Lecture 5.2 - Haemostasis.pdf

Overview: ◦Haem = blood stasis = stop ◦The first part consists of mainly platelets and endothelial cells ◦The second part consists of plasma proteins which participate in clot formation (coagulation) - promotes anti-thrombin Endothelium regulation of haemostasis: ◦Normal,...

Overview: ◦Haem = blood stasis = stop ◦The first part consists of mainly platelets and endothelial cells ◦The second part consists of plasma proteins which participate in clot formation (coagulation) - promotes anti-thrombin Endothelium regulation of haemostasis: ◦Normal, undamaged endothelium is non-thrombogenic surface ‣ Inhibits platelet aggregation ‣ Prevents activation and propagation of coagulation ‣ Enhances fibrinolysis ◦Antiplatelet endothelial functions ‣ Secretion of prostacyclin (PGI2) ‣ Inhibits platelet activation, secretion, aggregation ◦Anticoagulant endothelial functions: ‣ Maintenance of vascular integrity to separate blood elements from TF and sub endothelium ‣ Produces heparins that enhance the function of antithrombin III to inactivate activated clotting cascade factors Primary haemostasis: ◦Platelets are derived from megakaryocytes and they form a discoid shape and have no nucleus ◦Contain many glycoproteins through which they can interact with specific ligands ◦Primary haemostasis is the initial response to vascular injury - involves interaction of platelets, subendothelium and fibrinogen ◦Granules present in platelets - three types Platelet adhesion: ◦Endothelial injury exposes the circulating blood to a number of subendothelial matrix proteins ◦Vasoconstriction occurs first - reduces blood loss ‣ Nitric oxide helps with vasoconstriction ◦VonWillebrand factor (vWF) is a glycoprotein released by endothelial cells that is important for platelet adhesion ◦Initial platelet adhesion involves interaction of vWF and the GP1b receptor found of platelet surface ◦The platelet then becomes stabilised on the subendothelial surface - will go on to induce a shape change  Platelet activation and shape change: ◦Binding of platelets to vWF triggers their activation, causing a number of effects: ‣ Thromboxane generation - vasoconstrictor ‣ Granule release - the two types are alpha and dense granules - containing ADP, PAF, fibrinogen ‣ Activation of GPIIb/IIIa receptor- allows platelets to bind to fibrinogen required for aggregation ‣ Platelet shape change - swell, form extension (pseudopods) and spread across the subendothelium (also helps in the aggregation stage) Platelet granules: Platelet aggregation: ◦Nearby platelets are recruited and become activated ◦Following activation of GPIIb/IIIa platelets will begin cross linking ◦Nearby platelets bind to each other via the GP IIb/IIIa receptor which is mediated by fibrinogen and form a plug Secondary haemostasis: ◦Forms fibrin from fibrinogen to produce a stable mesh network and a permanent thrombus ◦In order to do this, a series of reactions occur involving coagulation factors (enzymes made by the liver) to produce thrombin ‣ Series of activation of different proteins ◦Thrombin cleaves fibrinogen and forms fibrin ◦Occurs concurrently alongside primary haemostasis Coagulation cascade - extrinsic pathway: ◦Activated by tissue factor (TF) released by endothelial cells after external damage ◦TF comes into contact with factor VII and activates it ◦Factor VIIa goes on to activate Factor X into Xa ◦The common pathway then begins  Coagulation cascade - intrinsic pathway: ◦Begins when factor XII becomes activated after exposure to subendothelial collagen ◦This causes the activation of factor XI ◦Together with calcium factor XIa activates factor IX ◦Factor IXa together with factor VIIIa form a complex (with calcium) to activate factor X ◦The common pathway then begins Coagulation cascade - common pathway: ◦The common pathway may result after activation of Factor X at the end of either the intrinsic or extrinsic pathway ◦Factor Xa, Va and calcium bind together to form a prothombinase complex ◦This activates prothrombin into thrombin ◦Thrombin cleaves fibrinogen to form fibrin - this is how the clot is formed ◦Factor XIIIa is known as the stabilising factor ◦Primary and secondary haemostasis are linked - need both processes to form a blood clot Fibrin clot: Fibrinolysis: ◦Haemostasis is a balance between thrombus formation and thrombus dissolution (fibrinolysis) ◦The central protein involved in fibrinolysis is plasminogen which is converted to its active form, plasmin ‣ Uses tissue plasminogen activator ◦Plasmin works by converting insoluble fibrin into soluble degradation products ‣ Anti-thrombin works to reverse the mechanism of clot formation ◦The process: ‣ Tissue plasminogen activator (tPA) is released from  endothelial cells - this is stimulated by thrombin ‣ tPA then activates plasmin from plasminogen ‣ Plasmin cleaves fibrin strands of the platelet plug and releases degradation products - this process can be targeted by therapy and drugs Bleeding disorders: ◦Coagulation disorders affect the coagulation cascade causing inadequate or excessive clotting ◦Bleeding disorders usually involve a deficiency in clotting factors/protein that is important in the cascade process ◦Clinically patients can present with: easy bruising, bleeding gums, prolonged nosebleeds, joint pain Von Willebrand disease: ◦Most common bleeding disorder ◦Deficiency in vWF caused by autosomal dominant genetic mutation ◦Primarily effects primary haemostasis but also vWF plays a role in stabilising factor VIII - small role in coagulation cascade ◦Range of clinical features from mild (e.g. patients won't know until they go through trauma) to more severe (e.g. 30 episodes of nose bleeds a year) Haemophilia: ◦Haemophilia A (factor 8 deficiency) and B (factor 9 deficiency) are inherited in an X-linked recessive manner whereas C (factor 11 deficiency) is autosomal recessive ◦All three types affect the intrinsic pathway - primarily affects secondary haemostasis ◦Presents with a range of severities  Thrombocytopenia: ◦The normal platelet count is 150-400 x 109/L. A count of less than 100 x 109/L is classified as thrombocytopenia. Spontaneous bleeding occurs with counts less than 20 x 109/L. Patients with such a low platelet count (or non-functional platelets) will lack step 2 of the three steps of haemostasis. In thrombocytopenia there is a prolonged bleeding time but normal PT and APTT (as these assess the clotting cascade and not platelet function). ◦In thrombocytopenia spontaneous bleeding is seen from small vessels in places such as the skin, gastrointestinal tract and genitourinary tract. Occasionally intracerebral bleeding can occur. The bleeding appears as petechiae. ◦The causes of thrombocytopenia can be classified as: ‣ Decreased production of platelets – e.g., due to bone marrow infiltration by malignancy; drugs, e.g., cytotoxic drugs; infections, e.g., measles and HIV; B12 and folate deficiency (which are needed for platelet production). ‣ Decreased platelet survival – e.g. due to immunologic destruction, e.g., immune thrombocytopenic purpura; non-immunologic destruction, e.g. disseminated intravascular coagulation ‣ Sequestration – in an enlarged spleen (hypersplenism) ‣ Dilutional – due to massive blood transfusions (blood stored for more than 24 hours does not contain platelets). Acquired bleeding disorders: ◦Liver disease (liver is the location where clotting factors are produced) ◦Vitamin K deficiency (Vitamin K is important for certain factors - can be seen in malnutrition) ◦Anti-coagulation induced e.g. warfarin - to help prevent clots forming Disseminated intravascular coagulopathy (DIC): ◦Rare condition causing both overactive clotting followed by bleeding ◦Caused by pathological activation of clotting due to inflammation ◦This might be secondary to: ‣ Sepsis - certain bacteria may be involved with DIC ‣ Major trauma/surgery ‣ Cancer ‣ Pregnancy related e.g. eclampsia Thrombophilia: ◦Thrombophilias are inherited or acquired defects of haemostasis resulting in a predisposition to thrombosis, e.g., deep vein thrombosis. Examples include factor V Leiden (in which there is an abnormal factor V which isn’t deactivated resulting in thrombosis), antithrombin deficiency, immobility, protein C or protein S deficiency and antiphospholipid syndrome. Laboratory testing: ◦Prothrombin time - measures extrinsic pathway (measures factors 2,5,7 and 10) ◦Activated partial thromboplastin time - measures intrinsic pathway (measures factors 8,9,11 and 12) ◦Platelet count ◦Bleeding time ◦D dimer - can be used to detect thrombus formation e.g. DVT, PE