1 Venous thrombosis Normal Hemostasis • A well regulated process • Maintains blood in a fluid, clot free state in normal vessels • Induces the rapid formation of a localized hemostatic plug at the site of vascular injury Primary haemostasis: • Vasoconstriction (immediate) • Platelet adhesion (within seconds) • Platelet aggregation and contraction (within minutes) Secondary haemostasis: • Activation of coagulation factors (within seconds) • Formation of fibrin (within minutes) Fibrinolysis: • Activation of fibrinolysis (within minutes) • Lysis of the plug (within hours) Hemostasis The Main Players in Hemostasis • Endothelial cells • Platelets • Coagulation cascade Endothelial Cells • Produce vWF (vonWillebrand factor) – A product of normal endothelium – found in the plasma in low concentration – essential for platelet binding to collagen and other surfaces • Secrete Tissue factor – induced by cytokines (TNF, IL-1) – activates the extrinsic clotting pathway Platelets • Express glycoprotein receptors on membranes. Gp Ib,IIb/IIIa • Have three types of granules – Alpha granules • Fibrinogen, fibronectin, factor V and VIII, PDGF, TGFb – Dense bodies or delta granules • ATP/ADP, ionized calcium, histamine, serotonin, epinephrine – Lysosomal granules Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2009. For permissions please email: journals.permissions@oxfordjournals.org Platelets continued • Upon encountering the ECM, platelets undergo three general reactions: 1. Adhesion and shape change mediated by vWF and glycoprotein Ib 2. Secretion (release reaction) – calcium required in coagulation cascade – ADP as mediator of platelet aggregation – Surface expression of phospholipid complex • Binding site for calcium ions and coagulation factors Platelets continued • 3. Aggregation – ADP and TXA2 (vasoconstrictor thromboxane A2) are the stimuli for the formation of the primary hemostatic plug • Aspirin inhibits synthesis of TXA2 – Fused mass of platelets • Created by coagulation cascade that produces thrombin • Thrombin also converts fibrinogen to fibrin cementing platelets in place Normal sequence of Hemostasis (4 steps) • 1. Arteriolar vasoconstriction (transient) – Reflex neurogenic mechanisms – It is important for the activation of platelets or coagulation systems • 2. Exposure of subendothelial ECM when there is endothelial injury – ECM, especially collagen, is highly thrombogenic – Platelets adhere and become activated • Change in shape • Release of secretory products – Aggregation of platelets forms hemostatic plug – This is primary hemostasis First two steps of normal hemostasis Secondary haemostasis ⚫ Secondary haemostasis involves a series of interactions between coagulation factors which occur on the surface of tissue-factor-bearing cells and activated platelets ⚫ This results in the generation of a thrombin burst and the formation of a haemostatic plug at the site of vascular injury ⚫ Based on the “cell-based model”, coagulation occurs in three overlapping phases – initiation, amplification and propagation Normal hemostasis continued • 3. Tissue factor released at the site of injury (by endothelial cells) – Works with secreted platelet factors – Activates coagulation cascade • A series of proteins where thrombin is activated • Induces further platelet recruitment and granule release – Ends in fibrin deposition – Called secondary hemostasis Normal hemostasis continued • 4. Formation of permanent plug – Prevents further hemorrhage – Polymerized fibrin and platelet aggregation Steps 3 and 4 Coagulation Cascade • A series of conversions of inactive proenzymes to activated enzymes, – culminating in the formation of thrombin • Thrombin then coverts the soluble plasma protein fibrinogen to insoluble fibrous protein fibrin Two pathways of coagulation cascade • Intrinsic –Surface contact • Extrinsic –Tissue injury Initiation phase • Upon vessel wall injury, tissue factor (TF) is exposed to circulating endogenous factor VII/VIIa – leading to the TF/VIIa complex which initiates coagulation • At the surface of TF-bearing cells the TF/VIIa complex activates: – Factor IX to IXa – Factor X to Xa • Factor Xa binds to factor Va on the cell surface Amplification phase • The factor Xa/Va complex activates small amounts of prothrombin to thrombin at the surface of subendothelial cells • This limited amount of thrombin activates factors V, VIII and platelets • The activated platelet binds factors Va, VIIIa and IXa Propagation phase • Thrombin-activated platelets change shape and expose negatively charged phospholipids to which the factor VIIIa/IXa complex binds – This results in factor X activation on the surface of activated platelets • The factor Xa/Va complex activates large amounts of prothrombin resulting in a “thrombin burst” which: – Converts fibrinogen to fibrin – Activates fibrin-stabilising factor XIII • The amount and rate of thrombin generation determines the strength of the haemostatic plug Trombin Control of cascade to prevent clotting elsewhere • Antithrombins – activated by heparin like molecules on endothelial cells • Clinical administration of heparin minimizes thrombosis • Proteins C and S – Vitamin K dependent – Inactivate cofactors Va and VIIIa • Plasminogenplasmin system – Breaks down fibrin and inhibits its polymerization – Products of split fibrin are anticoagulants Factors that favor or inhibit thrombosis Fibrinolytic system Conditions Causing Bleeding • Incomplete hemostasis is most common cause of bleeding. • Vitamin K deficiency – severe coagulation defect – Required for synthesis of prothrombin and factors VII, IX and X • Parenchymal diseases of the liver – Liver synthesizes several coagulation factors Signs and Symptoms of 1o Hemostasis Problems • Ecchymoses • Petechiae • Mucus membrane bleeding • Hematoma • Prolonged bleeding after minor surgery Hereditary Vascular Problems • Hereditary (spider) telangiectasis (Osler-RenduWeber): dilated superficial capillaries • Ehlers-Danlos: collagen disorder • Marfan syndrome: connective tissue • Osteogenesis imperfecta Acquired Vascular Problems • Senile purpura (Bateman’s): altered connective tissue support • Cushing syndrome: metabolic • Scurvy: abnormal collagen • Allergy: vascular inflammation • Viral infection Quantitative Platelet Disorders • Thrombocytopenia <100,000/ml BT prolonged ≈10,000 Bleeding in trauma or OR <10,000 Spontaneous, CNS bleeding • Thrombocytopenia due to destruction – ITP (acute in children, chronic in young women) with anti-glycoprotein – Drug reaction – Heparin induced thrombocytopenia – DIC and TTP About Thrombotic Thrombocytopeneic Purpura (TTP) • Disorder of systemic platelet aggregation in microvasculature • Stimulus: unusually large vWf • In children: likely to be deficiency in vWf metalloproteinase to break down vWf • In adults: vWf metalloproteinase inhibited by autoantibodies • Low PLT count, intravascular hemolysis, RBC fragmentation, high LDH Quantitative Platelet Disorders • Thrombocytopenia due to decreased production – Aplastic anemia (e.g., Fanconi’s) – Fibrosis – Acute leukemia – Megaloblastic anemia – Hereditary (e.g., May-Hegglin, Wiscott-Aldrich, Bernard-Soulier) • Splenic sequestration • HELLP syndrome (hemolysis, elevated liver enzyme, low PLT) in pre-eclampsia • Dilution (massive transfusion) Quantitative Platelet Disorders • Thrombocytosis – Primary with dysfunctions (e.g., CML, ET) – Post splenectomy: also see HJ, etc. – Hemolytic anemia – Acute hemorrhage and surgery Hereditary deficiencies • Hemophilia A--factor VIII deficiency – Sex-linked recessive – 30% due to new mutations and don’t have family link – Infuse patient with factor VIII from human blood or cryoprecipitate. • Hemophilia B--factor IX deficiency – Clinically indistinguishable from Hemophilia A – Sex-linked recessive Von Willebrand’s Disease Von Willebrand’s Disease- Most common congenital bleeding disorder. • Types I, II, and III. • PT normal PTT normal or elevated • Prolonged bleeding time • Type I most common (70%) • Type III causes most bleeding Thrombosis • Pathological state • Inappropriate activation of the normal hemostatic process – within the non-interrupted vascular system. • Thrombus (blood clots) formation – Blocks blood flow to vital areas DVT Hypercoagulability • Leiden Factor- 30% spontaneous venous thrombosis. • Most common congenital disorder. • Resistance to Protein C, defect on factor V • TX: heparin, warfarin. • Protein C, S deficiency-5% venous thromboses. TX: heparin, warfarin. Hypercoagulability • Antithrombin III deficiency- 2-3% thrombosis. Heparin doesn’t work. Can develop after previous heparin exposure. Heparin vs. Warfarin Warfarin activity D-dimers Hemocoagulative examination • aPTT = activate partial thromboplastin time ▪ The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT or APTT) is a performance indicator measuring the efficacy of both the "intrinsic" (now referred to as the contact activation pathway) and the common coagulation pathways. ▪ Apart from detecting abnormalities in blood clotting, it is also used to monitor the treatment effects with heparin, a major anticoagulant.