Haemostasis Flashcards
1
Q
General steps of haemostasis
A
1) Trauma/breach of vascular barrier triggers reflex vascular spasm -> slow down blood flow towards puncture site -> decreased blood loss
2) Primary haemostasis -> formation of platelet plug
3) Secondary haemostasis -> formation of blood clot
4) Clot dissolution
2
Q
What is thrombopoiesis?
A
Formation of platelets
3
Q
What does thrombopoietin (TPO) stimulate?
A
Thrombopoiesis
4
Q
Which organs produce TPO?
A
Liver and kidney
5
Q
What is a megakaryocyte?
A
Precursor for platelets
- can’t enter circulation directly -> turn into fragments first
6
Q
What kind of feedback loop does thrombopoiesis utilise?
A
Neg feedback loop
7
Q
How is TPO removed?
A
Via circulating platelets
8
Q
What is the lifespan of platelets?
A
~ 10 days
9
Q
Where are old platelets removed?
A
Spleen
10
Q
How is a platelet plug formed?
A
1) Platelet adhesion to exposed collagen and subsequent activation (change morphology into ‘spiky’ form)
2) Release of granules containing platelet agonists -> attract more platelet toward puncture site -> encourage further aggregation
3) Platelet aggregates to form plug and release more platelet attracting substances
4) Intact endothelial cells releases NO and prostacyclins that inhibits unintentional platelet activation/aggregation
11
Q
How does body inhibit uncontrolled platelet formation?
A
Intact endothelial cells releases NO and prostacyclins that inhibits unintentional platelet activation/aggregation
12
Q
What is Von Willebrand’s factor (vWF)?
A
Plasma protein produced by platelets that mediate platelet adhesion by binding platelets to exposed collagen
13
Q
How does vWF bind platelets to exposed collagen?
A
Via glycoprotein receptor found on surface of aggregated platelets
14
Q
What does adhesion do to platelets?
A
Activates platelets -> morphological change; spikes -> greater surface area for attachment of platelet
15
Q
How does aggregation of platelets occur?
A
Adhesion triggers release of granules containing platelet agonists -> aggregation
16
Q
Eg of platelet agonists
A
ADP, thromboxane A2 (Tbx A2)
17
Q
What does ADP do?
A
Attracts and activates more platelets
18
Q
What does Tbx A2 do?
A
Promote aggregation and further vasoconstriction at puncture site
19
Q
What does fibrinogen do?
A
Links platelets through glycoprotein receptors
20
Q
Characteristic of fibrinogen links
A
Weak, temporary -> need further reinforcement via secondary haemostasis
21
Q
Describe the process of secondary haemostasis
A
Formation of strong and insoluble fibrin mesh -> trap all the blood cells and clotting factors that try to escape from damaged vessel
22
Q
What does thrombin do?
A
It cleaves and activate fibrinogen as well as factor 13 -> factor 13a
23
Q
What is fibrinogen?
A
Insoluble protein where fibrin mesh originates from
24
Q
What does fibrinogen do?
A
Crosslinks platelets tgt
25
How is a fibrin strand formed?
Fibrinogen (soluble) cleaves and activate into fibrin monomer (insoluble) which polymerise into fibrin strand
26
What is a fibrin mesh?
Many fibrin strands crosslinked tgt
27
What crosslinks fibrin strands?
Factor 13a (fibrin stabilising factor)
28
What is prothrombin?
The inactivated form of thrombin that is present in circulation
29
How is prothrombin activated into thrombin?
Factor 10 -> factor 10a which cleaves and activates prothrombin to form thrombin in the presence of Ca2+ and factor 5a
30
What are the diff pathways of the coagulation cascade?
Extrinsic
Intrinsic
Common pathway
31
How many common coagulation factors are there?
12
32
Which coagulation factors are involved in the extrinsic pathway (exclude the common pathway)?
Factor 7
33
What are the steps to the common pathway?
Factor 10 -> factor 10a which cleaves and activate prothrombin to thrombin in the presence of Ca2+ and factor 5a
Thrombin cleave and activate factor 13 -> factor 13a
Thrombin cleave and activate fibrinogen -> fibrin ---> fibrin strand -> fibrin mesh (crosslinked by factor 13a)
34
What are the steps to the extrinsic pathway (exclude common)?
Damaged tissue releases tissue factor/thromboplastin -> bind to and activate factor 7 to form 7a -> cleave and activate factor 10 in the presence of Ca2+ to form factor 10a -> common pathway
35
Why is it called the extrinsic pathway?
Tissue factor comes from external sources
36
What are the steps to the intrinsic pathway?
Exposure to collagen fibres/foreign surface cause factor 12 to undergo conformational change in the presence of platelet phospholipids -> factor 12 activated to factor 12a
Factor 12a cleave and activate factor 11 -> factor 11a
Factor 11a cleaves and activate factor 9 -> factor 9a in the presence of Ca2+
Factor 9a cleaves and activates factor 10 -> factor 10a in the presence of Ca2+ and factor 8a -> common pathway
37
Why is it called the intrinsic pathway?
Factor 12 initiates the pathway and is a circulating plasma protein (endogenous part of circulatory sys)
38
Why is haemostatic dressing encouraged?
It's a foreign surface -> initiate intrinsic pathway
39
What happens when Ca2+ is absent and it's application?
Clotting pathway is impaired as many of the pathways req Ca2+
Application: prevent blood in test tube from clotting by adding agent to remove Ca2+
40
Which factors are not plasma proteins?
Factor 3 and Ca2+
41
Why does liver damage cause bleeding issues?
Most of the coagulation factors (except thromboplastin and Ca2+) are produced by the liver
Liver damage -> production of clotting factors impaired -> bleeding
42
Which factors req Vit K for production?
Factors 2, 7, 9 and 10
43
Consequence of Vit K deficiency
Bleeding issues
- Vit K needed to produce some clotting factors
- deficiency -> impaired production of some clotting factors
44
What is factor 1, 2, 3 and 4?
Fibrinogen, prothrombin, thromboplastin/tissue factor and Ca2+ respectively
45
Primary vs secondary haemostasis
Primary -> rapid but weak
Secondary -> slower but stronger
46
Why does the body need a complicated cascade for haemostasis?
Amplification -> speed up process
- cascade expands the no. of fibrin activated
Regulation
- multiple feedback loop
47
Why is there a need for 2 pathways?
Stop bleeding -> needs it to be fast
- extrinsic is only 1 step -> extremely fast -> initial clotting
48
How to assess the coagulation sys?
Quick screens
- FBC -> platelet count
- prothrombin time (PT)
- partial thromboplastin time (aPTT)
- activator added to trigger test
Specific/confirmatory tests
- coagulation factor assays
- platelet aggregation test (assess fn of platelet in cases whr platelet count is normal)
- vWF antigen assay
49
What does PT measure?
Time taken for fibrin clot to form after addition of agents
50
Which pathway does PT evaluate?
Extrinsic pathway
51
How is PT time interpreted?
Normal -> ~10s
- time >> than 10s -> extrinsic pathway is impaired
52
What is International Normalised ratio (INR) and what is the normal range?
Corrected ratio of pt's PT vs normal PT
Normal = 0.8 - 1.2
53
Which pathway does aPTT assess?
Intrinsic pathway
54
What is the normal result for aPTT?
25-35s
55
Application of aPTT?
Screen bleeding disorders
Monitor anti-coagulant therapy
- eg: heparin therapy -> make sure blood doesn't clot during surgery
56
What do citrated tubes do to the blood collected?
Remove Ca2+ via Ca2+ chelators to stop coagulation pathway
57
What agents are added to the citrated plasma during PT and why?
Thromboplastin -> activate extrinsic pathway
Ca2+ -> replace Ca2+ that was removed; so that factors can work
58
What agents are added to the citrated plasma during aPTT?
Kaolin -> activator; for contact-dependent activation of factor 12
Phospholipids
Ca2+
59
Is Ca2+ deficiency a common cause for bleeding disorders?
No, a lot of Ca in the body (eg: bones) -> can't reach a level that affects coagulation pathway, hence it is ok to remove Ca2+ from the blood during collection.
60
What kind of genetic disorder is hemophilia?
X-linked recessive disorder
61
Difference btw hemophilia A vs B
A -> factor 8 deficient
B -> factor 9 deficient
62
What test can be used to identify deficiency in a specific coagulation factor?
Coagulation factor assay
63
Treatment options for hemophilia
Activated prothrombin complex concentrates (APCC) = cryoprecipitate + activated FVII
- not long term treatment as it's permanently activated
Recombinant coagulation factors
- long term
- replace defective factors
64
Clinical relevance of presence of petechiae
Indicated low platelet count
65
What does splenomegaly indicate?
Thrombocytopenia
- spleen removes platelet; large spleen -> spleen has been working hard to remove platelets
66
Causes of thrombocytopenia
Dengue
Chemotherapy
- stops proliferation of cell
Immune
- attack any step in thrombopoiesis
67
What is Von Willebrand disease?
vWF deficiency
Autosomal dominant mutation
68
How is Von Willebrand disease detected?
vWF antigen assay
69
Why does Von Willebrand disease prolong aPTT?
vWF binds and stabilise factor 8, vWF deficiency -> factor degrade easily -> intrinsic pathway affected
70
Purpose of anti-coagulation system
Prevents aberrant clot formation
Removal of clots once they're no longer needed
71
General steps of anti-haemostasis
Plug prevention -> inhibit pri haemostasis
Clot prevention -> inhibit sec haemostasis
Clot removal -> fibrinolysis
72
How is plug formation inhibited?
Nitric oxide and prostacyclins inhibit platelet activation and aggregation
- restrict plug formation to damaged wall
73
How is aberrant clot formation inhibited?
Tissue factor pathway inhibitor (TFPI)
- binds/inhibits TF-Viia which is needed for activation of extrinsic pathway
74
How is clot formation inhibited via it's surface receptors?
Thrombomodulin and protein C receptor
- sequesters/inactivate thrombin
- activated protein C/S
75
What are the steps to protein C and S activation?
1) Thrombomodulin sequesters thrombin -> prevent thrombin from cleaving and activating fibrinogens not at site of wound
- EPCR binds protein C -> bring to thrombin-thrombomodulin complex -> protein C cleaved and activated by thrombin
2) Protein C activated by thrombin-thrombomodulin complex -> binds to circulating protein S -> form APC-PS complex (activated protein C-protein S complex)
3) APC-PS complex is active protease -> cleaves and inactivates factor 5a and 8a -> prevent clotting
76
What are the 3 main players for anti-coagulation and what they inhibit?
Tissue factor pathway inhibitor (TFPI)
- inhibit factor 7
Activated protein C and S
- inhibit factor 5a and 8a
Anti-thrombin
77
What produced anti-thrombin (AT)?
Liver
78
What is AT?
Serine protease inhibitors
79
What does AT do?
Sequesters thrombin that leaked from clots -> prevent clotting cascade
Binds/inhibits factor 10a and 9a
Ensures clotting is localised to damaged site
80
What initiates clot removal after wound healing?
Degredation of fibrin mesh
81
What mediates fibrinolysis?
Plasmin
82
How is plasmin regulated?
Circulatory plasminogen = zymogen of plasmin
83
How is plasminogen activated into plasmin?
Activated by tissue plasminogen activator (TPA) in presence of fibrin
84
Where does TPA come from?
Released from endothelial cells
- hence, if wound is fresh -> endothelial cells absent -> TPA not released -> plasmin not activated
85
What is released into the circulation once fibrin mesh is degraded?
Fibrin degradation pdts (FDP)
86
How is fibrinolysis regulated?
No TPA from damaged vessels
- not circulating ard damaged vessels
Low TPA affinity to plasminogen in the absence of fibrin
- w/o fibrin -> plasmin can't be activated -> won't cleave clot when it is not healed
Presence of circulatory plasmin activator inhibitor (PAI) (TPA inhibitor) and anti-plasmin
- TPA and plasmin that strays into circulation rapidly inhibited/sequestered by PAI and anti-plasmin -> fibrinolysis restricted to clots at wounds that are fully recovered
87
Where are fibrins crosslinked?
D-domains
- released in this form upon fibrinolysis
88
What is D-dimer?
Fibrin fragments consisting 2 D-domains + crosslink
89
What does elevated serum D-dimer indicate?
Thrombotic event
- if pt has huge clots w/o fresh wound
90
Concept of homeostasis regarding haemostasis
Increased clotting and decreased anti-clotting -> thrombosis
Decreased clotting and increased anti-clotting -> bleeding issues
91
What is thrombosis?
Clot formation which results in the occlusion of a blood vessel
92
What are the factors that increase the risk of thrombus formation?
Virchow's Triad
- venous stasis
- endothelial damage
- hypercoagulability
93
How does venous stasis increase risk of thrombus formation? Some eg of risk factors
Slow blood flow -> reduce propensity of anti-coagulants to interact w/ coagulation factors
Eg of risk factors:
- post-surgery/immobilisation -> muscle contraction serve as pump to move blood
- trauma -> both endothelial and venous stasis -> clotting to stop bleeding -> block blood vessel -> reduce circulation
94
How does endothelial damage increase risk of thrombus formation? Some eg of risk factors
Intact and smooth endothelial wall inhibits plug/clot formation
Eg of risk factors:
- smoking -> pro-inflammatory damage on vascular wall
- high BP -> force of BP on vascular wall cause damage -> endothelial cells damaged (surface receptors affected)
- fatty food/atherosclerosis -> fatty plaque deposited on surface of endothelial cells block surface + calcification causes damage
95
How does hypercoagulabiltiy increase risk of thrombus formation? Eg of some risk factors
Thrombophilia -> conditions that increase tendency for clot formation
Eg of risk factors:
- hereditary disorders (i.e: hereditary thrombophilia) -> inherit defect that causes defective anti-coagulation factor/increase coagulation factor
- tumor/cancer -> tilt balance of clotting/anti-clotting factors
- oral contraceptive -> increase pdtn of clotting factors
96
Eg of hereditary thrombophilia
Factor V Leiden (most impt)
- significant precedence
- factor 5 become resistant to degradation by APC-PS complex -> continue clotting -> hypercoagulability
Anti-thrombin 3 deficiency
Protein C deficiency
Protein S deficiency
97
What does PTT + activated protein C test for? What are the expected results?
Anti-clotting ability
PTT should be prolonged as activated protein C promotes the degradation of activated factor 5 and 8
- if PTT not prolonged -> protein C is not forming well -> factor 5 not responding well to degradation -> factor 5 leiden
98
How should factor 5 leiden diagnosis be validated?
Confirm w/ factor 5 leiden assay test
99
What are good physiological targets for anti-coagulation?
Inhibit factors that are involved in both intrinsic and extrinsic pathway
- factor 10
- thrombin
100
What are some good drug targets to treat thrombophilia? Eg of drugs
Inhibit Vit K -> affect pdtion of several clotting factors
- eg: warfarin
Inhibit factor 10
- eg: rivaroxaban
Inhibit thrombin
- eg: rivaroxaban and dabigatran
Activate anti-clotting factors (eg: AT)
- eg: heparin
101
Eg of anti-platelet therapeutic drugs and their targets
Aspirin -> inhibit Tbx A2
Clopidrogel -> inhibit ADP
Abciximab (monoclonal Ab) -> inhibit fibrinogen and vWF
102
What is disseminated intravascular coagulation (DIC)?
Serious condition that causes abnormal blood clotting throughout the body's blood vessels
103
Triggers of DIC
Widespread RBC hemolysis
Sepsis -> widespread intro of foreign substances into circulation -> factor 12 activated throughout circulation -> clot form throughout
104
What kind of disorder is DIC and why?
Thrombohaemorrhagic
- widespread thrombus -> increased D-dimers -> thrombotic
- depleted platelets and coagulation factors -> prolonged PT and aPTT -> bleeding
105
Treatment for DIC
Transfusion of coagulation factors
- replace plasma (plasmaphoresis)
