelet function.
  • Screening test for acquired and congenital platelet defects.
  • Do not perform if platelet count is < 75,000
  • Reference Range:
    • Ivy: 1-7 minutes
    • Duke: 1-6 minutes
• Sources of Error:
  • False Positive:
    • BP cuff maintained too high.
    • Incision too deep.
    • Disturbing clot with filter paper (blotting it frequently).
    • Touching incision wound (touch only the tip of the blood during blotting).

Coagulation Tests

• Lee-White Method:
  • Used for assessing the ability of whole blood to form a clot.
• aPTT (Activated Partial Thromboplastin Time):
  • Measures the time it takes for plasma to clot following the addition of a reagent.
  • Includes the intrinsic and common pathways
  • Called "activated" since negatively-charged surface activator (glass or silica) speeds up the rate of reaction.
  • Normal reference range: 20–35 seconds
• PT (Prothrombin Time):
  • Measures the time taken for plasma to clot after addition of tissue thromboplastin and calcium.
  • Includes the extrinsic and common pathways.
• INR (International Normalized Ratio):
  • Standardized measure of the extrinsic coagulation pathway.
  • Used to monitor warfarin therapy.
• TT (Thrombin Time):
  • Measures the time it takes for fibrinogen to clot after the addition of thrombin.
  • Tests the final phase of the coagulation cascade.
• Test for Fibrinolytic Pathway:
  • Assesses the activity of the fibrinolytic system.
  • Measures plasminogen activator inhibitor-1 (PAI-1) activity.
  • Monitors therapy with fibrinolytic agents.

Overview of Hemostasis

• Platelet Activation:
  • Leads to shape change (round discs to flat plates with spiky protrusions).
  • Release of secretory granules.
  • Platelet aggregation occurs.
• Primary Hemostatic Plug Formation:
  • Forms within minutes.
• Secondary Hemostasis:
  • Concerned with platelet aggregation and formation of a stable fibrin clot.
  • Involves the interplay of coagulation factors.
• Glanzmann's Thrombasthenia:
  • A disease involving secondary hemostasis.

Endothelium

• Central regulators of hemostasis:
  • Balance between antithrombotic and prothrombotic activities determines thrombus formation or dissolution. -Activated by:
  • Trauma
  • Infectious agents
  • Hemodynamic forces
  • Plasma mediators
  • Cytokines

Prothrombotic Properties of Endothelium

• Prostacyclin:
  • Produced by COX-1.
• Nitric Oxide:
  • Product of eNOS.
• Adenosine diphosphatase:
  • Degrades ADP.

Anticoagulant Effects of Endothelium

• Normal endothelium:
  • Shields coagulation factors from tissue factor in vessel walls..
• Thrombomodulin and Endothelial Protein C Receptor:
  • Bind to thrombin and protein C, respectively.
  • Thrombin loses its ability to activate coagulation factors and platelets.
  • Thrombin cleaves and activates protein C.
• Protein C/S complex:
  • Potent inhibitor of coagulation cofactors (Va and VIIIa).
• Heparin-like molecules:
  • Bind and activate antithrombin III.
  • Inhibits thrombin, factors IXa, Xa, XIa, and XIIa.
• Tissue Factor Pathway Inhibitor:
  • Binds and inhibits tissue factor/factor VIIa complexes.
  • Requires protein S as a cofactor.

Fibrinolytic Effects of Endothelium:

• tPA (tissue plasminogen activator):
  • Counterregulatory mechanism synthesized by endothelium.
  • Converts plasminogen to plasmin, which breaks down the fibrin mesh within the clot.
  • Prevents excessive clot formation and maintains healthy blood flow.

Exogenous Anticoagulants

• Antithrombin III:
  • Inhibits the activity of thrombin and other serine proteases (IXa, Xa, XIa, and XIIa).
  • Activated by binding to heparin-like molecules on endothelial cells.
• Protein C and Protein S:
  • Vitamin K–dependent proteins that act in a complex to inactivate factors Va and VIIIa.
• Tissue Factor Pathway Inhibitor (TFPI):
  • Produced by endothelium.
  • Inactivates tissue factor–factor VIIa complexes.

Coagulation Cascade

• Third arm of homeostatic process:
  • Series of amplifying enzymatic reactions that lead to the deposition of an insoluble fibrin clot.
• Steps:
  • Each step proteolytically cleaves an inactive proenzyme into an active enzyme, culminating in thrombin formation.
• Formation of Stable Fibrin Clot:
  • Result of the coagulation cascade.

Factor Classification

• Can be:
  • Intrinsic
  • Extrinsic
  • Common
• Or can be:
  • Surface-bound zymogens
  • Vitamin K-Dependent zymogens
  • Cofactor/Substrates

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