Haematology Flashcards
How do platelets bind to exposed subendothelial cells/matrix and what does this trigger?
- GlpIa to exposed collagen - GlpIb to Von Willebrand factor - Release of ADP and thromboxane A2 trigger platelet aggregation
What factors are involved in platelet aggregation?
GlpIIb/IIIa crosslink platelets via fibrinogen
Which arachidonic acid metabolites affect platelet aggregation?
Thromboxane A2 induces platelet aggregation Prostacyclin PGI2 inhibits platelet agggregation
What actions does thrombin have?
- Cleaves Fibrinogen - Activates Platelets - Activates procofactors (FV and FVIII) - Activates zymogens (FVII, FXI and FXIII)
Describe the initiation phase?
- Injury of vessels wall leads to contact between blood and subendothelial cells - Tissue factor (TF) is exposed and binds to FVIIa or FVII which is subsequently converted to FVIIa - The complex between TF and FVIIa activates FIX and FX - FXa binds to FVa on the cell surface
Describe the amplification phase?
- The FXa/FVa complex converts small amounts of prothrombin into thrombin - The small amount of thrombin generated activates FVIII, FV, FXI and platelets locally. FXIa converts FIX to FIXa - Activated platelets bind FVa, FVIIIa and FIXa
Describe the propagation phase?
- The FVIIIa/FIXa complex activates FX on the surfaces of activated platelets - FXa in association with FVa converts large amounts of prothrombin into thrombin creating a “thrombin burst”. - The “thrombin burst”leads to the formation of a stable fibrin clot.
What factor is activated to degrade fibrin?
Plasminogen is converted by tissue plasminogen activator (tPA) to plasmin which degrades fibrin.
What physiological anticoagulants are there?
- Antithrombin III: main inhibitor of activated coagulation serine proteases. Also inactivates thrombin and activated factors X, IX, XI. ATIII activity is greatly accelerated (2000-fold) by heparin -Heparin co-factor II: complexes with thrombin inactivating it. Activity is amplified 1000-fold by heparin - Protein C + S: upon activation by thrombin in the presence of thrombomodulin, activated protein C inactivates VIIIa and Va, thus reducing the rate of thrombin generation. Protein S is a cofactor - Tissue factor pathway inhibitor: binds to Xa and TF:VIIa complex, inhibiting their proteloytic activity
Which substances mediate vasoconstriction in haemostasis?
- adrenaline - ADP - kinins - thromboaxanes
Which coagulation factors are dependent on vitamin K for their normal function?
II, VII, IX, X, protein C and protein S
How does clopidogrel inhibit platelet aggregation?
Clopidogrel is a specifc, non-competive inhibitor of ADP- induced platelet aggregation, irreversibly inhibiting the binding of ADP to its platelet membrane receptors. Ultimately it inhibits the activation of the GPIIb/IIIa receptor, its binding to fibrinogen and further platelet aggregation.
What are the common causes of thrombocytopenia?
Immune-mediated Idiopathic Drug-induced Connective tissue disease Lymphoproliferative disease Sarcoidosis Non-immune mediated DIC Microangiopathic hemolytic anemia
What is the treatment for ITP?
>50,000 - Nothing 20-50,000 - Not bleeding: None - Bleeding : Steroids + IVIG <20,000 - Not bleeding: Steroids - Bleeding : Steroids, IVIG, Hospitalization
What are the clinical features of haemophilia?
X-linked congenital deficiency - Factor 8 (A) or 9 (B) Bleeding – Haematoma, joint etc. Prolonged aPTT but normal PT. Clotting factor replacement-Life long.
What are the clinical features of von Willebrand Disease?
Inheritance - autosomal dominant Incidence - 1/10,000 Clinical features - mucocutaneous bleeding Classification: Type 1 - Partial quantitative deficiency Type 2 - Qualitative deficiency Type 3 - Total quantitative deficiency
What are the components of Virchow’s triad?
Vessel wall, blood, flow.
What inherited thrombophilia risk factors are there?
- Anticoagulant deficiency (All rare): Protein C (0.3%), Protein S, Antithrombin (<0.2%) - Procoagulant excess (common): Factor VIII (10% of population), Factor II (2%), Fibrinogen - Additional factors: Factor V Leiden (5% caucasians), Lupus anticoagulant, Prothrombin G20210A
What is Lupus anticoagulant?
An immunoglobulin that binds to phospholipids and proteins associated with the cell membrane and increases the risk of thrombosis.
What acquired thrombophilia risk factors are there?
Age, obesity, previous DVT or PE, immobilisation, major surgery, long distance travel, malignancy (esp. pancreatic), pregnancy, COCP, HRT, antiphospholipid syndrome, polycythaemia, thrombocythaemia
What anticoagulant molecules are normally expressed by blood vessel walls?
- Thrombomodulin: Directs thrombin to activate protein C - Endothelial protein C receptor: Presents PC to Thrombomodulin - Tissue factor pathway inhibitor: Inhibits Tissue factor - Prostacyclin (PGI2): Inhibits platelet activation - Nitirc Oxide: Inhibits platelet activation
What is used as DVT prophylaxis?
Chemical: low molecular weight heparin e.g. Tinzaparin, Clexane Mechanical: TED stockings or intermittent compression devices
What is the treatment for a DVT/PE?
Immediate: Low molecular weight heparin until 2 days therapeutic INR Ongoing: warfarin for 3-6 months, started simultaneously In cancer patients continue LMWH not warfarin THrombolysis reserved for life threatening PE or limb threatening DVT
How does heparin work, how is it administered and what long-term side effects are there?
Potentates antithrombin III which inactivates thrombin and factors 9, 10, 11 LMWH: SC OD, no monitoring (anti-Xa assay) except in renal failure and late pregnancy Unfractionated: IV infusion, monitor APTT Side effects: heparin induced thrombocytopenia and osteoporosis (more common with UFH)