Haemostasis Flashcards
What is haemostasis?
The cellular and biochemical processes that enables both the specific and regulated cessation of bleeding in response to vascular insult
What is haemostasis for?
Prevent blood loss from intact vessels
Arrest bleeding from injured vessels
Enable tissue repair
What are the stages of haemostasis?
Primary - vessel constriction and formation of unstable platelet plug
Secondary - stable platelet plug
Fibrinolysis - vessel repair and breakdown of blood clot
What is the mechanism of haemostasis?
[4 steps]
Vessel constriction = limits blood flow to site of injury
Formation of an unstable platelet plug = platelet adhesion, platelet aggregation, limits blood loss and provides surface for coagulation
Stabilisation of plug with fibrin = blood coagulation to stop blood loss
Vessel repair and dissolution of clot = cell migration / proliferation and fibrinolysis to restore vessel integrity
Why is it important to understand haemostatic mechanisms?
Diagnose and treat bleeding disorders
Control bleeding in individuals who do not have an underlying bleeding disorder
Identify risk factors for thrombosis
Treat thrombotic disorders
Monitor the drugs that are used to treat bleeding and thrombotic disorders
What is the balance model of coagulation?
Normal haemostasis = delicate balance between bleeding and thrombosis
Equilibrium established between thrombosis factors (coagulant factors and platelets) and bleeding factors (fibrinolytic factors and anticoagulant proteins)
Why might the balance be tipped towards the bleeding side of the scale? (i.e. more bleeding occuring that thrombosis)
Lack of a specific factor e.g. failure of production OR increased consumption / clearance
Defective function of a specific factor - can be genetic or acquired (drugs, synthetic defect, inhibition)
What occurs in primary haemostasis?
3 main factors: platelets, VWF, and vessel wall
Damage to endothelium of vessel = exposure of collagen (in the vessel wall)
Platelets can attach directly to the collagen via the glycoprotein 1A (GlpIa) receptor
OR
Through a viable Von Willebrand Factor via Glp1b receptor
Platelets release their granular contents, which combine with thromboxane = activated platelets
Leads to activation of GlpIIb and GlpIIIa receptors on the platelets
Which 3 factors affected can cause disorders of primary haemostasis?
Primary haemostasis requires 3 main factors: platelets, VWF, and vessel wall
So if either of these 3 is in low production or not working, it can lead to primary haemostasis disorders
What is a low platelet count known as?
What causes low platelets?
Thrombocytopenia
- Production issue: bone marrow failure e.g. leukaemia, B12 deficiency
- Accelerated clearance issue: eg: immune thrombocytopenic papura (ITP), Disseminated Intravascular Coagulation (DIC)
- Pooling and destruction in an enlarged spleen (splenomegaly has many causes)
What is ITP?
Common cause of thrombocytopenia
Immune thrombocytopenic purpura = immune system forms antiplatelet autoantibodies
AutoAbs = bind to sensitised platelets
Platelets cleared by macrophages
What causes impaired function of platelets leading to primary haemostasis?
- Hereditary: absence of glycoproteins or storage granules (rare)
- Acquired due to drugs: aspirin, NSAIDs, clopidogrel (common)
What are conditions that impaired platelet function?
Glanzmann’s thrombasthenia = absence of GlpIIa and GlpIIIa receptors
Bernard Soulier syndrome = absence of GlpIb receptors
Storage Pool disease = group of disorders refering to reduction in granular content in the platelets
How can drugs impair the function of platelets?
Antiplatelet therapy, e.g. Aspirin, often used in treatmment of CVD
Aspirin blocks the cyclo-oxygenase enzyme to prevemt thromboxane production from arachidonic acid
What are the two functions of VWF?
Binding to collagen and capturing platelets
Stabilising co-agulation Factor VIII
Factor VIII may be low if VWF is very low
Why may VWF be reduced leading to primary haemostasis?
Low VWF is called Von Willebrand disease (VWD)
1: Hereditary: decrease of quantity +/ function (common)
2: Acquired due to antibody (rare)
What hereditary pattern is VWD?
Autosomal inheritance pattern
Hereditary VWD is cassified into types:
Types 1 and 3 lead to deficiency of VWF
Type 2 leads to VWF with abnormal function
How can the vessel wall be affected to cause primary haemostasis?
1: Inherited (rare) - abnormalities e.g. Hereditary haemorrhagic telangiectasia, Ehlers-Danlos syndrome and other connective tissue disorders
2: Acquired (common): steroid therapy, ageing (senile purpura), vasculitis, scurvy (Vit C deficiency)
Summary of Disorders of Primary Haemostasis:
- Platelets: thrombocytopenia; drugs
- VWF - VWD
- Vessel wall - hereditry vascular disorders; steroids, age, vaculitis, scurvy
What are the clinical features of typical primary haemostasis bleeding?
Immediate
Prolonged bleeding from cuts
Nose bleeds (epistaxis): prolonged > 20 mins
Gum bleeding: prolonged
Heavy menstrual bleeding (menorrhagia)
Bruising (ecchymosis), may be spontaneous/easy
Prolonged bleeding after trauma or surgery
What common feature is seen in patients with thrombocytopenia?
Petechiae and purpura (bleeding under the skin)
Purpura = does not blanch under pressure
What does purpura only (no petechiae) indicate clinically?
Platelet (thrombocytopenic purpura) or vascular disorders
Why might VWD (von willebrand disease) cause haemophilliac-like bleeding?
Low FVIII
What are tests for disorders of primary haemostasis?
Platelet count, platelet morphology (electron microscpe used)
Bleeding time (now replaced with platelet function analysis in lab)
Measure levels and functions of of von Willebrand Factor (VWF)
Clinical observation
Note – coagulation screen (PT, APTT) is normal in disorders of primary haemostasis so cannot conclusively be used for a diagnosis (except more severe VWD cases where FVIII is low)
What are the typical ranges for platelet counts?
[x10^9]
150 - 400 = normal range
<100 = bleeding with trauma
<40 = spontaneous bleeding
<10 = severe spontaneous bleeding
What are the treatments if it is failure of production leading to primary haemostasis disorder?
Replace missing factor/platelets e.g. VWF containing concentrates can be used for prevention and treatment
Stop drugs e.g. aspirin/NSAIDs
What are treatments for immune destruction?
Immunosuppression via steroids (e.g. prednisolone)
Splenectomy for ITP
What are treatments for increased consumption?
Treat the underlying cause
In the meatime - continue with replacement therapy
What are some additional haemostatic treatments?
- Desmopressin (DDAVP)
Vasopressin analogue
2-5 fold increase in VWF (and FVIII)
releases endogenous stores (so only useful in mild disorders) - Tranexamic acid = antifibrinolytic (i.e. stops blood clots from breaking down)
- Fibrin glue/spray used in surgery
- Other approaches e.g hormonal (oral contraceptive pill for menorrhagia)
What is secondary haemostasis?
Coagulation of blood
By stabilising the platelet plug from primary haemostatic using fibrin
What are the causes of disorders of coagulation (secondary haemostasis)?
Coagulation generates thrombin (IIa), which then converts fibrinogen to fibrin
So deficiency in any of the coagulation factors result in failure of thrombin production and so fibrin formation
How can the balance between thrombosis and bleeding be tipped to cause bleeding?
An increase in fibrinolytic factors or anticoagulant proteins
A decrease in coagulation factors
What are the causes of coagulant factor deficiences?
- Hereditary deficiency of coagulation production e.g. Factor VIII/IX: haemophilia A/B
- Acquired deficiency of coagulation production e.g. liver disease, anticoagulant drugs, warfarin, direct Oral Anticoagulants (DOACs)
- Dilution e.g. blood transfusions
- Increased consumptions e.g. Disseminated intravascular coagulation (DIC) – common
(Immune – autoantibodies – rare)
What are some examples of inherited coagulation disorders?
How does it lead to bleeding?
Haemophilia A (Factor VIII deficiency) Haemophilia B (Factor IX deficiency) sex linked 1 in 10^4 births Others are very rare (autosomal recessive)
Although the platelet plug is formed, due to missing coagulation factors, fibrin cannot be formed so the unstable platelet plug breaks apart - leads to bleeding
What is a characteristic clinical feature of haemophilia?
Haemarthrosis = spontaneous joint bleeding
Can lead to joint deformity and myscle wasting
What should be avoided in patients with haemophilia?
Intramuscular injections - leads to extensive haematoma
Why are different coagulation factor deficiencies not all the same in terms of symptoms and quality of life?
What are the differences between absence of: Factor VIII and IX (haemophilia) Prothrombin (Factor II) Factor XI Factor XII
Factor VIII and IX (Haemophilia)
Severe but compatible with life
Spontaneous joint and muscle bleeding
Prothrombin (Factor II)
Lethal
Factor XI
Bleed after trauma but not spontaneously
Factor XII
No bleeding at all
Why does liver failure cause coagulation disorders?
Most coagulation factors are synthesised in the liver
Except VWF (synthesised in the endothelial cells lining the vessels) and Factor V (synthesised in platelets)
Why does dilution produce coagulation disorders?
Red cell transfusions no longer contain plasma
Major haemorrhage requires transfusion of plasma as well as red cells and platelets
What are the features of increased consumption e.g. in DIC?
Generalised rather than localised activation of coagulation – tissue factor comes into contact with factorVIIa leads to widespread and unregulated activation of coagulation
Can be triggered by e.g. sepsis, major tissue damage, inflammation
Consumes and depletes coagulation factors
Platelets consumed - thrombocytopenia
Activation of fibrinolysis depletes fibrinogen – raised D-dimer (a breakdown product of fibrin)
Deposition of fibrin in vessels causes organ failure and sheering of blood vessels
How is DIC treated?
Increased consumption issue
So replacement therapy with fresh plasma transfers until underlying cause is found and treated
What are the clinical features of coagulation disorders?
Superficial cuts do not bleed (platelets are working fine and platelet plug does not need stabilsing)
Bruising is common, nosebleeds are rare
spontaneous bleeding is deep, into muscles and joints
Bleeding after trauma may be delayed and is prolonged
Bleeding frequently restarts after stopping
So what is the difference in bleeding in primary haemostasis compared to secondary haemostasis (coagulation)?
Platelet/Vascular = superficial bleeding into skin, mucosal membranes; bleeding immediate after injur
Coagulation - bleeding into deep tissues, muscles, joints; delayed but severe bleeding after injury; prolonged bleeding
What are the tests for coagulation disorders?
Screening tests (‘clotting screen’):
Prothrombin time (PT)
Activated partial thromboplastin time (APTT)
Full blood count (platelets)
Coagulation factor assays (for Factor VIII etc)
Tests for inhibitors
What could cause prolonged APTT but normal PT?
What may cause prolonged PT but normal APTT?
What may cause prolonged PT and APTT?
Prolonged APTT, normal PT: Haemophilia A Haemophilia B Factor XI deficiency Factor XII deficiency
Prolonged PT, normal APTT:
Factor VII deficiency
Prolonged PT, prolonger APTT: Liver disease Anticogulant drugs DIC Dilution following RBC transfusion
What are the different types of factor replacement therapy?
Fresh Frozen Plasma - contains all coagulation factors
Cryoprecipitate - rich in fibrinogen, FVIII, VWF, FXIII
Factor Concentrates - available for all factors except V (so prothrombin complex concentrates, FII, FVII, FIX, FX)
Recombinate forms of FVIII and FIX - good for ‘on demand’ to treat bleeds, and to prevent bleeds
How common is an increase in fibrinolytic factor and anticoagulant factors that lead to bleeding disorders?
V. rare
Bleeding disorders caused by increased fibrinolytic factors and anticoagulant proteins are usually due to drugs: tPA (used to bust clots in strokes) or heparin
What is meant by thrombosis?
Intravascular coagulation Inappropriate coagulation Venous or arterial Obstructs flow May embolise to lungs
What are the 2 main venous disorders of thrombosis?
And how do they present clinically?
Pulmonary embolism (PE) = Tachycardia Hypoxia Shortness of breath Chest pain Haemopysis Sudden death
Deep Vein Thrombosis (DVT) = Painful leg Swelling Red Warm May embolise to lungs Post thrombotic syndrome - longstanding pain and swelling
What are 2 main arterial disorders of thrombosis?
MI
Cerebral vascular disease - stroke
What is Virchow’s triad?
Considers 3 contibutory factors to thrombosis:
Changes in the blood = dominant in venous thrombosis
Changes in vessel wall = dominant in arterial thrombosis
Changes in blood flow (in particular slowing) = contributes to both thromboses
What is thrombophilia?
How might it present clinically?
Increased risk of venous thrombosis due to imbalance in clotting factors - can be genetic or acquired
Thrombosis at young age
‘spontaneous thrombosis’ - unprovoked e.g. after sugery, pregnancy etc.
Multiple thromboses
Thrombosis whilst anti-coagulated
What are examples of anticoagulant proteins?
Antithrombin
Protein C
Protein S
What may cause the thrombosis bleeding balance to tip towards thrombosis?
Decrease in anticoagulant factors
OR
Increase in coagulant factors
What is the role of the vessel wall in thrombosis?
Many proteins active in coagulation are expressed on the surface of endothelial cells and their expression is altered in inflammation (TM, EPCR, TF)
What is the role of blood flow in thrombosis?
Reduced flow (stasis) increases risk e.g. surgery, long flight, pregnancy
What are the risk factors for venous thrombosis?
Age
Genetics
Environment
Interactions between all 3 of these
How can you prevent venous thrombosis?
- Prevention - thromboprophylaxis = especially during times of high risk
- Reduce risk of recurrence / extension - lower procoagulant factors e.g. warfarin, DOACs; increase anticoagulant activity e.g. heparin
When is therapeutic anticoagulation used?
Venous thrombosis:
Initial treatment to minimise clot extension/embolisation (< 3 months)
Long term treatment to reduce risk of recurrence
Atrial fibrillation:
To reduce risk of embolic stroke
Mechanical prosthetic heart valve: reduce thrombosis risk
When is thromboprophylaxis (preventative) used?
E.g. following surgery, during hospital admission, during pregnancy
What is heparin?
Where is it found naturally?
What are the different types of heparin used in the UK and where are they administered?
Naturally occurring glycosaminoglycan
Produced by mast cells of most species
Porcine products used in UK
Varying numbers of saccharides in chains of heparin – differing lengths:
- Long chains = Unfractionated (UFH) – intravenous administration, short half life
- Low molecular weight (LMWH) – subcutaneous administration
How does unfractionated heparin work?
Enhances the effects of antithrombin
Changes active site of antithrombin = when the pentasaccharide sequence on the unfractionated heparin binds to the antithrombin, it gives it a far greater affinity for its target proteases (FXa and thrombin)
This allows the antithrombin to bind more securely and more quickly to thrombin and FXa leading to their inactivation
For the inactivation of thrombin, heparin binds and wraps itself around antithrombin and thrombin
But for the inactivation of FXa, heparin only binds itself to antithrombin
How does low molecular weight heparin (LMWH) work?
Predominantly works against FXa
Shorter chains in the LMWH are not long enough to wrap around antithrombin AND thrombin
What is the effect of unfractionated heparin on the APTT test?
Prolongation of APTT - exponential increase in APTT time (s)
So APTT can be used to monitor a patient on unfractionated heparin
Why is the APTT test not used to monitor LMWH?
The APTT time is more predictable, it only increases slightly and steadily
Instead, measure anti-Xa to monitor LMWH
How does Warfarin work?
Vitamin K antagonist - competes with Vitamin K
It blocks the recycling of vitamin K, and so enzymes dependent on vitamin K (e.g. vitamin K dependent carboxylase) no longer function for the activation of FII, FVII, FIX, FX, Protein C and Protein S
Therefore warfarin reduced the production of functional coagulation factprs and induces an anticoagulated state slowly
What are the issues with administering Warfarin?
Has a very complicated metabolism = many dietary, physiological and drug interactions to keep in mind
This narrows the therapeutic index and requires monitoring
How can the effects of Warfarin be reversed?
Reversed slowly by Vit K administration – takes several hours to work
Reversed rapidly by infusion of coagulation factors:
PCC (Prothrombin Complex Concentrate- contains Factors II, VII, IX and X)
FFP (Fresh Frozen Plasma)
What is the order of half-lives, shortest to longest, of the factors: II, VII, IX, X?
[i.e. after the administration of wafarin, which factors fall from the fastest to the slowest?]
Shortest
FVII
FIX
FX
FII
Longest
What are side effects of Warfarin?
Bleeding:
Minor 5%
Major 0.9 – 3.0%
Fatal 0.25%
Skin Necrosis - due to severe protein C deficiency (as Protein C has a shorter half life than the clotting factors, it falls before the warfarin has an effect on the clotting factors)
Purple toe syndrome - disrupted atheromatous plaques bleed = cholesterol emboli lodge in extremities
Embryopathy – Early fusion of epiphyses, warfarin is teratogenic (cause abnormal fetal growth) in 1st trimester Chondrodysplasia punctata
How do you monitor warfarin?
Calculate INR (from PT) Un-anticoagulated normal INR = 1.0 Target with warfarin = 2-3
Can also use the ISI = International Sensitivity Index, this indicates the sensitivity of a particular thromboplastin for warfarin
What can cause resistance to warfarin?
Lack of patient compliance
Dietary factors e.g. increased Vit K intake
Other drug interactions - may ead to increased metabolism of warfarin
Reduced binding
What is DOAC?
Direct Oral Anticoagulant
Can directly interact with factors
e. g. Rivaroxaban, Apixaban, Edoxaban = inhibit FXa
e. g. Dabigatran = inhibit FIIa
How do Warfarin and DOACs compare?
Drug: Warfarin | DOACs Onset: slow | rapid Dosing: variable (monitor using INR) | fixed Food effect: yes | no Interactions: many | few Monitoring required: yes | no Renal dependence: no | some Reversibility: Vit K / PCCs | few specific antidotes available (expensive)
What is the initial treatment for a venous thrombosis?
DOAC or LMWH for first few days
To minimise clot extension (< 3 months)
Then give warfarin or DOAC
What is the long term treatment to reduce risk of recurrence?
DOAC or warfarin
What is the treatment for AF?
DOAC or warfarin - reduces risk of stroke
What is the long term treatment for mechanical prosthetic heart valves?
Warfarin (DOACs not effective and should be avoided)
What treatment is given following surgery?
LMWH or DOAC
What treatment is given during pregnancy?
LMWH (DOACs not safe in pregnancy)
