5.11 - 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? (3)
- prevention of blood loss from intact vessels
- arrest bleeding from injured vessels
- enable tissue repair
What is the overall mechanism of haemostasis?
- injury to endothelial lining
- vessel constriction, VSMCs contract locally to limit blood flow to injured vessel
- primary haemostasis - formation of an unstable platelet plug - platelet adhesion (to vessel wall via VWF) and aggregation (to each other) = limits blood loss + provides surface for coagulation
- secondary haemostasis - stabilisation of the plug with fibrin - causes blood coagulation to stop blood loss
- fibrinolysis - vessel repair and dissolution of clot - cell migration/proliferation and fibrinolysis to restore vessel integrity
Why do we need to understand haemostatic mechanisms? (5)
- diagnose and treat bleeding disorders
- control bleeding in individuals who do not have 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 haemostasis a balance between?
Bleeding (fibrinolytic factors, anticoagulant proteins) and thrombosis (coagulant factors, platelets)
When can the haemostatic balance be tipped towards bleeding?
- too many fibrinolytic factors / anticoagulant proteins
- not enough coagulant factors / platelets
What are the causes of reduced coagulant factors / platelets?
Lack of a specific factor:
- failure of production - congenital and acquired
- increased consumption/clearance
Defective function of a specific factor:
- genetic
- acquired - drugs, synthesis defect, inhibition
What is happening during platelet adhesion and aggregation?
- damage to endothelium wall
- platelets bind to exposed collagen - either directly to vessel wall through GPIa receptor, or indirectly via VWF to GPIb receptor
- platelets release granular contents (ADP which binds P2Y12 on other platelets to activate them, VWF) and become activated along with thromboxane A2 release which activates other platelets (also release Ca2+)
- leads to flip flopping and activation of GPIIb/IIIa receptors on platelets (allows fibrinogen to bind)
What about platelets can cause a problem in primary haemostasis? (2)
- low numbers - thrombocytopenia
- impaired function
What can cause a low number of platelets (thrombocytopenia)? (3)
- bone marrow failure e.g. leukaemia, B12 deficiency
- accelerated clearance e.g. immune (ITP), disseminated intravascular coagulation (DIC)
- pooling and destruction in an enlarged spleen (splenomegaly)
What is auto-ITP (autoimmune thrombocytopenic purpura)?
- antiplatelet antibodies bind to sensitised platelets
- macrophages of reticular endothelial system in spleen clear these platelets
- ITP is a very common cause of thrombocytopenia
(Treatment: splenectomy)
What can impaired function of platelets be due to? (2)
- hereditary absence of glycoproteins or storage granules (rare) - e.g. Glanzmann’s thrombasthenia, Bernard-Soulier syndrome, storage pool disease
- acquired due to drugs: aspirin, NSAIDs, clopidogrel (common)
What are some examples of hereditary platelet defects causing impaired function? (3)
- Glanzmann’s thrombasthenia - absence of GPIIb/IIIa
- Bernard Soulier syndrome - absence of GPIb
- storage pool disease - reduction in contents of dense granules of platelets (ADP, ATP, serotonin, Ca2+)
What is antiplatelet therapy used for?
Prevention and treatment of cardiovascular and cerebrovascular disease
How does aspirin work (antiplatelet)?
- irreversibly binds to COX
- reduction in thromboxane A2 production therefore reduction in platelet aggregation (platelets)
- even though prostacyclin production is inhibited too (prostacyclin inhibits platelet aggregation), the endothelial cells can produce more unlike the non-nucleated platelets
How does clopidogrel work (antiplatelet)?
Blocks ADP receptor P2Y12 on platelets (reduce platelet recruitment + aggregation etc)
What about Von Willebrand Factor can cause a problem in primary haemostasis?
Von Willebrand disease
What are the causes of Von Willebrand disease? (2)
- hereditary disease of quantity and/or function (common)
- acquired due to antibody (rare)
What are the two functions of VWF in haemostasis?
- binding to collagen and capturing platelets
- stabilising factor VIII (factor VIII may be low if VWF is very low)
Describe the inheritance of hereditary VWD?
- autosomal inheritance pattern
- deficiency of VWF is type 1 or 3
- VWF with abnormal function is type 2
What about the vessel wall can cause a problem in primary haemostasis? (2)
- inherited (rare) - e.g. hereditary haemorrhagic telangiectasia, Ehlers-Danlos syndrome and other connective tissue disorders
- acquired (common) - e.g. steroid therapy, ageing (‘senile’ purpura), vasculitis, scurvy (vitamin C deficiency)
What happens in VWD?
Failure of primary haemostasis - platelet plug cannot form
What are the clinical features of bleeding in primary haemostasis? (7)
- immediate bleeding
- prolonged bleeding from cuts
- nosebleeds (epistaxis): prolonged >20min
- gum bleeding: prolonged
- heavy menstrual bleeding (menorrhagia)
- bruising (ecchymosis), may be spontaneous/easy
- prolonged bleeding after trauma/surgery
What is a particular feature of thrombocytopenia?
Petechiae - small spots under skin caused by bleeding under skin
In what types of disorders do we see purpura?
- platelet (thrombocytopenic purpura)
- vascular disorders (wet purpura if over mucosal surfaces like gums)
How can we tell the difference between petechiae and purpura?
- both are caused by bleeding under the skin
- purpura do not blanch when pressure is applied
- petechia <3mm, purpura 3-10mm, bruise >10mm
What is the bleeding pattern like in severe VWD?
Haemophilia-like bleeding (due to low FVIII too)
What are the tests for disorders of primary haemostasis? (4)
- platelet count, platelet morphology
- bleeding time (PFA100 in lab - platelet function analysis)
- assays of VWF
- clinical observation
- note - coagulation screen (PT, APTT) is normal, except more severe VWD where FVIII is low
What are the platelet count ranges for normal and thrombocytopenia?
- normal range: 150-400 x10^9/l
- no spontaneous bleeding, but bleeding with trauma: 40-100 x10^9/l
- spontaneous bleeding common: 10-40 x10^9/l
- severe spontaneous bleeding: <10 x10^9/l
How do we treat failure of production/function in primary haemostasis?
- replace missing factors/platelets e.g. VWF containing concentrates
- can be prophylactic (e.g. before surgery) or therapeutic (e.g. after bleeding)
- stop drugs e.g. aspirin/NSAIDs
How do we treat immune destruction in primary haemostasis?
- immunosuppression e.g. prednisolone
- splenectomy for ITP
How do we treat increased consumption (e.g. in DIC) in primary haemostasis?
- treat underlying cause
- replacement therapy as necessary
What additional haemostatic treatments are there for primary haemostasis disorders? (4)
- desmopressin (ddAVP) - vasopressin analogue that causes 2-5x increase in VWF (and FVIII) - releases endogenous stores so only useful in mild disorders
- tranexamic acid - antifibrinolytic
- fibrin glue/spray
- other approaches e.g. hormonal (oral contraceptive pill for menorrhagia)
What is the role of coagulation?
To generate thrombin (IIa), which will convert fibrinogen to fibrin
What would a deficiency of any coagulation factor cause?
A failure of thrombin generation and hence fibrin formation
What are the main causes of disorders of coagulation? (3)
- deficiency of coagulation factor production
- dilution
- increased consumption
What are the two types of deficiency of coagulation factor production and what makes these up?
- hereditary - factor VIII/IX (haemophilia A/B)
- acquired - liver disease, anticoagulant drugs (warfarin, DOACs)
What can dilution be caused by?
- acquired - through red blood cell transfusions that do not have plasma
- a major haemorrhage requires a transfusion of plasma as well as red cells and platelets to avoid dilutional effect with reduction in coagulation factors
What are the common and rare causes of increased consumption?
- acquired
- disseminated intravascular coagulation (DIC) - common
- immune - antibodies - rare
What is haemophilia A?
Factor VIII deficiency, sex-linked
What is haemophilia B?
Factor IX deficiency, sex-linked
What do both haemophilias A&B do mainly?
- cause a failure to generate fibrin to stabilise platelet plug
- unstable platelet plug breaks away leading to bleeding
What is the hallmark of haemophilia?
- haemarthrosis - spontaneous bleeding of joints when factors are low
- usually seen in more developing countries because factor replacement therapy is more accessible in developed countries
- long term - chronic haemoarthritis with target joints having recurrent bleeds and damaging the synovial lining leading to joint deformity and muscle wasting
What happens if an intramuscular injection is given to a haemophilia patient?
There is extensive haematoma that occurs - avoid!!
Is haemophilia (A/B) compatible with life?
Yes - severe but compatible with life, spontaneous joint and muscle bleeding
Is prothrombin (factor II) deficiency compatible with life?
No - lethal
What does factor XI deficiency lead to?
Bleed after trauma but not spontaneously (so less severe than haemophilia)
What does factor XII deficiency lead to?
No bleeding at all
How can liver failure cause decreased production of coagulation factors?
Liver synthesises most coagulation factors (apart from VWF made by endothelial cells and factor V made by platelets) so liver failure reduces production
What happens in disseminated intravascular coagulation (DIC)?
- generalised activation of coagulation due to tissue factor (normally does not contact factor VIIa)
- consumes and depletes coagulation factors and platelets (thrombocytopenia)
- activation of fibrinolysis depletes fibrinogen –> raised D-dimer (breakdown product of fibrin)
- deposition of fibrin in vessels causes organ failure and shearing of RBCs causing RBC fragmentation
What does activation of fibrinolysis raise (DIC)?
D-dimer (breakdown product of fibrin)
What does deposition of fibrin in vessels cause (DIC)?
- organ failure
- shearing of RBCs causing red cell fragmentation
What can DIC be triggered by? (4)
- sepsis
- major tissue damage
- inflammation
- pre-eclampsia
How do we treat DIC? (2)
- treat underlying cause
- meanwhile give supportive treatment with replacement of missing coagulation factors e.g. give FFP and platelets
What are the clinical features of coagulation disorders? (5)
- superficial cuts do not bleed (platelets functioning and platelet plug enough for small cuts)
- 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
How do you clinically distinguish between platelet/vascular deficiencies and coagulation deficiencies? (platelet/vascular vs coagulation)
- superficial bleeding into skin, mucosal membranes VS bleeding into deep tissues, muscles, joints
- bleeding immediate after injury VS delayed, but severe bleeding after injury, often prolonged
What tests are there for coagulation disorders? (3)
- screening tests (‘clotting screen’) - prothrombin time (PT), activated partial thromboplastin time (APTT), full blood count (platelets)
- coagulation factor assays (for FVIII etc)
- tests for inhibitors
Describe the extrinsic pathway (tests for coagulation disorders, recap phase 1a)
- trauma exposes the cells below the endothelial layer e.g. smooth muscle cells
- these cells have tissue factor (factor III) in their membrane
- factor VII is floating in the blood - some is already active (VIIa) so is set to proteolytically cleave other proteins
- factor VIIa binds to a TF and Ca2+ –> VIIa-TF complex on surface of smooth muscle cells
- [TF and Ca2+ (released by nearby activated platelets) are cofactors - must bind to VIIa for it to function]
- VIIa-TF complex cleaves factor X –> factor Xa
- factor Xa cleaves factor V –> factor Va
- factor Xa uses factor Va and Ca2+ as cofactors to form the prothrombinase complex (one can activate thousands of thrombins = enormous amplification)
- activates prothrombin (factor II) –> thrombin (factor IIa)
Describe the intrinsic pathway (tests for coagulation disorders, recap phase 1a)
- circulating factor XII comes into contact with negatively charged phosphates on membranes of activated platelets or subendothelial collagen exposed by trauma
- undergoes conformational change –> activated into factor XIIa
- factor XIIa proteolytically cleaves factor XI –> factor XIa
- factor XIa combines with Ca2+ –> proteolytically cleaves factor IX –> IXa
- factor IXa forms a complex with Ca2+ and factor VIIIa –> proteolytically cleaves factor X –> factor Xa
- (factor VIII degrades rapidly in blood unless bound to VWF)
- Xa –> Va –> Xa + Va + Ca2+ –> IIa
What is the role of thrombin? (recap phase 1a)
- thrombin uses Ca2+ as a cofactor and has many procoagulative effects:
1. thrombin binds to receptors on platelets and activates them
2. activates three cofactors: factor V (common pathway), factor VIII, fabin - cleaves soluble fibrinogen (I) –> insoluble fibrin (Ia) - fibrin precipitates out of plasma and forms long rope-like protein chains
3. proteolytically cleaves stabilising factor (XIII) –> XIIIa - combines with Ca2+ cofactor to form cross links between fibrin chains –> further reinforcing the fibrin mesh
What is the currently accepted model for coagulation (recap phase 1a)?
- initiation: TF combines with FVIIa (and Ca2+), IX–>IXa, X–>Xa, II–>IIa (small amount of thrombin)
- amplification: thrombin causes cofactor V–>Va, VIII–>VIIIa, IX–>IXa (increased by XIa), platelet activation, IXa with VIIIa and Ca2+ amplifies X–>Xa
- propagation: amplified X–>Xa causes rapid burst in thrombin production which cleaves fibrinogen (I)–> fibrin (Ia)
What does prothrombin time (PT) measure?
Extrinsic pathway
What does activated partial thromboplastin time (APTT) measure?
Intrinsic pathway
What could cause an increased APTT but normal PT?
- deficiencies in intrinsic pathway factors
- haemophilia A (factor VIII)
- haemophilia B (factor IX)
- factor XI deficiency
- factor XII deficiency (does not cause bleeding)
What could cause an increased PT but normal APTT?
- deficiencies in extrinsic pathway factors
- factor VII deficiency
What could cause an increased APTT and PT? (5)
- liver disease
- anticoagulants like warfarin
- DIC (platelets and D-dimer)
- dilution following red cell transfusion
- deficiency of factors II, V, X (common factors in both pathways)
What are the treatments for coagulation disorders? (3)
Factor replacement therapy:
- plasma (fresh frozen plasma FFP)
- cryoprecipitate
- factor concentrates
What does FFP/plasma contain?
All coagulation factors
What does cryoprecipitate contain?
Rich in fibrinogen, FVIII, VWF, FXIII
What do factor concentrates include?
- concentrates available for all factors except factor V (FFP/platelets needed)
- prothrombin complex concentrates (PCCs) - factors II, VII, IX, X
What are recombinant forms of FVIII and FIX used to treat?
- ‘on demand’ to treat bleeds
- prophylaxis to prevent bleeds
What has the evolution of haemophilia treatment been like?
- plasma-derived clotting factors (1969) –> widespread viral contamination
- currently improved recombinant clotting factors VIII, IX, VIIa (1990s) - improved safety, eliminated potential for transmission of blood borne pathogens
- investigational therapies (2008+) - prolonged half-life (VIII/IX), gene therapy, novel agents
What novel treatments are emerging for haemophilia? (3)
- gene therapy (for both A and B)
- bispecific antibodies (for haemophilia A e.g. Emicizumab) - binds FIXa and FX, mimics procoagulant function of FVIII
- RNA silencing (for both A and B) - targets antithrombin (natural anticoagulant)
What other treatments can be used alongside factor replacement for coagulation disorders? (2)
- desmopressin (ddAVP)
- tranexamic acid - antifibrinolytic
When can fibrinolytic factors and anticoagulant proteins be increased?
- extremely rare except when induced by drugs:
- tPA (stroke)
- heparin
What are two examples of venous thrombosis?
- pulmonary embolism (PE)
- deep vein thrombosis (DVT)
What are the symptoms of pulmonary embolism (PE)? (6)
- tachycardia
- hypoxia
- shortness of breath
- chest pain
- haemoptysis
- sudden death
What are the symptoms of deep vein thrombosis (DVT)? (6)
- painful leg
- swelling
- red
- warm
- may embolise to lungs –> PE
- post thrombotic syndrome - damage to valves causing long term damage
When do most patients with thrombosis die?
At the haemostatic endpoint
What is thrombosis? (4)
- intravascular and inappropriate coagulation
- venous (or arterial)
- obstructs flow
- may embolise to lungs
What is Virchow’s triad?
The three contributory factors to thrombosis:
- blood - dominant in venous thrombosis
- vessel wall - dominant in arterial thrombosis
- blood flow - contributes to both venous and arterial thrombosis
What is thrombophilia?
Increased risk of venous thrombosis
How might thrombophilia present? (4)
- thrombosis at young age
- ‘spontaneous’ thrombosis (unprovoked)
- multiple thromboses
- thrombosis whilst anticoagulated
What does the balance of haemostasis look like in venous thrombosis?
- reduced fibrinolytic factors + anticoagulant proteins
- increased coagulant factors + platelets
What anticoagulant proteins would decrease to cause thrombosis? (3)
- antithrombin (most powerful)
- protein C
- protein S
What coagulant factors would increase to cause thrombosis? (3)
- factor VIII
- factor II
- factor V Leiden (increase activity due to activated protein C resistance)
What would cause an increase of platelets (leading to thrombosis)?
Myeloproliferative disorders
What is the role of the vessel wall in venous thrombosis?
- many proteins active in coagulation are expressed on the surface of endothelial cells and their expression altered in inflammation:
- thrombomodulin receptor
- endothelial protein C receptor
- tissue factor
What are examples of situations where reduced blood flow (stasis) increase the risk of thrombosis? (3)
- surgery
- long haul flight
- pregnancy (compression from foetus)
How do we treat venous thrombosis? (2 areas)
- prevention
- assess and prevent risks
- prophylactic anticoagulant therapy
- reduce risk of recurrence/extension
- lower procoagulant factors e.g. warfarin, DOACs
- increase anticoagulant activity e.g. heparin
How can we prevent venous thrombosis?
- assess and prevent risks
- prophylactic anticoagulant therapy
How can we reduce risk of recurrence/extension of venous thrombosis?
- lower procoagulant factors e.g. anticoagulants like warfarin, DOACs
- increase anticoagulant activity e.g. heparin
What coagulopathies are involved in SARS-CoV-2? (5)
- DIC - disseminated intravascular coagulation
- SIC - sepsis-induced coagulopathy
- HPS - haemophagocytic syndrome
- APS - antiphospholipid syndrome
- TMA - thrombotic microangiopathy
What are some therapeutic indications for anticoagulation?
- venous thrombosis - initial treatment to minimise clot extension/embolisation (<3 months), long term treatment to reduce risk of recurrence
- atrial fibrillation - 800/100k potentially eligible in one year, to reduce risk of embolic stroke
- mechanical prosthetic heart valve (increased stroke risk)
What are some preventative (thromboprophylaxis) indications for anticoagulation?
E.g. following surgery, during hospital admission, during pregnancy
What is heparin?
- naturally occurring glycosaminoglycan
- produced by mast cells of most species
- porcine products used in UK
- varying number of saccharides in chains - differing lengths
What are the two chain lengths of heparin?
- long chains - unfractionated (UFH) - intravenous administration, short half -life
- low molecular weight heparin (LMWH) - subcutaneous administration
What are the actions of unfractionated heparin? (4)
- enhancement of antithrombin
- inactivation of thrombin (hep binds AT and thrombin)
- inactivation of FXa (hep binds AT only)
- (inactivation of FIXa, FXIa, FXIIa)
What are the actions of low molecular weight heparin (LMWH)? (2)
- contain pentasaccharide sequence for binding antithrombin
- predictable dose response in most cases so does not require monitoring (compared to UFH) - if required, measure anti-Xa
How were coumarins (vitamin K antagonists) discovered?
- 1920s - fatal haemorrhagic disease in cattle fed mouldy sweet clover
- 1930s - haemorrhagic cattle ‘deficient in prothrombin’, sterol (vitamin K) cures similar disease in chickens, dicoumarol isolated from spoilt sweet clover in 1939
- 1940s - dicoumarol effective in VTE and MI, warfarin marketed as rat poison in 1948
- 1950s - warfarin more effective anticoagulant than dicoumarol
How does warfarin work?
- warfarin blocks recycling of vitamin K
- inactivates factors II, VII, IX, X, protein C and protein S
- competes with vitamin K - complicated metabolism (many dietary, physiological and drug interactions, narrow therapeutic index and needs monitoring)
- reduces production of functional coagulation factors
- induces an anticoagulated state slowly
- reversible
How is warfarin reversible?
- reversed slowly by vitamin K administration - takes several hours to work
- reversed rapidly by infusion of coagulation factors:
- PCC - contains factors II, VII, IX, X
- FFP
What are the side effects of warfarin? (4)
- bleeding - minor 5%, major 0.9-3%, fatal 0.25%
- skin necrosis (mitigate by starting heparin)
- purple toe syndrome
- embryopathy - chondrodysplasia punctata - avoid in first trimester of pregnancy
What happens in skin necrosis (warfarin side effect)?
Reduce with heparin
- severe protein C deficiency
- 2-3 days after starting warfarin
- thrombosis predominantly in adipose tissues
What happens in purple toe syndrome (side effect of warfarin)?
- disrupted atheromatous plaques bleed
- cholesterol emboli lodge in extremities
What happens in chondrodysplasia punctata (warfarin side effects)?
- early fusion of epiphyses
- warfarin teratogenic in 1st trimester
How do we monitor warfarin?
INR = (PT / PTc)^ISI
- INR = international normalised ratio
- ISI = international sensitivity index
What are the ranges for unanticoagulated normal INR and target INR?
- unanticoagulated normal INR = 1.0
- target INR usually 2-3
What can cause resistance to warfarin? (4)
- lack of patient compliance (measure warfarin levels, proteins induced by vitamin K absence PIVKA)
- diet, increased vitamin K intake
- increased metabolism Cyt P450 (CYP2C9)
- reduced binding (VKORC1)
What are some examples of DOACs and what factors they inhibit?
- rivaroxaban, apixaban, edoxaban - Xa
- dabigatran - IIa
What are the differences between warfarin VS DOACs?
- onset/offset - slow vs rapid
- dosing - variable vs fixed
- food effect - yes vs no
- interactions - many vs few
- monitoring required - yes vs no
- renal dependence - no vs some
- reversibility - vitamin K/PCCs vs specific antibodies available for dabigatran and in development for FXa inhibitors
What anticoagulants should be used for therapeutic management of venous thrombosis?
- initial treatment to minimise clot/embolisation - DOAC/LMWH for first few days followed by DOAC/warfarin
- long term to reduce risk of recurrence - DOAC/warfarin
What anticoagulants should be used for therapeutic management of atrial fibrillation?
DOAC/warfarin to reduce risk of embolic stroke
What anticoagulants should be used for therapeutic management of mechanical prosthetic heart valves?
Warfarin (DOACs not effective and should be avoided)
What anticoagulants should be used for preventative (thromboprophylaxis) measures? (note lower doses used)
- following surgery - LMWH/DOAC
- during hospital admission (DOACs not effective for use as medical thromboprophylaxis, heparin used)
- during pregnancy - LMWH (DOACs not safe)
