Review of Haemostasis for Medical Students

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Review of Haemostasis for Medical Students
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Topics

• Physiology of haemostasis
• Bleeding disorders (congenital)
• Massive blood loss
• DIC
• Anticoagulant drugs
• rVIIa

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Haemostasis needs

• Platelets
• Vessel Wall
• Clotting Factors

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FIG 1 Normal blood vessel
erythrocyte platelet lumen leukocyte
Intima/media with endothelial cell basal
membrane smooth muscle cells collagen fibers
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• FIG 2
• Injured blood vessel
• Exposure of collagen
• Adhesion of platelets mediated by vWF
• Activation and de-granulation of platelets
• Aggregation of platelets

erythrocyte non-activatedplatelet lumen endotheli
al cell basal membrane activated
collagen-boundplatelet collagen fibers smooth
muscle cells
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FIG 3 Platelet plug stabilization by fibrin
lumen endothelial cell basal membrane erythrocytes
activatedplatelets plug sealing and
stabilization by fibrin formation
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Normal Platelet / vWF Function

• Release of
• ADP
• serotonin
• calcium
• etc

• Release of
• glycoproteins
• vWF
• fibrinogen
• coagulation factors

platelet
d-granule
a -granule
GPIb-IX receptor-complex
von Willebrand - Factor
fibrinogen
GPIIb-IIIa receptor-complex
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Von Willebrand factor

• VWF is a large complex molecule. It has three
  main functions
• 1)It sticks to platelets (via GpIb)
• 2)It sticks to collagen 1) 2) means it lets
  platelets stick to collagen
• 3)It binds VIII and protects it from degrading
  and therefore increases its half-life
• VWF is made by endothelial cells and
  megakaryocytes (not the liver like the other
  factors).
• When released into the circulation it is in the
  form of Ultra-large molecular weight multimers
  (like a large bundle of straw).
• Ultra-large multimers of VWF are processed in the
  circulation into smaller High-molecular weight
  multimers, these are important for platelet
  interactions. Smaller multimers are also found in
  the circulation as a result of continued
  processing.

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Von Willebrand factor

• A lack of High-molecular weight multimers of VWF
  can reduce the platelet-collagen interaction and
  lead to a bleeding diathesis.
• Some patients with VWD have low (type I) or very
  low (type III) levels of a normal VWF molecule.
  Other patients have normal levels of a VWF
  molecule which lacks one (or many) of the three
  functions above or cannot form multimers properly
  (type II).

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Traditional view
Intrinsic System
Extrinsic System
Collagen / Kallikrein
Prekallikrein
HMWK
XII
XIIa
VII
XI
XIa
Tissue Factor
X
Phospholipid VIII
VIIa Tissue Factor
IX
IXa
2
2
Ca
Ca
Fibrinogen
Xa
2
V Phospholipid Ca
Final
Prothrombin
Thrombin
Common
Pathway
Fibrin
XIII
XIIIa
Ca
2
Cross Linked Fibrin
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New view
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PT
The starter motor..
Switched off by Tissue Factor Pathway Inhibitor
Provides initial Thrombin burst
Factors measured in Prothrombin Time
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The engine.. Thrombin from initial burst back
activates intrinsic system
APTT
TT
Fibrin then cross linked by XIII
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Natural anticoagulants

• Protein C (activated by thrombin/thrombomodulin)
• Protein S - cofactor for protein C
• Protein C and S cleave factors V and VIII
• Antithrombin inhibits Thrombin and Xa
• TAT complexes removed by liver
• Activity increased 000s by heparin

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Global coagulation tests

• APTT Kallikrein, HMWK, XII, XI, IX, VIII, X, V,
  II, I
• intrinsic system
• PT VII, X, V, II, I
• extrinsic system
• TT I
• NB reason for 5050 mix (inhibitors versus
  deficiency)
• Protamine correction
• Reptilase time
• Specific factor assays
• Bleeding time, PFA-100, TEG

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CAUSES OF A PROLONGED PROTHROMBIN TIME

• CONGENITAL
• Coagulation factor deficiencies VII, X, V, II, I
• ACQUIRED
• Hepatocellular disease
• Vitamin K deficiency (II, VII, IX, X)
  obstructive jaundice, haemorrhagic disease of the
  newborn
• Disseminated intravascular coagulation (DIC)
• Massive blood transfusion
• Warfarin (monitoring test based on PT)
• Gross overheparinisation, some lupus
  anticoagulants

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CAUSES OF A PROLONGED ACTIVATED PARTIAL
THROMBOPLASTIN TIME

• CONGENITAL
• Coagulation factor deficiencies XII, XI, IX,
  VIII, X, V, II, I
• ACQUIRED
• Hepatocellular disease
• Vitamin K deficiency
• Disseminated intravascular coagulation
• Massive blood transfusion
• Heparin (monitoring test based on APTT)
• Lupus anticoagulants

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CAUSES OF A PROLONGED THROMBIN TIME

• CONGENITAL
• Dys/hypofibrinogenaemia
• ACQUIRED
• Hepatocellular disease dys/hypofibrinogenaemia
• Disseminated intravascular coagulation
• hypofibrinogenaemia
• FDPs
• Heparin

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Fibrinolysis
Circulating antiplasmin and PAI I inhibit this
system. Their effect is not present in the milieu
of the clot which protects tPA and Plasmin
form their actions
Plasmin interaction with Fibrin is Lys residue
dependent. TAFI removes such residues. TA and
EACA act as Lys analogues.
tPA or uPA
Plasminogen
Plasmin
Fibrin degradation products
Fibrin
(If x-linked get D-D dimers)
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Platelets

• The job of platelets is to
• Form a plug in a breech in a blood vessel wall to
  arrest haemorrhage
• Provide a phospholipid surface rich in negatively
  charged phospholipid for coagulation factors to
  interact (tenase and prothrombinase complexes).

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Platelets

• Platelets have receptors for components of the
  subendothelium, which is exposed when a blood
  vessel is cut. A very important initial
  interaction is that between platelet glycoprotein
  IbIX-V, VWF and collagen.
• The above triggers activation of the platelets,
  in particular IIb-IIIa receptors in an active
  conformation are moved to the surface. This
  receptor facilitates platelet cross-linking via
  fibrinogen.
• Activated platelets release contents from their
  granules including ADP, which in turn activate
  more platelets.
• Activation of the platelet Arachadonic acid
  pathway results in the release of thromboxane A2.
• Activated platelets flip-flop their membrane
  phospholipids to provide a negatively charged
  surface.

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Haemophilia

• A reduced VIII B reduced IX
• Both sex-linked recessive
• Queen Victoria
• Intronic rearrangements, point mutations, gene
  deletions
• Haemophilia A 1 in 10,000 male infants
• Prolonged APTT (normal PT and TT)
• Mild Rx Tranexamic acid, DDAVP (not HB)
• Severe Rx factor replacement recombinant or
  pooled donor
• Home prophylaxis
• Past problem with HIV, now HCV ??? CJD

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Von Willebrands Disease

• Functions of VWF
• 1. Sticks to platelets (GPIb)
• 2. Sticks to collagen in subendothelium
• (Important in small blood vessel lesions high
  shear stress)
• 3. Binds to and protects VIII (labile)
• Therefore in VWD see long APTT (low VIII) and
  bleeding where VWF platelet interaction important

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Von Willebrands Disease

• Type 1 mild- moderate quantitative deficiency
• Type 2 qualitative changes (functional)
• Type 3 severe deficiency
• Type 2N reduced VIII binding in isolation
• Type 2A absent HMW multimers
• Type 2B increased affinity for platelet GPIb
• Type 2M HMW multimers present

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Von Willebrands Disease

• Treatment options
• DDAVP, antifibrinolytics
• NB DDAVP causes fluid retention. NOT for type 2B
• Intermediate purity VIII concentrate
• VWF concentrate (NB takes hours for VIII to
  follow so may need to give both)
• (NB type I may auto-correct in pregnancy)

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Massive blood loss

• Defined as loss of gt one circulating volume in 24
  hours
• Coagulopathy is multifactorial
• Loss of factors only once 80 of volume replaced
• Dilution of factors during fluid resuscitation
• Inhibitory effect of some colloids on clotting
  factors
• DIC secondary to trauma
• Acidosis
• Hypothermia (enzymes) (blood warmer)

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Massive blood loss

• Regular checks of FBC and PT,APTT,TT and
  Fibrinogen
• Aim for platelets gt 50x109/l or gt100x109/l if
  polytrauma or CNS injury
• Aim for fibrinogen gt1g/l
• Aim for PT and APTT lt1.5x control times
• FFP 12-15ml/kg
• Cryoprecipitate 1-1.5 packs /10kg if fibrinogen
  fails to correct with FFP
• ? rVIIa
• Stainsby D, MacLennan S, Hamilton PJ. Management
  of massive blood loss a template guideline.Br J
  Anaesth. 2000 Sep85(3)487-91.

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Disseminated intravascular Coagulation

• ..an acquired syndrome characterised by the
  intravascular activation of coagulation with loss
  of localisation arising from different causes.
• DIC can originate from and cause severe damage to
  the microvasculature, which if sufficiently
  severe, can produce organ dysfunction.
• ISTH definition

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Causes of DIC

• Conditions associated with overt DIC
• sepsis/severe infection (any organism)
• trauma (e.g. polytrauma, neurotrauma, fat
  embolism)
• organ destruction (e.g. severe pancreatitis)
• malignancy
• massive blood loss with inadequate fluid
  replacement therapy
• vascular abnormalities
• (e.g. Kassbach-Merrit syndrome)
• severe hepatic failure
• severe toxic or immunological reactions
• (e.g. recreational drugs, transfusion reactions,
  transplant rejection)

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Pathogenesis of DIC

• Basically.
• Excess thrombin generation
• Reduced natural anticoagulant activity
• Decreased fibinolysis

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Excess thrombin generation

• Increased tissue factor (monocytes and
  endothelial cells due to pro-inflammatory
  cytokines.
• Tissue thromboplasin (the stuff in the PT reagent
  from damaged tissue or malignant tissue)
• Direct activation of clotting factors by snake
  venoms

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Reduced natural anticoagulant activity

• Low antithrombin (increased TAT clearance and
  decreased liver biosynthesis)
• Lower protein C activity (reduced thrombomodulin,
  low protein S (bound to c4b binding protein),
  reduced synthesis)

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Decreased fibinolysis

• Increased PAI I
• Levels correlate with outcome in meningococcal
  sepsis

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Clinical features

• Mucosal oozing, bleeding from surgical wounds or
  indwelling canulae
• Multi organ failure secondary to microthrobi (and
  hypovolaemia)

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Diagnosis
Diagnostic algorithm for the diagnosis of overt
DIC Does the patient have an underlying disorder
known to be associated with overt DIC? (If yes,
proceed if no, do not use this algorithm)
Order global coagulation tests (platelet
count, PT, fibrinogen, soluble fibrin monomers
(SFM) or fibrin degradation products
(FDP). Score coagulation test results Platelet
count (gt1000 lt1001, lt502) Elevated FDP or SFM
(no increase0 moderate increase2 strong
increase3) Prolonged PT (by lt3 seconds0 gt3 but
lt6 seconds1 gt6 seconds2) Fibrinogen level
(gt1g/l0 lt1g/l1) Score gt5 compatible with
overt DIC Score lt5 suggestive (not affirmative)
of non-overt DIC repeat tests in 1-2 days
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Management of DIC

• TREAT THE CAUSE
• Fluid resus as needed, antibiotics if sepsis
• If bleeding or need surgery give FFP, Platelets,
  Cryoprecipitate
• (Aim Plateletsgt50x109/l, PT and APTT lt 1.5x
  normal)
• If thrombotic manifestations eg. Dermal
  ischaemia consider low dose heparin infusion.

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Heparin
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Heparin

• Unfractionated
• monitor with APTT (ratio 1.5-2.5 - base on
  patients baseline)
• reversal by stopping infusion and very rapid with
  protamine sulphate
• can be hard to anticoagulate some children due to
  low antithrombin levels
• bolus then continuous infusion
• Low molecular weight
• less monitoring
• once or twice daily administration
• more reliable pharmacokinetics (NB renal
  excretion)
• anti-Xa levels - 4h post dose
• Treatment 0.5-1 Prophylaxis 0.1-0.3 IU/ml

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Warfarin
II, VII, IX, X, protein C and S are vitamin K
dependent. NB C and S fall first so overlap
with heparin (Warfarin induced skin
necrosis) Peri-surgical management of patients
on warfarin
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Other anticoagulant agents

• Aspirin
• Clopidogrel
• Abciximab
• NSAIDS
• Fibrinolytics

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rVIIa
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rVIIa

• VIII inhibitors
• massive blood loss
• inherited platelet disorders
• Jehovahs witnesses
• 90 microg/Kg every 2 hours
• normalise other things (platelets, Hb,
  fibrinogen, factors) as possible
• consider mega-dose
• NB consent for non-licensed indications
• EXPENSIVE

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rVIIa

• 1 vial of rVIIa (1.2 mg) 702
• 1 vial of rVIIa (2.4 mg) 1404
• 1 unit of blood from 100
• 1 pool of platelets from 190
• So 35 Kg boy needs 3150 microg so 3 vials opened
  each dose
• 3 x 702 2,106
• 2 hourly for one day 2106 x 12 25,272

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Questions