Haemostasis & Blood Coagulation

Haemostasis
1. Vascular constriction
a. Local myogenic spasm/contraction
b. Vasoconstrictor thromboxane A2 released by PLTs
2. Platelet plug
a. Contact with subendothelium/collagen  swell, pseudopod formation, release granules,
adhere to vWF which leaks from subendothelium, release thromboxane A2
b. ADP and thromboxane activate nearby platelets
c. Later, fibrin threads form
3. Formation of clot as a result of coagulation
a. Begins to develop in 15-20 seconds (1-2 mins if trauma was minor)
b. Traumatised vessel wall releases activator substances
c. Clot retracts
4. Eventual fibrous tissue growth into clot

Platelets
- Actin and myosin in cytoplasm, and thrombosthenin (also contractile)
- Can form ATP and ADP
- Synthesise prostaglandins
- Fibrin-stabilising factor
- Growth factor
- Surface glycoprotein – adheres to damaged endothelium and exposed collagen
- Phospholipids (activate clotting later)
- ½ life of 8-12 days
- Eliminated by tissue macrophages (>50% removed in spleen)

Clotting factors

Clotting factor Synonyms Implicated in..

Fibrinogen Factor I
Prothrombin Factor II
Tissue factor Factor III (tissue thromboplastin)
Calcium Factor IV
Factor V Proaccelerin
Labile factor
Ac-globulin
Factor VII Serum prothrombin conversion
accelerator; proconvertin; stable
factor
Factor VIII Antihaemophilic factor; Haemophilia
antihaemophilic globulin;
antihaemophilic factor A
Factor XI Plasma thromboplastin Christmas disease
component; Christmas factor,
antihaemophilic factor B
Factor X Stuart factor
Factor XI Antihaemophilic factor C; plasma
thromboplastin antecedent
Factor XII Hageman factor
Factor XIII Fibrin stabilising factor
Prekallikrein Fletcher factor
HMW kininogen Fitzgerald Factor (HMWK)
Platelets
Prothrombin, thrombin, fibrinogen, fibrin
1. Prothrombin activator (complex of activated substances) is formed after rupture of vessel
2. In the presence of enough Ca++, prothrombin activator converts prothrombin to thrombin
3. Thrombin causes polymerisation of fibrinogen into fibrin fibres within 10-15s i.e. quickly
4. Therefore formation of prothrombin activator is the rate limiting step
5. Prothrombin (factor II) is a plasma protein, formed in the liver, continuously. Falls within a day or two of
liver failure. Vitamin K is required in the formation of prothrombin and factors 7, 9, 10
6. Fibrinogen (factor I) is also formed in the liver, so liver disease can also decrease circulating fibrinogen
7. Thrombin acts on fibrinogen to form fibrin monomer, which polymerises with other fibrin monomers to
form polymers making the reticulum of the blood clot
8. Fibrin stabilising factor stabilises the reticulum (found in plasma and released by platelets)

Prothrombin activator: extrinsic and intrinsic pathways

Extrinsic
- Trauma to the vascular wall and surrounding tissues
- Release of tissue factor (factor III)
- Tissue factor complexes with factor VII and together activates factor X ( factora Xa)
- Factor Xa combines with factor V to form prothrombin activator (other components involved), and in the
presence of calcium ions, prothrombin is split to form thrombin  fibrinogen to fibrin
- Thrombin activates factor V, which then accelerates prothrombin activation
- i.e. factor X causes splitting of prothrombin to thrombin, and factor V accelerates this process
The prothrombin time is a measure derived from the prothrombin ratio (PR) and the international normalised
ratio (INR). This is used in checking if the dose of warfarin is correct. The prothrombin time measures the
extrinsic pathway of coagulation; this includes measuring clotting factors 2, 5, 7, 10 and fibrinogen. The normal
value is 12-15 seconds (depending on your control).

Intrinsic
- Trauma to the blood itself or exposure to collagen
- Blood trauma  activation of factor XII and release of platelet phospholipids
- Factor XII also activated by contact with collagen or wettable surface e.g. glass
- Factor XIIa activates XI
- Factor Xia activates factor IX
- Factor IX acts along with factor VIII and with platelet phospholipids to activate factor X
- When factor VIII or platelets are deficient, the above step is affected
- Activated factor X combines with factor V and with platelet/tissue phospholipids to form prothrombin
activator (same step as in extrinsic pathway)
 -

The activated partial thromboplastin time measures the intrinsic pathway of clotting. It measures the clotting
factors - 8, 9, 11, 12, and is used in von Willebrand's disease. It is used to check if the dose of heparinis correct.
The normal value is 25-39 seconds (depending on your control).

Intrinsic vs extrinsic
- Extrinsic triggered by tissue factor from tissue
- Intrinsic triggered by contact of factor XII and platelets with collagen
- Extrinsic can be explosive, within 15 seconds
- Intrinsic pathway is much slower, requiring 1-6 minutes to cause clotting

Prevention of haemostasis
- Endothelial surface: Thrombomodulin: binds to thrombin and inhibitis it, also activates protein C, which
inactivates Factor V and Factor VIII
- Fibrin: thrombin is adsorbed to fibrin fibres, helping to limit the clot
- Antithrombin III (antithrombin-heparin cofactor): combines to thrombin and blocks its effect on
fibrinogen, and inactivates the thrombin shortly after

Heparin
- When combined with antithrombin III, the effectiveness of antithrombin III increases a thousandfold (in
removing thrombin). Together, they also remove other factors
- Heparin is produced by many tissues, especially mast cells, especially in the lungs and liver

Lysis of clots
- Plasminogen, when activated, becomes plasmin (or fibrinolysin) and digests fibrin, fibrinogen and
certain factors
- Therefore plasmin can lyse clots
- Plasminogen is in the plasma, and becomes trapped in a clot
- Injured tissues and vascular endothelium slowly release t-PA that works a few days later, converting
plasminogen to plasmin, which removes the remaining clot
Bleeding Disorders
1. Vitamin K deficiency
2. Haemophilia
3. Thrombocytopenia
4. vWBF deficiency
5. Disorders of endothelium

1. Vitamin K deficiency
Leads to ↓ prothrombin (factor II), factor VII, factor IX and factor X
Also involved in production of protein C (anticoagulant)
Vitamin K is synthesised in the intestinal tract by bacteria
Therefore deficiency is caused by:
1. Vitamin K absence from diet
2. In the neonate before they establish gut flora
3. GI disease, where there is poor absorption of fats from the GIT (since vitamin K is fat soluble and usually
absorbed along with fat)
- This can include biliary obstruction or liver disease, because lack of bile prevents fat
digestion/absorption
- When vitamin K is given, it takes 4-8 hours before sufficient clotting factors are produced, e.g. to go
ahead with a procedure

2. Haemophilia
X-linked and occurs almost exclusively in males
- Females can be affected in X chr lyonisation or gene abnormalities e.g. Turner syndrome
Incidence 1:5000 male births
Can be deletions, insertions, missense, nonsense, splice site mutations or inversions
In 85% it is caused by an abnormality or deficiency in Factor VIII  haemophilia A or classic haemophilia
The other 15% have Christmas disease (factor IX deficiency) – 1:25,000
Haemophilia C: AR, decreased factor XI, 1:100,000

Pathophysiology
- Factor VIII is a plasma glycoprotein, synthesised in liver endothelial cells
- Upon release, immediately linked to vWF preventing degradation until needed
- Factor IX is vitamin K dependent, also produced in liver, and is responsible for activating factor VIII

Presentation
- Bleeding usually doesn’t occur unless there is trauma, but in some patients it can occur with very
minor/unnoticeable trauma
- Hallmark is delayed bleeding, joint and muscle haemorrhage
- E.g. tooth extraction can lead to bleeding for days
- Factor VIII has two active components: large component and small
o Small is important in intrinsic factor, and this component is deficient in haemophilia
o The large component is lost in vWBD
- Treament is purified factor VIII
- In neonate can cause intracranial bleeding, and bleeding after immunisation etc.
- Older children: bruising, haematomas, intracranial, joint, muscle and mouth bleeding

Complications
1. Haemophilic arthropathy
o Single largest preventable cause of morbidity
2. Inhibitor development
o Inhibitory antibodies develop to the infused factor in 30% of pts with haemophilia A
o May have a strong FHx
o Occur within the first 20 exposures to FVIII
o Antibody measured as Bethesda units, may need to treat with high dose recombinant FVIIa or
prothrombinex

Classification
1. Mild = >5% factor levels
2. Moderate: 1-5%  bleeding with trauma or surgery
3. Severe <1%  spontaneous bleding

Diagnosis
- N.B. factor VIII is involved in the INTRINSIC not the extrinstic pathway, therefore haemophilia will cause
↑ aPTT
- ↓ plasma factor VIII, IX or XI assay (for A, B and C respectively)

Management
- IV factor concentrates
o Demand therapy (after bleeding: ASAP)
o Prophylaxis to keep levels >1%
 Primary: when commenced before arthropathy develops
o Can give continuous infusion e.g. after a CNS bleed
- Desmopressin (DDAVP)
o Synthetic vasopressin
o Causes release of FVIII and vWF from storage sites in endothelium and in platelet granules
o Try on pts with mild-mod haemophilia A
o Repeated use  depletion of storage pools
o Complications; severe hyponatraemia, so monitor fluid intake
- Antifibrinolytic therapy if bleeding symptoms e.g. in mouth, GIT, uterus
- Haemophilia C: give FFP, antifibrinolytic agents
- Thrombin powder (topical e.g. for oral bleeds)
- Epistaxis: packing, cautery, vasoconstrictor spray
- Glucocorticoids for haematuria
- Joints/muscles: RICE

3. Thrombocytopenia
Presentation
- Bleeding is from many small venules or capillaries  small punctuate haemorrhages throughout the
body “thrombocytopenic purpure”
- Usually bleeding doesn’t occur until platelets are < 50,000/uL

Cause
- Usually idiopathic thrombocytopenia – due to antibodies against platelets

Management
- Can manage with fresh whole blood transfusions containing large numbers of platelets
- Splenectomy sometimes helpful

Acquired platelet pathology

- Antiplatelet drugs e.g. aspirin (blocks COX-1 and thromboxane A2 synthesis, impairing platelet
aggregation)  aspirin has permanent effects
- NSAIDs also inhibit COX-1 but are reversible
- Valproate can cause platelet dysfunction, thrombocytopenia
-

4. von Willebrand disease

Epidemiology
Most prevalent hereditary bleeding disorder (up to 1% of population)

Pathophysiology
- vWF has two functions
o mediates adhesion of PLTs to endothelium and to each other
o Carrier protein for FVIII, prevents its degradation
- vWF gene is on chr 12p
- usually AD, but can be recessive/doubly heterozygous

3 types:
o Type 1: reduced amount (partial quantitative deficiency)
o Type 2: qualitative defect (4 variants)
o Type 3: absence

Presentation
 - Mucosal bleeding symptoms are prominent
o Bruising, epistaxis, oral, menorrhagia
- Bleeding post-procedure or surgery
- Usually will have a positive FHx

Diagnosis
- Screening tests (PT, aPTT, fibrinogen, thrombin time, platelet functions) are often NORMAL
- Can look for qualitative/quantitative vWF defects in lab
- vWF antigen, ristocetin cofactor, FVIII activity, vWF multimer tests (looking for low levels)

DDx:
- Type O blood and hypothyroidism can be assoc with low vWF
- Pregnancy, menstrual cycle, exercise and stress or blood loss can lead to high vWF

5. Disorders of endothelium
(a) Osler-Weber-Rendu Syndrome aka hereditary haemorrhagic telangiectasia
- Autosomal dominant
- Abnormal development of vasculature  skin and mucosal telangiectasias, AVMs in internal organs,
epistaxis, aneurysms
- Leads to a bleeding tendency including epistaxis or bleeding from GIT, lungs, kidney, spleen, bladder,
liver, brain
- Can be life-threatening but usually a favourable prognosis

(b) Ehlers-Danlos Syndrome

- Autosomal dominant (but can be other types of inheritance)
- Classic (types I and II) 
o Soft, elastic, velvety skin, prone to tearing, bruising or scarring
o Slow healing time
o Joint disease, cardiac complications
o Hypermobility, dislocation, subluxation
o All due to improper collagen synthesis
o Pain is a major complication
- Type IV EDS (vascular EDS)
o Autosomal dominant
o Fragile vessels and organ membranes  risk of rupture, aneurysms
o Can bleed anywhere, can cause sudden death
- DDx:
o Coagulation disorders
o Silverman syndrome

(c) Scurvy
- Vitamin C required for synthesis of collagen, so deficiency can lead to spongy gums, bleeding mucosal
membranes
- Advanced scurvy: open, suppurating wounds, loss of teeth
- Bruises, fractures, sores that do not heal

6. Rare factor deficiencies

- Usually autosomal recessive
-

DIC
Clotting mechanism activated in widespread areas of circulation  DIC

Can be due to large amounts of traumatised or dying tissue in the body, releasing great quantities of tissue factor
into the blood
Clots often small but numerous, and plug peripheral vessels
e.g. in septicaemia, where endotoxins from bacteria activate clotting

Clots will block blood supply and therefore exacerbates circulatory shock

DIC can lead to bleeding due to consumption of procoagulants

Investigations
1. Bleeding time
a. Prolonged especially by platelet deficiency, but also by other deficiencies
2. Prothrombin time
a. Reflects concentration of prothrombin in the blood
b. Time taken for coagulation to take place via extrinsic pathway (tissue factor and calcium is
mixed into blood)
c. Normal is ~12 seonds
3. APTT
a. The activated partial thromboplastin time measures the intrinsic pathway of clotting. It
measures the clotting factors - 8, 9, 11, 12, and is used in von Willebrand's disease. It is used
to check if the dose of heparin is correct. The normal value is 25-39 seconds (depending on
your control).