Haemostasis Flashcards
How does a clot form (simple overview)?
-Vessel wall becomes damaged
=Injured endothelial cells actively promote clot formation via exposure of subendothelial collagen and release of tissue factor
-Von Willebrand factor (circulating protein) binds to components in blood vessel wall
-Capture platelets to activate coagulation factor cascade (exposure to ADP and thrombin) = recruit even more platelets
=Thrombus/ plug
=Clot dissolved to restore blood flow (fibrinolysis)
What is primary haemostasis?
-Involves the interactions between vessel wall, platelets and vWF to produce the initial barrier to blood loss, the primary haemostatic plug
Actions in primary haemostasis
- Vasoconstriction (immediate)
- Platelet adhesion (seconds)
=via vWF and collagen
=Produce and release thromboxane A to cause vasoconstriction to reduce blood flow to damaged area
=Platelets activated through various agonists then release further agonists like ADP for further activation - Platelet aggregation (minutes)
=Platelet to platelet linkages formed through binding of mainly soluble fibrinogen to activated Gp2b/3a on adjacent cells
=Strengthened when fibrinogen converted to fibrin
What is secondary haemostasis?
-Involves coagulation factors acting in concert to generate fibrin to strengthen the primary haemostatic plug
=Key enzyme: thrombin
Phases of in vivo coagulation
- Initiation: generates a small amount of thrombin from tissue factor and Factor VII interactions.
- Amplification: Thrombin causes positive feedback with large scale generation of thrombin on platelet surfaces
- Propagation: thrombin cleaves fibrinogen to fibrin. The clot is stabilised by cross-links via Factor XIIIa
What regulates secondary haemostasis?
Inhibitors (antithrombin, protein C, protein S)
Actions in secondary haemostasis timeline
- Activation of coagulation factors (seconds)
- Formation of fibrin (minutes)
What is fibrinolysis?
-A process which degrades the fibrin-bound clot to prevent vascular occlusion and remove the clot once the wound has healed
Actions in fibrinolysis (timeline)
- Activation of fibrinolysis (minutes)
- Lysis of the plug (hours)
Key enzyme in fibrinolysis
-Fibrinolytic plasmin (fibrin-splitting protease).
=Generated from plasminogen via activators such as tissue plasminogen activator (tPA) from endothelial cells.
=Plasmin fibrin into fibrin degradation products such as DDimers.
=Fibrinolysis also needs to be regulated via inhibitors like anti-plasmin.
Where do drugs act in haemostasis?
-Antiplatelet drugs: primary haemostasis
-Anticoagulant drugs: secondary
-Thrombolytic drugs: fibrinolysis
Von Willebrand Factor
This multimeric plasma protein binds to exposed collagen and is crucial for platelet adherence and activation
What is coagulation?
Mechanisms leading to the conversion of soluble fibrinogen to insoluble fibrin.
Describe the coagulation system
-The coagulation system consists of a cascade of proteolytic enzymes called coagulation factors.
-The suffix “a” refers to the
activated factor.
-These factors are synthesised in the liver, though Factor V is also made in endothelium and platelets.
-Synthesis of Factors II, VII, IX and X is Vitamin K dependent
What to involve in a bleeding history
-Have you bled?
=Challenges: birth, operations, dentistry, trauma
=Pattern: primary vs secondary (examination)
-PMH (liver disease)
-Family (haemophilia, vWD)
-Drugs (warfarin, DOA, aspirin)
Pattern in primary haemostasis bleeding
-Easy bruising and bleeding from small vessels (petechiae)
-Mucocutaneous
=mucous membranes: nose, GI and GU tracts) and into skin (purpura and ecchymoses)
Pattern in secondary haemostasis
-Prolonged or delayed bleeding (wounds, muscles, joints, cannula sites)
Simple tests investigating bleeding
-FBC and film
=Platelets (how many, morphology)
-Coagulation screens
What is a coagulation screen?
-Simple tests that addresses some (but not all) aspects of coagulation. Identifying clotting factor problems
-It consists of the prothrombin time (PT- extrinsic pathway/ PeT ), activated partial thromboplastin time (aPTT- intrinsic pathway APinTT) and often fibrinogen concentration.
-These tests are performed on plasma obtained from citrate-anticoagulated blood.
How is the extrinsic pathway measured?
-Adding tissue factor (and calcium) to blood and measuring PT time
=Time taken for sample to clot
=Normal range: 10-13.5 seconds
-Activate common pathway (factors 2,5, 10)
=Fibrinogen to fibrin
-Change in viscosity and density of clot
-Abnormalities= prolongation of PT
How is the intrinsic pathway measured?
-APTT= time taken for sample of blood to clot after a contact activator and Ca2+ added
=Normal range: 25-35 seconds
-Factors, 8,9,11,12
-Converge into common pathway
=Abnormalities: prolonged APTT
What is PT prolonged in?
• Deficiency / inhibitors of Factors II, V, VII, X or fibrinogen
• Warfarin use / Vitamin K deficiency
• Liver disease +++
• Disseminated Intravascular Coagulation (DIC)
What is APTT prolonged in?
• Deficiency / inhibitors of Factors II, V, VIII, IX, X, XII or
fibrinogen
• Heparin therapy (unfractionated/ DOACs)
• ‘Lupus anticoagulant’ / Antiphospholipid antibodies
• Liver disease +
• Disseminated Intravascular Coagulation
PT/APTT interpretation
-Both prolonged= global defect or common pathway
-Just PT prolonged= extrinsic pathway defect
-Just APTT prolonged= intrinsic pathway defect
Thrombin time and what causes it to prolong?
-Thrombin added to blood, effect on fibrinogen to fibrin conversion
-Prolonged:
=Heparin
=Low/abnormal fibrinogen
=High D-Dimer
What does the coagulation screen not cover?
-Von Willebrand disease
-Many drugs
-Congenital disease
Specialist tests for bleeding
-Coagulation Factor Assays
-Von Willebrand profile
-Platelet function tests
-Thromboelastography
Describe the Clauss fibrinogen assay
-Diluting plasma, adding thrombin and comparing clotting time to a reference standard
-Normal range: 1.5-4.0 g/L
-Raised levels in acute phase response
-Low levels in severe sepsis, DIC, rare congenital states
Causes of bleeding in terms of the blood vessel wall
-Missing: Trauma
-Dysfunctional: Ehlers Danlos/ abnormalities
Causes of bleeding in terms of von Willebrand factor
-Missing: Von Willebrand Disease (deficient)
-Dysfunctional: Von Willebrand disease (dysfunctional protein)
Causes of bleeding in terms of platelets
-Missing: Many states
-Dysfunctional: Drugs (aspirin, clopidogrel), uraemia, congenital
Causes of bleeding in terms of coagulation factors
-Missig: warfarin, liver disease, DIC, haemophilia
-Dysfunctional: anticoagulants, congenital
