Introduction to Haemostasis and Platelets Flashcards
Haemostasis
The process whereby
haemorrhage following vascular
injury is stopped
List the Major Role players in Haemostasis
Blood vessel endothelium
Blood Coagulation factors
Blood platelets
Inhibitors of coagulation
Fibrinolysis
Blood Vessel Wall/Endothelium
Injury to the vessel wall:
- causes vasoconstriction
- activates membrane bound tissue factor which initiates coagulation
- exposes subendothelial connective tissue which allows binding of platelets to subendothelial collagen via von Willebrand factor (vWF)
- vWF mediates platelet adhesion to sub-endothelium and carries coagulation factor VIII in plasma
Phases in Haemostasis
- Vascular Phase
- Platelet Phase
- Coagulation Phase
- Fibrinolytic Phase
Haemostasis is dependent on which factors:
Vessel Wall
Adequate Platelet Numbers
Proper Platelet Function
Adequate Coagulation Factor Levels
Proper Coagulation Factor Function
Proper Function of Fibrinolytic Pathway
Platelets:
Produced by megakaryocytes in bone
marrow
Circulate in blood as disclike spheres, 1-2 μm in
diameter
Normal count:150-400 X 10 /l
Mean platelet lifespan is 10-12 days
Third of total platelet mass is pooled in the spleen
Platelets:
Formation
Megakaryocyte:
–One of largest cells in body
–Produces platelets by fragmentation of the cytoplasm
– +- 2000-3000 platelets per megakaryocyte
Production is regulated by the cytokine thrombopoietin (TPO)
Platelets:
Structure
Anucleate, small & discoid
Surface receptors for binding to:
– Vessel wall
– vWF
– Fibrinogen
Storage granules:
– Dense granules: calcium
– α-granules: vWF, fibrinogen, some coagulation factors
– Lysosomes: enzymes
Platelet:
Morphology
Peripheral zone:
– site of platelet adhesion and aggregation
– surface contains receptors for thrombin, collagen, ADP,
adrenalin, serotonin and others: platelet agonists
– many plasma proteins and coagulation factors V, XI and
fibrinogen are bound to the surface
Sol-gel zone:
-Composed of fibrous elements, some of which are
arranged along the greatest circumference of the
platelet
– these elements contain a contractile protein,
thrombastenin which is responsible for platelet
contraction and clot retraction
Organelle zone:
– contains dense bodies, granules, glycogen particles and mitochondria
– dense bodies and granules release their contents into the plasma during the release reaction through a system of tortuous channels
Platelet Functions
Formation of plug to arrest bleeding
– Platelet-vessel wall binding (adhesion)
– Platelet-platelet binding (aggregation)
Platelet provides negatively charged phospholipid
membrane
– Activates & promotes coagulation
Initiate haemostasis - (temporary plug)
– adhere to exposed subendothelium
– aggregate to form a plug
Localise the coagulation process
– provide a phospholipid surface for the clotting factors to attach
End result: stabilisation of the platelet plug
Phase of Plate Function
Adhesion
Secretion
Aggregation
Contraction
Phases of Platelet Function:
Adhesion
Circulating vWF binds to exposed sub-endothelial collagen
• vWF undergoes a conformational change, allowing it to
now bind to Glycoprotein (GP)Ibα on platelet membrane
• Platelet therefore binds to subendothelium via vWF
• GPIbα-vWF interaction particularly important in areas of
high fluid shear stress, where it is the primary initiator of
haemostasis
• Other GPs (e.g. GPIa/IIa and GPVI) bind directly to
collagen at low shear stress areas
• GPIbα-vWF interaction induces intracellular signalling that
activates membrane GPIIb/IIIa
• GP IIb/IIIa is a receptor for fibrinogen and is important in
platelet-platelet adhesion
Phases of Platelet Function:
Secretion
• After initial stimulation, platelets release their granule
contents, e.g. ADP, thrombin, fibrinogen, vWF, etc.
• Platelet prostaglandin synthesis is activated to form
thromboxane A2, a potent stimulator of further platelet
activation
Phase of Platelet Function:
Aggregation
Additional platelets accumulate, become activated, and adhere to one another
Active metabolic process: agonist binding initiates
signalling pathways that convert GPIIb/IIIa to its activated state
GPIIb/IIIa on platelets now able to bind to fibrinogen and vWF in plasma, leading to aggregation of activated platelets at site of vessel damage
Platelet Function:
Coagulation Cascade
• Platelets also participate in the coagulation cascade
– secrete coagulation factors such as fibrinogen, and
factors V and XIII
– provide a negatively charged surface that binds the
vitamin K dependent coagulation factors to localise the
coagulation process
Phases of Platelet Function:
Contraction
• After formation of stable secondary platelet-fibrin
haemostatic plug, the clot is reduced in volume and
becomes more compact through process of clot retraction
• Forces responsible for clot retraction are generated by
platelet actin-myosin cytoskeleton
• Actin filaments are anchored to membrane GPIIb/IIIa
receptors, which in turn are linked to fibrin strands outside
platelet
• As clot contracts serum is extruded from the fibrin mesh
with reduction in clot volume
Clinical Manifestations of Bleeding Tendencies
PETECCHIAE
– Red to purple spots smaller than 3 mm in diameter
– Prominent on extremities due to increased venous
pressure
• ECCHYMOSES
– Blue to purple subcutaneous haemorrhages larger than
3 mm
• PURPURA
– Purple appearance of the skin due to petecchiae or
ecchymoses
• HAEMATOMA
– Large collection of clotted blood in tissues
• HAEMARTHROSES
– Joint bleeds
Thrombocytopenia/Platelet Function Abnormality
Mucocutaneous bleeding: – Excessive bleeding after dental extraction & gingival bleeding – Petechiae & ecchymoses – Menorrhagia, often worse at menarche – Epistaxis
Platelet Abnormalities
Qualitative(Numbers)-Thrombocytopenia and Thrombocytosis
Quantitative(Functional Abnormality)-Inherited and Acquired
Thrombocytopenia:
Causes
Decreased Production
Increased peripheral destruction/consumption/Loss
Thrombocytopenia:
Decreased Production
Generalised bone marrow failure: – Drugs – Aplastic anaemia – Leukaemia – HIV – MDS
Affecting only megakaryocytes:
– Drugs, viruses
– Rare congenital disorders
Inherited
Thrombocytopenia:
Increased peripheral destruction/consumption/Loss
Immune:
– ITP
– Infections: viruses (HIV), malaria
– Drugs
TTP
DIC
Massive splenomegaly (↑ pooling)
Extracorporeal circulation (platelet loss)
Immune Thrombocytopenia:
Def
Clinical Variant
Clinical Features
Treatment
Thrombocytopenia due to an immunologic destruction of circulating platelets
Megakaryocytes present in BM
CLINICAL VARIANTS
• Acute self-limiting (children, infections)
• Chronic (adults)
HAEMATOLOGICAL AND CLINICAL FEATURES • Thrombocytopenic haemorrhages (mucocutaneous) • Platelet lifespan shortened • Platelets are destroyed in the RES • Platelet antibodies
TREATMENT
• General: Treat patient and not platelet count.
• Children: Conservative, > 90% spontaneous cure
• Adults:
– Steroids - remission in 75%
– Splenectomy - remission in 75% of those not responding to steroids
– Chemotherapy/other immunosuppressive
therapy/thrombopoietin receptor agonists, for pts not
responding to splenectomy
– Exclude SLE
Value of a Peripheral Blood Smear
Normal
Bernard Soulier Syndrome(BBS)
Gray Platelet Syndrome(GPS)
NB: Thrombocytopenia + RBC fragments = haematological emergency!! •TTP •DIC •HELLP syndrome •Severe sepsis
Inherited Thrombocytopenia:
Peripheral Blood Smear
Bernard-Soulier Syndrome-Large Platelets
May-Hegglin anomaly:
- Large Platelets
- Inclusions-Dohl-like bodies
Exclude Pseudothrombocytopenia
Thrombocytopenia-Treatment Principles
Types
Normal
- > 50 x 109/l - safe to undergo operation
- > 20 x 109/l - low danger of severe spontaneous bleeding
Stress Platelets:
->5 x 109/l - prevent spontaneous bleeding
ASSOCIATED DYSFUNCTION (uraemia, drugs, sepsis) -Do Bleeding Time to determine platelet function
Types:
General Measures
RCC Transfusions
Platelet Transfusions
Thrombocytopenia: Treatment Principles
General Measures
Reduce risk of intracranial haemorrhage – bed rest – ensure soft stools – cough suppression – control hypertension
Avoid aspirin containing drugs
Avoid intramuscular injections
Suppress menses
Thrombocytopenia: Treatment Principles
RCC Transfusions
Maintain normal Hb level
Thrombocytopenia: Treatment Principles
Platelet Transfusions:
Standard treatment for bleeding associated with
thrombocytopenia and /or defective platelet function in conditions such as:
– Bone marrow failure e.g. aplastic anaemia, acute
leukaemia
– Massive transfusion with dilutional thrombocytopenia
– Acute DIC
– Congenital disorders of platelet function
Role of prophylactic platelet transfusions less well defined
Spontaneous bleeding unusual at counts >5x109/l
Widely accepted transfusion triggers:
- Threshold of 10x109 /l for adult stable patients
- Threshold of 20x109 /l for patients at increased bleeding risk:
-Anatomic lesions e.g. peptic ulcer
-Fever/sepsis
-Recent severe haemorrhage/bleeding from mucous
membranes
-Anticoagulant therapy
-On drugs affecting platelet function
-Severe anaemia
Threshold of 50 x109 /l for most surgical procedures e.g. laparotomy, liver biopsy
100 x109 /l for CNS surgery, ocular surgery
Massive transfusion: maintain platelet count at >50x 109 /l
Multiple trauma and head injury, maintain platelet count at > 100x109 /l
Cardiopulmonary bypass - transfuse only in the presence of microvascular bleeding and platelet count < 100 x109 /l
Chronic stable thrombocytopenia e.g. aplastic anaemia, prophylactic transfusions are generally not indicated
Not required for BM aspirate/biopsy. Application of local pressure is sufficient.
Thrombocytosis:
Causes
- Myeloproliferative Disorders
- Reactive
- Post-splenectomy
Thrombocytosis: Causes
Myeloproliferative Disorders
Essential thrombocythaemia (ET)
Polycythaemia vera (PV)
Chronic myelogenous leukaemia (CML)
Myelofibrosis
Thrombocytosis: Causes
Reactive
Infection, inflammation
Bleeding
Surgery, trauma
Iron deficiency
Malignancy
Thrombocytosis: Causes
Post-Splenectomy
Large platelets,Howell-Jolly bodies)
Platelet Function Abnormalities
Inherited
Acquired:
- Drugs
- Uraemia-Renal Failure
Von Willebrand Disease
Deficiency of VWF amount or function
Lab Results - Prolonged BT, abnormal
Von Willebrand Factor testing
Von Willebrand Disease:
Etiology
Von Willebrand Factor – Synthesis in endothelium and megakaryocytes – Forms large multimeres – Carrier of factor VIII – Anchors platelets to sub-endothelium – Bridge between platelets
Incidence - 1/10,000
Clinical features - mucocutaneous bleeding
le is controversial
Von Willebrand Disease:
Laboratory Evalutation
Based on [VWF Ag], functional assays and multimere
pattern
Classification
– Type 1 Partial quantitative deficiency
– Type 2 Qualitative deficiency
– Type 3 Total quantitative deficiency
Blood Vessel Abnormalities
INFECTIONS
– Typhoid fever, meningococcal septicaemia
SCURVY
CUSHING’S DISEASE AND STEROID THERAPY
PURPURA SIMPLEX
SENILE PURPURA
ALLERGIC PURPURA
– Henoch-Schönlein-purpura
DRUGS
Von Willebrand Disease:
Diagnosis
- Clinical History
- FBC and Peripheral Smear
- Often Normal
- Morphology can give a clue
Von Willebrand Disease:
Diagnosis-Bleeding Time
µL)Hereditary and acquired platelet dysfunctionsVon Willebrand Disease (VWD)Afibrinogenaemia, severe hypofibrinogenaemiaSome vascular bleeding disorders e.g. hereditarycollagen abnormalities
Von Willebrand Disease:
Diagnosis-Platelet Function Analyser(PFA-100)
Whole blood
Measures platelet function under high shear
conditions similar to the environment of a partially occluded blood vessel
•
Von Willebrand Disease:
Diagnosis-Platelet Aggregometry
Platelet rich plasma, whole blood
Von Willebrand Disease:
Diagnosis-Flow Cytometry
Tests for specific platelet receptors:
CD41 (GPIIb/IIIa)
CD42b (GPIb)
CD61 (GPIIIa)
Anti-Platelet Drugs
Aspirin (Disprin®): irreversibly inhibits cyclo-oxygenase
Clopidogrel (Plavix®), prasugrel (Efient®): irreversibly
inhibits platelet’s ADP receptor (P2Y12 receptor)
Abciximab (ReoPro®), Eptifibatide (Integrilin®), Tirofiban
(Aggrastet®): Glycoprotein IIb/IIIa antagonists
Dipyridamole (Persantin®): ↑ platelet [c-AMP] thereby
inhibiting platelet function, but role is controversial
