1. Haemostasis and thrombosis Flashcards
The body achieve haemostasis by balancing pro-coagulant and anti-coagulant factors. What are the PRO-coagulant factors?
Primary haemostasis: platelets, endothelium, vWF. Coagulation cascade.
The body achieve haemostasis by balancing pro-coagulant and anti-coagulant factors. What are the ANTI-coagulant factors?
Fibrinolysis. Natural inhibitors of thrombosis: anti-thrombins, protein C/S, tissue factor pathway inhibitor (TFPI)
What three responses does vessel injury stimulate?
- Vasoconstriction - in order to minimise blood loss.
- Platelet activation - forms the primary haemostatic plug.
- Activation of the coagulation cascade
Coordinated haemostasis - what are the components of blood clot formation?
Vascular endothelium; platelets; coagulation proteins; white blood cells
What is the endothelium composed of?***
Endothelial cells, basement membrane, smooth muscles, collagen, elastin, glycosaminoglycans???
What are the functions of the endothelium?
Synthesis of PGI2, vWF, plasminogen activators, thrombomodulin. Maintains a barrier between blood and procoagulant subendothelial structures.
What happens as a result of endothelial damage?
Endothelial damage will expose those pro-coagulant substances which then triggers a haemostatic response.
What do endothelial cells also produce?
Prostaglandins, vWD, plasminogen activators (important for activating fibrinolysis), thrombomodulin
What does the exposure of subendothelial pro-coagulant factors lead to?
Platelet aggregation at the site of damage
Explain how platelets are produced?
Produced in the bone marrow and originate from megakaryocytes.
- Stem cell precursors (2n) undergo nuclear replication to form megakaryocytes and become multinucleate.
- Maturation with granulation occurs.
- The megakaryocytes enter circulation.
- Each megakaryocyte produces ~4000 platelets.
- Lifespan ~10 days, 1/3 stored in spleen.
What is the relevance of the lifespan of platelets?
NOTE: this is significant because once anti-platelet drugs halt platelet activity, its effect will last for 10 days.
Clinical relevance: if someone on aspirin needs to have surgery, they need to stop aspirin 7-10 days before surgery.
How is the production of platelets regulated?
By a range of thrombopoietic factors (e.g. thrombopoietin, IL-6, IL-12). These can be given therapeutically to stimulate platelet production.
What are glycoproteins on platelets?
Glycoproteins are cell surface proteins via which platelets can interact with the endothelium, vWF and other platelets
Why do platelets have dense granules?
Dense granules contain energy stores (in the form of ATP and ADP)
What is in the dense granules of platelets?
ADP, ATP, serotonin, Ca 2+
What does the presence of open cannalicular system, microtubules and actomyosin mean?
Platelets are capable of massively expanding their surface area
How do platelets adhere to the exposed sub-endothelial structures, directly and indirectly?
DIRECTLY - through GlpIa. INDIRECTLY - by binding to vWF via GlpIb (this is the MORE IMPORTANT route).
What is the adhesion of platelets to the exposed subendothelial structures followed by?
It is followed by release of various mediators such as ADP and thromboxane A2
What do ADP and thromboxane A2 promote?
Platelet aggregation
How do platelets attach to each other? And what attaches to it?
Platelets attach to each other via GlpIIb/IIIa (aka fibrinogen receptor). Fibrinogen also binds to this receptor.
Aspirin reversibly inhibits COX, true or false?
False, aspirin irreversibly inhibits COX
NSAIDs are different from aspirin because they reversibly block COX, true or false?
True
ADP receptors also important for platelet aggregation. What are some examples of inhibitors?
Clopidogrel, ticagrelor
Which pathway mainly occurs in vitro during clotting studies?
intrinsic pathway
Is the intrinsic or extrinsic pathway more important in the body?
Extrinsic
What is the rate limiting step for fibrin formtion?
Factor Xa
What is the coagulation cascade triggered by?
The pathway is triggered by trace amounts of thrombin (which is formed following the activation of the platelet plug)
What are the effects of thrombin?
Activates fibrinogen, activates platelets, activates pro-cofactors (Factor 5 and Factor 8), activates zymogens (Factor 7, 11 and 13).
These will all link together to form a prothrombinase complex which results in the activation of prothrombin to thrombin.
What is the most important step of the coagulation cascade?
Generation of THROMBIN
What does thrombin catalyse in the final step of the coagulation cascade?
Thrombin will catalyse the breakdown of fibrinogen to FIBRIN which is the final step in the coagulation cascade
Describe the initiation phase of the coagulation cascade:
- Damage to the endothelial results in the exposure of Tissue Factor which binds to Factor 7, thereby activating it to Factor 7a
- The tissue factor/factor 7a complex will result in activation of Factor 9 and Factor 10
- Factor 10a binds to Factor 5a which results in the FIRST STEP OF THE COAGULATION CASCADE
What can people with Factor V Leiden not do?
NOTE: people with Factor V Leiden will not be able to bind their Factor 5a to Factor 10a
Describe the amplification phase of the clotting cascade:
- The activated factors 10 + 5 will result in the formation of a small amount of thrombin.
- Once the thrombin has been generated, it will activate platelets.
- The thrombin will also activate Factor 11 which will activate Factor 9
- Thrombin will also activate Factor 8 and recruit more Factor 5a
- Factors 5a, 8a and 9a will bind to the activated platelet which then goes on to perform the last phase of the clotting cascade
Describe the propagation phase of the clotting cascade:
- The activate platelet with factors 5, 8 and 9 will recruit Factor 10a
- This will then result in the generation of large amounts of thrombin (THROMBIN BURST)
- The high levels of thrombin will convert fibrinogen to FIBRIN
- This enables the formation of a stable fibrin clot
Why is the prothrombinase complex important?
The formation of thrombin is influenced by the presence or absence of the above component. If all the components are present, you will be able to activate prothrombin at 300,000 times the rate of Factor 10a alone
Which factors are vitamin K dependent?
2, 7, 9, 10
Where are vitamin K dependent factors produced?
In the liver
How can we activate factors 2, 7, 9 and 10?
To biologically activate these factors, vitamin K is required as a co-enzyme for the gamma-carboxylation of the clotting factors
What can reduce your vitamin K absorption?
Bacteria are important for the production of vitamin K, so taking antibiotics and harming your gut flora can reduce your vitamin K absorption.
Similarly, vitamin K is fat soluble so you need bile to be able to absorb vitamin K (if you have an obstruction of the biliary tree, it can cause vitamin K deficiency)
What is the most common cause of vitamin K deficiency?
Warfarin
What process causes blood clot removal?
Fibrinolysis
What is tissue plasminogen activator (tPA) produced by and what does it do?
Tissue plasminogen activator (tPA) is produced by the endothelium and it converts plasminogen to plasmin
What is the clinical relevance of tPA?
tPA is sometimes given in stroke, MI and peripheral vascular disease
Apart from tPA what else activates plasminogen to plasmin?
Urokinase
What does plasmin break down fibrin into?
Fibrin degradation products (FDP)
What are tPA and urokinase inhibited by?
Plasminogen activator inhibitor 1 & 2.
What is plasmin inhibited by?
Alpha-2 antiplasmin and alpha-2 microglobulin
What does Thrombin-activatable fibrinolysis inhibit?
Fibrin breakdown
What are three physiological anticoagulants?
Antithrombins, protein C and protein S, tissue factor pathway inhibitor
How do antithrombins work?
Antithrombins will bind to thrombin on a 1:1 ratio and it will then be excreted in the urine. There are FIVE types of antithrombin but the most active is antithrombin-III
What is the most thrombogenic condition?
The lack or deficiency of antithrombin is the MOST THROMBOGENIC condition
What is the clinical relevance of antithrombin?
Clinical Relevance: heparin augments the effect of antithrombin-III
What do protein C and protein S do?
In order to stop thrombin generation, activated Factors 5 and 8 need to be in. These factors are inactivated through the protein C and protein S pathway
What activates thrombomodulin (transmembrane receptor)?
The trace amounts of thrombin generated at the start of the cascade will activate thrombomodulin (transmembrane receptor)
What does activation of thrombomodulin do?
Activation of thrombomodulin will open up the receptor for thrombomodulin to bind to Protein C through endothelial protein C receptor (EPCR)
What do you call protein C which is bound to thrombomodulin through the endothelial protein C receptor (EPCR)?
Activated protein C (APC)
In the presence of what does protein C fully activate?*
Protein S (acting as a non-enzymatic co-factor)
What does the fully activated protein C do?
It will inactivate factor 5a and factor 8a
In which condition do you have activated protein C resistance?
Factor V Leiden
What does activated protein C resistance in factor V Leiden mean?
This means that the factor 5a in people with Factor V Leiden, will be resistant to breakdown by activated protein C. This results in a prothrombotic state
What are the two causes of activated protein C resistance?
Mutated Factor 5 (e.g. factor V leiden), or high levels of Factors 8
What step of the coagulation cascade does tissue factor pathway inhibitor target?
The first step of the coagulation cascade (initiation phase)
What factor is released in the first step of the coagulation cascade?
Tissue factor
As soon as the coagulation cascade is initiated and tissue factor is released, tissue factor pathway inhibitor is activated to neutralise what?
TF-factor 7a complex
The TF-factor 7a complex is NOT needed for very long - what is it only needed to activate?
Factor 10 and factor 9
The TF-factor 7a complex is NOT needed for very long, what is it neutralised by?
Tissue factor pathway inhibitor (TFPI)
What are examples of genetic defects that cause excessive bleeding?
Platelet abnormalities, vessel wall abnormalities, clotting factor deficiencies and excess clot breakdown
What are acquired defects that cause excessive bleeding?
Liver disease, vitamin K deficiency, autoimmune disease (platelet destruction), trauma and anticoagulants/antiplatelets
What are examples of genetic defects that cause excessive thrombosis?
Clotting factor inhibitor deficiencies, decreased fibrinolysis
What is an example of an acquired defect that cause excessive thrombosis?
Atherosclerosis
What are the types of disorders of haemostasis?
Vascular disorders (e.g. scurvy), platelet disorders, (e.g. low number or abnormal function), coagulation disorders (e.g. factor deficiency), mixed/ consumption (e.g. DIC)
What is an example of a vascular disorder causing haemostasis
Scurvy
What is an example of a mixed/consumption disorder causing haemostasis?
E.g. DIC
What are clinical features of bleeding disorders?
Local vs general (spontaneous), haematoma or bleeding joint, skin/mucous petechiae and purpura, wound/surgical bleeding
What is haematoma or joint bleed associated with?
Haemophilia
What is skin/mucous petechiae and purpura associated with?
Suggestive of platelet deficiency or von Willebrand disease
What does the timing of bleeding indicate?
Immediate - issue with the primary haemostatic plug (platelet, endothelium, vWF). Delayed - issue with coagulation cascade
What is the difference in the site of bleeding between platelet disorders and coagulation disorders?
Platelet disorders: skin, mucous membranes (epistaxis, gum, vaginal, GI tract).
Coagulation factor disorders: soft tissues, joints, muscles
How do petechiae present as a platelet disorder or a coagulation factor disorder?
Platelet disorder
How do ecchymoses (“bruises”) present as a platelet disorder or a coagulation factor disorder?
As a platelet disorder they are small and superficial and as a coagulation factor disorder they are large and deep
How do haemarthrosis / muscle bleeding present as a platelet disorder or coagulation disorder?
As a platelet disorder - extremely rare. As a coagulation factor disorder - common
How does bleeding after cuts and scratches present as a platelet disorder or coagulation disorder?
Platelet disorder only
How does bleeding after surgery or trauma present as a platelet disorder or coagulation disorder?
Platelet disorder - Immediate, usually mild. Coagulation factor disorder - delayed (1-2 often severe)
When the platelet drops below what level is treatment required?
Treatment is required when platelet count drops below 30 x 109/L. This is associated with spontaneous haemorrhage
Why is it important to look at platelets under the microscope?
Because you could get pseudothrombocytopaenia where the platelets clump together creating an erroneously low platelet count. Microscopy also allows you to see other abnormalities such as Grey Platelet Syndrome (you see large platelets)
What are reasons for disorders of platelets due to decreased number?
Decreased production, decreased survival (ITP), increased consumption (DIC) and dilution
What are reasons for disorders of platelets due to decreased platelet function?
Acquired (e.g. aspirin, end-stage renal failure), congenital (e.g. thrombasthenia), cardiopulmonary bypass
What is the mode of action of antiplatelet drugs?
The main aim of platelet activation if GlpIIb/IIIa activation.
Clopidogrel prevents blood clots by irreversibly binding to the P2Y12 receptor on platelets, preventing adenosine diphosphate (ADP) from activating platelets.
What are the causes immune-mediated thrombocytopaenia?
Idiopathic, drug-induced (e.g. quinine, rifampicin, vancomycin), connective tissue disease (e.g. rheumatoid arthritis, SLE), lymphoproliferative disease, sarcoidosis
What are the causes of non-immune mediated thrombocytopenia?
DIC and MAHA
What happens in idiopathic immune thrombocytopaenic purpura (ITP)?
- ITP is a very common cause of thrombocytopenia.
- Autoantibodies are generated against platelets
- Platelets are tagged by antibodies and are then destroyed in the reticuloendothelial system (liver, spleen, bone marrow - anywhere where you find macrophages)
What is the peak age of acute ITP and chronic ITP?
Acute: children (2-6 years). Chronic: adults (Peak age 20-40 years)
What is the female:male ratio of acute and chronic ITP?
Acute: 1:1, chronic: 3:1
Is there a preceding infection in acute ITP and chronic ITP?
Acute: common. Chronic: rare
What is the onset of symptoms like in acute and chronic ITP?
Acute: abrupt. Chronic: abrupt-indolent
What is the platelet count at presentation in acute and chronic ITP?
Acute: <20,000. Chronic: <50,000
What is the duration of acute ITP and chronic ITP?
Acute: 2-6 weeks and chronic: long-term
Does spontaneous remission occur in acute or chronic ITP?
Acute ITP: common, chronic ITP: uncommon
What is ITP like in adults and in childhood?
Childhood ITP is usually acute, SEVERE but it is self-limiting and resolves without any treatment. In adults, ITP is usually chronic and indolent.
What does treatment of ITP depend on?
Platelet count and symptoms
How does IVIG work in ITP?
Works by competing with the anti-platelet antibodies
How is ITP with platelet count > 50,000 and no symptoms treated?
No treatment
How is ITP with platelet count 20-50,000 and not bleeding treated?
No treatment
How is ITP with platelet count 20-50,000 with bleeding treated?
Steroids and IVIG
How is ITP with platelet count <20,000 and bleeding treated?
Steroids, IVIG, and hospitalisation
Are petechiae in thrombocytopaenia blanching or non-blanching?
They do not blanch
What are features to look for in thrombocytopenia?
Haematomas and subconjunctival haemorrhages
Why is it important to look at the blood film in patients with thrombocytopenia?
There are various causes of thrombocytopaenia that can be diagnosed from the blood film: Vitamin B12 deficiency, acute leukaemia
What are examples of coagulation factor disorders that are inherited?
Haemophilia A and B, Von Willebrand disease, and other factor deficiencies
What are examples of coagulation factor disorders that are acquired?
Liver disease, vitamin K deficiency/warfarin, DIC
What is haemophilia a deficiency of?
Congenital deficiency of factor 8 or 9
What is haemophilia characterised by?
Deep bleeding into joints and muscles
What is the inheritance pattern of haemophilia?
X-linked disease
What pathway is haemophilia a disorder of?
It is caused by an isolated abnormality in the INTRINSIC pathway
What is APTT and PT like in haemophilia?
Prolonged APTT, normal PT
In haemophilia, clotting factor replacement is required temporarily - true or false?
False, required for life
What are clinical features of haemophilia?
Haemarthroses (MOST COMMON), soft tissue haematomas (e.g. muscle atrophy, shortened tendons), other sites of bleeding (e.g. urinary tract, CNS, neck), prolonged bleeding after surgery or dental extractions
What is the factor deficiency in haemophilia A and B?
A: factor VIII. B: factor IX
What is the inheritance pattern in haemophilia A and B?
A and B: X-linked recessive
What is the incidence of haemophilia A and B?
A: 1/10,000 males. B: 1/50,000 males
What is the severity of haemophilia A and B?
In both, related to factor level.
<1% = severe - spontaneous bleeding 1-5% = moderate - bleeding with mild injury 5-25% = mild - bleeding with surgery or trauma
What are the complications of haemophilia A and B?
Soft tissue bleeding. Ecchymoses are typical of coagulation disorders
What is the most common coagulation disorder?
Von Willebrand disease
What is the incidence of VWd?
1/10,000
What is the inheritance pattern of vWD?
Autosomal dominant
What are the clinical features of Von Willebrand disease?
Mucocutaneous bleeding
What is the classification for vWD?
Type 1 - PARTIAL quantitative deficiency
Type 2 - QUALITATIVE deficiency
Type 3 - TOTAL quantitative deficiency
Why is the classification of vWD similar to haemophilia A?
Because there is a strong relationship between vWF and factor 8. Binding of factor 8 to vWF protects factor 8 from being destroyed in the circulation
In Type 1 vWD, what are the results of the diagnostic laboratory tests?
PARTIAL quantitative deficiency. So vWF is low, vWF activity is low, and multimer is normal.
In Type 2 vWD, what are the results of the diagnostic laboratory tests?
Qualitative deficiency. So vWF antigen is normal, vWF activity is low, multimer is normal
In Type 3 vWD, what are the results of the diagnostic laboratory tests?
TOTAL quantitative deficiency. So, vWF antigen is very low, vWF activity is very low and multimer is absent
What are sources of vitamin K?
Green vegetables. Synthesised by intestinal flora
What is vitamin K required for the synthesis of?
Factors 2, 7, 9 and 10 and Protein C, S and Z
What are the causes of vitamin K deficiency?
Malnutrition; biliary obstruction (reduces absorption of vitamin K); malabsorption; and antibiotic therapy (kills gut flora)
What is the treatment for vitamin K deficiency?
Vitamin K, FFP
DIC is an EMERGENCY. What is activation of both coagulation and fibrinolysis is triggered by?
Sepsis (MOST COMMON); trauma (e.g. head injury, fat embolism); obstetric complications (abruptio placentae, amniotic fluid embolism); malignancy; vascular disorders; reaction to toxin (e.g. snake venom); immunological disorders (e.g. severe allergic reaction, transplant rejection)
What is the mechanism of DIC?
- There is systemic activation of coagulation.
- This results in intravascular deposition of fibrin and then thrombosis of small, midsize vessels with organ failure.
- Systemic activation also leads to depletion of platelets and coagulation factors. This leads to bleeding.
What is the pathogenesis of DIC?
Release of thromboplastic material into the cogaulation leads to activation of thrombin which activates the coagulation cascade. (look at diagram in notes)
In DIC, what happens to APTT, PT, TT and fibrinogen?
Increased APTT, increased PT, increased TT, decreased fibrinogen
Plasmin (as part of the fibrinolytic pathway) degrades fibrin (and fibrinogen), resulting in…
FDPs
In DIC what happens to FDP and what does it indicate?
FDP increases. Indicates the presence of plasmin.
What does the release of thromboplastic material into the coagulation activate?
Leads to activation of thrombin
What are the clotting study results in DIC (APTT, PT, TT, fibrinogen, FDP, platelets, schistocytes)
o Prolonged APTT o Prolonged PT o Prolonged TT o Decreased fibrinogen o Increased FDP o Decreased platelets o Schistocytes
Why does DIC produce schistocytes?
Due to fragmentation of red blood cells as they pass through the fibrin mesh in the small blood vessels
What is the treatment of DIC?
Treatment of underlying disorder; anticoagulation with heparin; platelet transfusion; FFP; coagulation inhibitor concentrate (APC concentrate)
Why does liver disease lead to bleeding disorders?
- Decreased synthesis of clotting factors 2, 7, 9, 10, 11 and fibrinogen
- Inadequate intake or absorption of dietary vitamin K deficiency (inadequate intake or absorption)
- Dysfibrinogenaemia
- Enhanced haemolysis (decreased alpha-2 antiplasmin)
- DIC
- Thrombocytopaenia due to hypersplenism
What to consider in the management of haemostatic defects in liver disease?
Treatment for prolonged PT/PTT; treatment for low fibrinogen; treatment for DIC
What is the treatment for prolonged PT/PTT in the management of haemostatic defects in liver disease?
Vitamin K 10mg o.d. x 3 days - usually ineffective; FFP infusion; 25-30% of plasma volume (1200-1500ml); immediate but temporary effect
What is the treatment for low fibrinogen in the management of haemostatic defects in liver disease?
Cryoprecipitate (1 unit/10kg)
What is the treatment for DIC (elevated D-dimer, low factor VII, thrombocytopenia) in the management of haemostatic defects in liver disease?
Replacement therapy
Oral vitamin K (2 mg) has also been shown to be effective in reducing raised INRs without omission of warfarin. IV vitamin K has also been shown to be a safe and effective method of warfarin reversal. True or false?
True
When there is vitamin K deficiency due to warfarin overdose, what should you do if INR is therapeutic - 5?
Lower or omit next dose of warfarin; resume warfarin therapy when INR is therapeutic
What is therapeutic INR for patients on anticoagulant therapy?
For patients who are on anticoagulant therapy, the therapeutic INR ranges between 2.0 to 3.0. INR levels above 4.9 are considered critical values and increase the risk of bleeding.
When there is vitamin K deficiency due to warfarin overdose, what should you do if INR is 5 - 9 and no bleeding?
Lower or omit next dose of warfarin; resume warfarin therapy when INR is therapeutic. Omit dose of warfarin and give vitamin K (1-2.5 mg PO). For rapid reversal: vitamin K 2-4 mg PO (repeat).
When there is vitamin K deficiency due to warfarin overdose, what should you do if INR is > 9 and no bleeding?
Omit dose of warfarin; vitamin K 3-5 mg PO; repeat as necessary. Resume therapy at lower dose when INR therapeutic
When there is vitamin K deficiency due to warfarin overdose, what should you do if INR is > 20 and bleeding?
Omit warfarin; vitamin K 10mg slow IV infusion; PCC (or recombinant human factor VIIa); repeat vitamin K injections every 12 hours as needed
What is management of vitamin K deficiency due to warfarin overdose dependent on?
INR
What contains the vitamin-K dependent clotting factors?
Prothrombin complex concentrate
Warfarin is on its way out, what is coming to the forefront?
NOACs/DOACs
What is the benefit of warfarin?
We can rapidly reverse the bleeding
What are tissue factor pathway inhibitors (initiation phase of coagulation pathway)?
TFPI, Nematode Anticoagulant Protein C2 (NAPc2).
What are factor IX a inhibitors (thrombin generation phase of coagulation pathway)?
IXa inhibitors, IXa antibody
What are protein C activators (thrombin generation phase of coagulation pathway)?
APC, thrombomodulin
What are examples of factor Xa inhibitors (thrombin generation phase of coagulation pathway)?
Fondaparinux, rivorxaban, abixaban, etc.
What are examples of thrombin inhibitors (thrombin activity phase of coagulation pathway)?
Hirudin, bivalirudin, argatroban, melagatran, dabigatran
