Fibrinolysis

• Fibrinolysis


  The process by which fibrin clots are broken down and the flow of blood is reestablished
• The body has a system for clot formation and a means by which the fibrin clots may be removed
• Fibrinolysis is initiated as soon as the clotting process has begun, to break down the fibrin clot that is formed
• Normally, the fibrinolytic system and the coagulation system are in equilibrium in normal persons
• Fibrinolysis is increased whenever coagulation is increased
• Plasmin


  The active enzyme that is responsible for digesting fibrin or fibrinogen
• Plasminogen


  The inactive form of plasmin, which is converted to plasmin by certain proteolytic enzymes called plasminogen activators
• Plasminogen activators


  • Tissue activators
  • Contact XI
  • XII
  • HMWK
  • Prekallikrein
  • Kallikrein
• Plasminogen


  • 92,000 Daltons; from the liver; a single chain protein with 5 glycosylated loops called kringles
  • Kringles enable plasminogen, along with activators TPA and UPA, to bind the lysine moieties on the fibrin molecule during the polymerization process. This fibrin-binding step is essential to fibrinolysis
  • Fibrin-bound plasminogen is converted into a two-chain active plasmin molecule when cleaved between arginine at position 561 and valine at position 562 by neighboring fibrin-bound TPA or UPA
• Plasmin


  • A serine protease that systematically digests fibrin polymer by the hydrolysis of arginine-related and lysine–related peptide bonds
  • As fibrin becomes digested, the exposed carboxy-terminal lysine residues bind additional plasminogen and TPA, which further accelerates clot digestion
  • Bound plasmin digests clot and restores blood vessel patency (openness). Its localization to fibrin through lysine binding prevents systemic activity. However, free plasmin can be found in the circulation and is capable of digesting Factors 1, V, VIII, and fibronectin. Plasma alpha2-antiplasmin rapidly binds and inactivates any free plasmin
• Tissue Plasminogen Activator (TPA)


  Secreted by the endothelial cells, hydrolyzes fibrin-bound plasminogen, converting it to plasmin. It has 2 glycosylated kringle regions, forms covalent lysine bonds with fibrin during polymerization and localizes at the surface of the thrombus with plasminogen to convert the latter to plasmin. Circulating TPA is bound to inhibitors such as PAI-1. These complexes are cleared from the circulation
• Urokinase Plasminogen Activator (UPA)


  Secreted by urinary tract epithelial cells, monocytes, and macrophages. Circulates in the plasma at a concentration of 2-4ng/mL and becomes incorporated into the mix of fibrin-bound plasminogen and TPA at the time of thrombus formation. UPA converts plasminogen to plasmin. Because it has only one (1) kringle region, UPA does not bind firmly to fibrin and has a relatively minor physiologic effect
• Exogenous plasminogen activators


  • Streptokinase
  • Acyl-plasminogen streptokinase activator complex (APSAC)
• Plasminogen Activator Inhibitor-1 (PAI-1)


  The principal inhibitor of plasminogen activation, inactivating both TPA and UPA, thus preventing them from converting plasminogen to plasmin. Produced by various cell types. Present in excess of the TPA concentration in plasma, and circulating TPA normally becomes bound to PAI-1. Only at times of EC activation, such as after trauma, does the level of TPA secretion exceed that of PAI-1 to initiate fibrinolysis
• Alpha-2 antiplasmin


  Synthesized in the liver and is the primary inhibitor of free plasmin. It is a SERPIN. Free plasmin produced by activation of plasminogen can bind either to fibrin, where it is protected by AP (alpha-2 antiplasmin)
• Thrombin Activatable Fibrinolysis Inhibitor (TAFI)


  A plasma procarboxypeptidase produced by the liver that becomes activated by the thrombin-thrombomodulin complex. It inhibits fibrinolysis by cleaving carboxy-terminal lysine residues from partially degraded fibrin, thereby preventing the binding of TPA and plasminogen to fibrin and blocking the formation of plasmin
• Fibrin Degradation Products (FDPs) or Fibrin Split Products (FSPs)


  The end products of fibrin and fibrinogen, formed during fibrinolysis and removed from the blood by the mononuclear phagocytic system (monocytes & macrophages)
• 1. dimer


  A specific product of digestion of cross-linked fibrin only and is therefore a marker of thrombosis and fibrinolysis. It is present only when there has been thrombin generation, polymerization of fibrin clot by FXIIIa, and fibrinolysis of the clot by plasmin
• Assessing D-dimer levels is an important diagnostic tool to identify DIC and to rule out venous thromboembolism and pulmonary embolism
• Plasminogen activators


  • Intrinsic activators (Factor XIIa, Kallikrein, HMWK)
  • Extrinsic activators (Tissue plasminogen activators, Urokinase plasminogen activators)
  • Exogenous activators (Urokinase, Streptokinase)
• Degradation of non-cross-linked fibrin by plasmin


  1. Fragment X

  2. Fragment Y

  3. Fragment D

  4. Fragment D

  5. Fragment E
• Degradation of cross-linked fibrin by plasmin


  1. Complex DD/E

  2. Complex YD/DY

  3. Complex YY/DXD
• 1. dimer immunoassay


  Detects the presence of FDPs which signifies fibrinolysis, and indicates acute and chronic DIC, systemic fibrinolysis, deep vein thrombosis, and pulmonary embolism
• Quantitative D-dimer assay


  Uses microlatex particles coated with monoclonal anti-D-dimer antibodies and the patient's plasma, measured by turbidimetry. Detects concentrations as low as 10ng/mL. Results may be reported in D-dimer units (DDU) or fibrinogen equivalent units (FEU)
• Fibrin Degradation Product Immunoassay


  A slide agglutination test using a latex suspension coated with polyclonal antibodies specific for D and E fragments, calibrated to detect FDPs at a concentration of 2 ug/mL or greater
• IC


  Intravascular coagulation
• Systemic fibrinolysis


  Breakdown of fibrin clots throughout the body
• Deep vein thrombosis


  Formation of a blood clot in a deep vein, usually in the leg
• Pulmonary embolism


  Blockage of the pulmonary artery by a blood clot
• Stroke


  Interruption of blood supply to the brain, often caused by a blood clot
• FDPs


  Fibrin degradation products
• 1. dimer


  A protein fragment produced when a blood clot is broken down
• Quantitative D-dimer assay


  1. Microlatex particles coated with anti-D-dimer antibodies + patient's plasma, measured by turbidimetry

  2. Results within 30 minutes
• Most methods detect D-dimer concentrations as low as 10ng/mL
• 1. dimer units (DDU)


  Units used to report D-dimer results
• Fibrinogen equivalent units (FEU)


  Units used to report D-dimer results
• Fibrin Degradation Product Immunoassay

  Replaced by automated D-dimer assay
• Thrombo-Wellcotest


  1 gtt patient's serum + 1 gtt latex suspension, visible agglutination indicates FDPs at 2 ug/mL or greater
• Qualitative FDP assays are confined to diagnosis and monitoring of DIC due to inadequate sensitivity at low concentrations
• Plasminogen


  Converted to plasmin by the action of TPA, bound plasmin degrades fibrin