Haemostasis Flashcards
The immediate arrest of haemorrhage depends on?
Vasoconstriction Adhesion Aggregation Leading to the platelet plug Then the activation of the coagulation system Formation of the fibrin clot
main points on what happens in primary haemostasis
vasoconstriction to slow blood flow
vasodilation and increased permeability to allow leukocytes to enter site of injury
platelets activate and become sticky due to activation factors (turbulence of blood flow, contact with damaged blood vessel)
von Willebrand factor mediates platelet linking to collagen via platelet receptor GPIb
GPIb causes platelet to change from discoid to spherical cells with pseudopodia, allowing interaction with other platelets
activated platelets release ADP and TXA2 which induces aggregation, recruitment of more platelets and upregulation of expression of glycoproteins
that causes the binding of fibrogen, allowing crosslinking and aggregation of platelets
main points on what happens in secondary haemostasis
extrinsic pathway activated by damaged cells, releasing tissue factor
Exposure of TF binds to factor VII activating factor X
intrinsic pathway activated by blood contacting collagen fibres from broken vessel wall
common pathway activated by prothrombin which is produced by extrinsic and intrinsic pathways
common pathway leads to thrombin production
thrombin stimulates further aggregation (positive feedback) and converts soluble fibrinogen to insoluble fibrin
this creates a mesh which traps leukocytes, erythrocytes and platelets = strengthened clot
Fibrinolysis occurs generating plasmin which dissolves the clot. The fibrinogen split products generated prevent further clotting by inhibiting fibrin polymerisation and platelet aggregation.
what is the aim of primary and secondary haemostasis
primary- slowing of blood preventing blood loss and formation of a plug
secondary- activation of the coagulation cascade and strengthening the plug
what does healthy endothelium express/produce in order to block platelet adhesion
expresses:
ecto-ADPase (CD39)
produces:
prostacyclin (PGI2)
nitric oxide (NO)
what drug inhibits platelets?
asprin
what is a quantitative platelet disorder of primary haemostasis
thrombocytopenia or thrombocytosis
what is a qualitative platelet disorder of primary haemostasis
functional defect can be inherited or acquired.
what is an acquired platelet disorder of primary haemostasis
drugs, alcohol, uremia and myeloproliferative disorders
facts about Thrombocytopenia
<150x109/L platelets
bleeding occurance:
Minor >10x109/L.
Spontaneous <10x109/L.
Clinical bleeding doesn’t always correlate with platelet count due to other factors e.g. endothelium integrity and platelet functionality.
what is Thrombocytopenia
Abnormally low levels of thrombocytes, also known as platelets, in the blood.
what is a Normal platelet count
150-350 x109/L
what are the platelet counts for people with mild, moderate and severe Thrombocytopenia
Mild (50-150),
moderate (20-50)
severe(<20) - more prone to bleeds
platelet numbers when bleeding rarely occurs?
Bleeding rarely occurs >50x109/L.
name all the main diseases of haemostasis and briefly outline what they are
Primary haemostasis
Thrombocytopenia - low (<150x109/L) platelets
Secondary haemostasis
Haemophilia (A/B/C) -
Cant form blood clots
how do we diagnose Thrombocytopenia, what do we look at?
Full blood count.
Blood film. Number, size, colour of platelets.
Platelet clumping – citrate sample
Large platelets – congenital thrombocytopenias
Small platelets – Wiskott-Aldrich
Red cell fragments – thrombotic microangiopathy. (thrombosis in capillaries)
Hypogranular neutrophils (no pink granules) – myelodysplasia (cancers where cells don’t mature)
thrombocytopenia etiology?
Exclude congenital thrombocytopenia’s - previous normal count.
Exposure to drugs e.g. Alcohol or quinine.
Viral infections e.g. HIV or CMV.
Hypothermia can cause this too as platelets will go into the spleen if too cold so not in circulation
Post operative – dilutional – will resolve, cardiac surgery – damage to platelets, may persist.
autoimmune – ITP, Anti-phospholipid syndrome and post transfusion purpura.
what are the treatments for Immune thrombocytopenic purpura (ITP) and how do they work
Anti-D
IgG
Rituximab
cyclophosphamide splenectomy
1/3 of platelets go into the spleen - allows them to go back into circulation
signs of Immune thrombocytopenic purpura (ITP)
Petechial rash or oral bleeding.
What is thrombotic microangiopathies
is a pathology that results in thrombosis in capillaries and arterioles, due to an endothelial injury
(Ischaemic injury to one or more organ or tissue.)
Thrombotic microangiopathies (TMAs) are a group of disorders characterized by microangiopathic hemolytic anemia (fragmented red cells, schistocytes, in peripheral blood) and thrombocytopenia.
what does Thrombotic microangiopathies include
TTP, HUS, HELLP(Haemolytic anaemia with Elevated Liver enzymes and Low Platelet count) and HIT.
what is the treatment for Thrombotic microangiopathies
TTP – plasma exchange
HUS – withdrawal of drugs
HELLP- delivery of fetus and placenta
HIT – cessation of heparin and administration of antithrombotic agents.
what is Bernard Soulier syndrome
rare autosomal recessive bleeding disorder that causes a deficiency of glycoprotein Ib (GpIb), the receptor for von Willebrand factor = Poor platelet adhesion and aggregation.
what is Glanzmann thrombasthenia
Glanzmann thrombasthenia is a platelet function disorder that is caused by an abnormality in the genes for glycoproteins IIb/IIIa.
= Fails to bind fibrinogen.
Platelet count is normal but no aggregation with agonists e.g. ADP, epinephrine and collagen.
positive feedback in the coagulation cascade
thrombin stimulates further aggregation (positive feedback)
Thrombin activates FXI on the platelet surface which can activate FIX to enhance FXa generation.
High levels of thrombin can cleave PAR4 – plt shape change – Stabilisation of platelet plug.
High levels of thrombin generated at propagation phase bind to fibrin and are protected from inhibition by antithrombin.
thrombin role in the coagulation cascade
localisation
Thrombin released from the platelet plug is swept downstream- antithrombin – half life of thrombin <1 minute.
FXa is rapidly inhibited by TFPI.
Thrombin on healthy endothelial cells participate in a negative feedback loop. Thrombin binds to thrombomodulin causing a conformational change – no longer cleave fibrinogen.
This is rapidly inhibited by protein C inhibitor. Rapidly dissociates so thrombomodulin can bind thrombin and activate protein C. Activated protein C and S inactivate factor Va and FVIIIa. This confines thrombin generation to site of injury in healthy endothelium.
Deficiencies of protein C and S or defects that prevent cleavage and inactivation of FV (FV Leiden) allow for what?
the spread of thrombi into the vasculature and are associated with venous thrombisis.
Fibrinolysis?
Plasmin cleaves the fibrin network and releases FDP (D and E). Also cleaves fibrinogen – fragment X and Y, can impair plt aggregation and fibrin polymerisation.
Fibrinolysis defects.
Hyperfibrinolysis – disposes to bleeding and thrombosis.
Hypofibrinolysis – deficiencies of t-PA or u-PA, plasminogen, contact factors. Associated with thromboembolic disease
which defect factor causes haemophilia A
factor VIII
X-linked (MEN effected, women carriers)
presentation of haemophilia A
Common sites - joints, muscles, brain.
Repeated hemarthroses leads to joint destruction. Compartment syndrome leading to necrosis and muscle shortening.
treatment of haemophilia A, complications and new studies
dependant on the patient - requires a lot of monitoring to ensure treatment still working
Treat until bleeding stops
Factor VIII Concentrates Recombinant Prophylaxis (Thrice weekly.)
Complication is the development of inhibitors (10-15%).
Studies underway with novel antibodies to mimic FVIII
Combined FV and FVIII defectiveness haemophilia A?
Moderate bleeding tendency.
FV and FVIII levels - 5-30%.
Normally due to a mutation in transport protein .
Treatment FVIII concentrate, FFP/ Octaplas
which defect factor causes haemophilia B
Factor IX.
Similar clinical picture to Haem A. Recurrent spontaneous joint bleeds occur.
Bleeding in carriers more common.
Treatment – Berifix. Once daily. Refixia –pegulated FIX – Different assay needed.
Inhibitors are less common.
One of the rarest autosomal recessive for haemophilia B
Factor X
which defect factor causes haemophilia C and the mutations?
Factor XI
Heterozygous have a partial deficiency, homozygous or compound heterozygous have severe deficiency.
3 mutations majority type II or III
Type II – stop codon in exon 5. homozygous state results in levels ~1%.
Type III Phe283 is replaced with Leu (missense mutation) results in levels ~10%.
Type II/III heterozygotes result in levels ~3%
what can FXI deficiency lead to
FXI deficiency can lead to excessive haemorrhage after surgery or trauma but does not cause bleeds into joints or muscles.
info on factor XIII
Rare autosomal recessive.
Homozygotes present with life long bleeding from the umbilical cord.
Spontaneous intracranial bleeds common. Spontaneous abortions in early pregnancy. Delayed wound healing.
Acquired Deficiencies? name them
Factor II – Autoantibody due to lupus- type anticoagulant.
Factor V – transient after surgery, transfusion or aminoglycoside antibiotics.
Factor VII – Very rare.
Factor IX – very rare associated with post partum and SLE.
Factor X – Amyloidosis.
acquired deficiency Factor VIII presentation and treatment
Presentation occurs with large haematomas. Treatment involves immunosuppressants
liver disease effects?
Prothrombin Time increases due to a decrease of extrinsic coagulation factors synthesised in the liver. As liver damage increases the APTT will also be affected. Also affects fibrinogen synthesis.
Vitamin K deficiency effects?
This affects the production of Vitamin K dependent factors ( FII, FVII, FIX, FX) causing an increase in the Prothrombin Time.
which haemophilia is not based on the amount of its factor but the functionality of it ? name factor
haemophilia C and factor XI
what environmental factors can effect the levels of coagulation factors in the blood
running around before sample extraction
Once abnormal coagulation results have been found second line tests can be done to investigate the cause what are these
Thrombin Time (TT). Reptilase Time (RT) Protamine Time. Correction Tests. D-Dimer assay. Factor Assays. Lupus anticoagulant.
what does warfarin do
Inhibits the vitamin K-dependent posttranslational modification of coagulation factors.
what does heparin do
Heparin complexes with Anti-Thrombin III to inhibit thrombin and increase the clotting time
what are some new oral anticoagulants
Thrombin inhibitors:
Dabigatran (14-17hrs)
PT^, APTT^, TT^
Xa inhibitors:
Rivaroxaban (7-11hrs)
Apixaban (8-15hrs)
PT^, APTT^, TT not affected
