Haemostasis and Thrombosis Flashcards
(47 cards)
1
Q
Functions of vWF
A
- Mediate platelet adhesion where it binds to platelet GPIb-V-IX
- Carrier protein for Factor VIII
Factor VIII is released from vWF by the action of thrombin
2
Q
What is haemostasis
A
Haemostasis = response to blood vessel injury and bleeding
PRIMARY HAEMOSTSIS
- Response to vascular wall injury
- Formation of PLATELET PLUG adhering to endothelial wall
- Limits bleeding immediately
SECONDARY HAEMOSTASIS
- End result is to generate a STABLE CLOT TO REINFORCE PLATELET PLUG
- Coagulation cascade
- Gradually stable plug will be dissolved by FIBRINOLYSIS
Requirements for a clot to form
- Negative charged phospholipid surface
- Replete with coagulation factors and fibrinogen
- Calcium and temperature are important
3
Q
Platelet adhesion receptors
A
GPVI: collagen
Protease activated receptors: thrombin
GP Ib-V-IX: VWF
GP IIb-IIIa: fibrin
4
Q
What is the role of factor XIII?
A
Factor XIII crosslinks fibrin polymers to stabilise the fibrin clot
5
Q
Factor XIII Deficiency
A
- Factor XIII deficiency is an AUTOSOMAL RECESSIVE disorder
- Individuals with factor XIII deficiency form blood clots like normal, but these clots are unstable and often break down, resulting in prolonged, uncontrolled bleeding episodes.
- FXIII consists of two subunits: subunit A and subunit B. Most of the Factor XIII deficiency states are caused by mutations in subunit A; very few have a mutation in subunit B.
CLINICAL FEATURES
- Chronic nosebleeds (epistaxis)
- bleeding from the gums
- discoloration of the skin due to bleeding underneath the skin (ecchymoses)
- solid swellings of congealed blood (hematomas).
- Bleeding into the joints (hemoarthrosis) is rare.
DIAGNOSIS History/Exam APTT and PT are NORMAL Factor XIII assay - diagnostic A clot solubility test - only effective when an affected individual has very low levels of factor XIII. During these tests, a clot is exposed to a solution of 1% monochloracetic acid or 5 m urea. In individuals with less than 1% factor XIII, the clots will breakdown. Most untreated individuals with factor XIII deficiency will have close to 0% factor XIII activity in the blood.
6
Q
What are the natural anticoagulants of the coagulation cascade?
A
- Anti-Thrombin: Factor 10 and Factor 2 (thrombin) (primarily) but also factor 9,11,12
- Protein C + Cofactor Protein S: Factor Va and VIIIa (5,8)
Tissue Factor Pathway Inhibitor: switches off Factor VII (initial factor in extrinsic pathway)
C1 esterase inhibitor - synthesised in liver, inhibits FXIIa, FXIa and PK and complement proteases (C1r, C1s)
Note: in factor V leiden, factor Va is no longer susceptible to cleavage by activated protein C, therefore inactivated more slowly resulting in a hypercoagulable state
7
Q
What are the inhibitors of the fibrinolytic pathway?
A
- Thrombin converts fibrinogen to fibrin
- Plasminogen is activated by tissue plasminogen activator (tPA) from endothelial cells and urokinase plasminogen activator (uPA) to form plasmin
- Plasmin converts fibrin to fibrin fragments
- Plasmin has short half life because it is rapidly inhibits by the alpha-2-antiplasmin
- Plasminogen activator inhibitor (PAI-1) inhibits tPA and uPA preventing the formation of plasmin
8
Q
Coagulation Cascade
A
EXTRINSIC Pathway:
- activated by THROMBOPLASTIN (tissue factor, phospholipid membrane, calcium)
- Factor 7
- PT (prothrombin time) - “play tennis outside”
- TF - FVIIa complex activates Factor X (10) and IX (9)
- Sustained generation of thrombin depends on the activation of factor IX (9) and VIII (8)
INTRINSIC Pathway
- Activated by CONTACT ACTIVATION
- Factor 12,11,9,8
- APTT (activated partial thromboplastin time)
COMMON Pathway
- Factor Xa, Va, II (10,5,2)
- APTT and PT changes
- Factor Va bind Factor Xa and together form the prothrombinase complex, which converts prothrombin (II) to thrombin (IIa)
- Thrombin converts fibrinogen to fibrin which undergoes polymerisation to form an insoluble fibrin clot
- Factor XIII stabilises and crosslinks overlapping fibrin strands
THROMBIN TIME Prolonged:
- Heparin (indirect thrombin inhibitor, reptilase normal)
- Dabigatran (direct thrombin
- Low fibinogen levels/dysfunctional fibrinogen, DIC
- Increased fibrin degradation products
INR = PT (patient)/PT (normal plasma)
REPTILASE TEST
- Normal < 24s
- NOT prolonged by an form of heparin or direct thrombin inhibitor
- All other causes of prolonged TT will also prolong reptilase
9
Q
Mixing Study
A
- Mixing study is performed when the APTT is prolonged
- It helps to differentiate between a prolonged clotting time due to a factor deficiency vs. factor inhibitor eg: acquired factor VIII inhibitor or in the laboratory test, eg: lupus anticoagulant.
- Other interfering substances that can also act as inhibitors include heparin fondaparinux, DOACs, elevated CRP
It involves mixing the patient’s plasma with the control plasma in 1:1
- If the prolonged APTT/PT CORRECTS when the control plasma is added, this suggests that there is a FACTOR DEFICIENCY or SLOW ACTING INHIBITOR
- If the APTT/PT remains PROLONGED when the control plasma is added, this indicates there is an inhibitor present in the sample.
LUPUS ANTICOAGULANT
TIME DEPENDENT INHIBITOR - ACQUIRED FACTOR VIII DEFICIENCY
- In factor VIII deficiency, the immediate mixing study will correct but when it is incubated for 1-2 hours at 37 degrees, it will show prolonged APTT. Delayed reactivity is characteristic of factor VIII inhibitors.
10
Q
Features of DIC
A
Prolonged PT/APTT/TT Low fibrinogen Low platelets Elevated D dimer Blood film: thrombocytopenia, red cell fragmentation
Syndrome of systemic intravascular activation of the coagulation system
- Fibrin deposition/microangiopathy
- Microvascular dysfunction/organ ischaemia
- Consumption of platelets and coagulation proteins with increased bleeding risk
CAUSE
- Sepsis
- Trauma
- Malignancy: carcinoma, APML, MDS
- Pancreatitis
- Obstetric: amniotic fluid embolus, abruption, HELLP
- Liver failure
- Snake venom
Mx
- Platelets
- FFP (coagulation factors)
- Cryoprecipitate (fibrinogen)
11
Q
Which medications are:
- Indirect Factor Xa/IIa Inhibitors
- Direct Factor Xa Inhibitors
- Direct thrombin inhibitors
A
- Indirect Factor Xa/IIa Inhibitors
Heparin: LMWH, UFH
Fondaparinux
Danaparoid - Direct Factor Xa Inhibitors
Apixaban
Rivaroxaban - Direct thrombin inhibitors (IIa)
Dabigatran
Bivalirudin
12
Q
Effect of DOACs on lab results
A
- Dabigatran: prolonged TT, APTT
- Rivaroxaban: Prolonged PT (could be normal), rivaroxaban assay
- Apixaban: Prolonged or normal PT, apixaban level. Normal PT does not exclude presence of therapeutic apixaban
- dRVVT - prolonged in all 3 agents
13
Q
Reversal of:
- Warfarin
- Dabigatran
- Apixaban
- Rivaroxaban
A
- Warfarin: vitamin K, prothrombinex
- Dabigatran: Idarucizumab
- Apixaban/Rivaroxaban: Andexanet alfa
Ciraparantag: reverse UFH, LMWH, direct thrombin + direct Xa inhibitors
14
Q
Reversal of:
- Warfarin
- Dabigatran
- Apixaban
- Rivaroxaban
A
- Warfarin: vitamin K, prothrombinex
- Dabigatran: Idarucizumab
- Apixaban/Rivaroxaban: Andexanet alfa
Ciraparantag: reverse UFH, LMWH, direct thrombin + direct Xa inhibitors
15
Q
Causes of isolated prolonged PT/INR
A
- Vitamin K antagonist (warfarin) or deficiency
- Liver disease
- Factor VII deficiency
- Rivaroxaban/Apixaban
16
Q
Causes of isolated prolonged APTT
A
- Heparin
- LMWH
- Deficiency of 12,11,9,8
- Von willebrand disease
- Lupus anticoagulant
17
Q
Causes of prolonged PT and APTT
A
- DIC: low fibrinogen, raised d dimer
- Liver disease
- Common pathway deficiency - 2, 5, 10, fibrinogen
18
Q
Causes of prolonged TT
A
- Heparin
- Dysfibrinogenemia
- Hypofibrinogenemia
- Thrombin inhibitor - dabigatran
19
Q
Following dyspnoea, what is the most common clinical feature of acute PE? A. Calf swelling or pain B. Haemoptysis C. Pleuritic pain D. Tachycardia E. Wheezing
A
D. Tachycardia
20
Q
Indications for thrombophilia screen
A
- Fam hx <45yo
- Multiple or recurrent thrombosis
- Thrombosis in unusual sites - portal vein thrombosis
21
Q
Management of VTE
A
- Strongly provoked (major surgery/trauma): 3 months
- Minimally provoked: 3 months and modify as per patient RF
- Unprovoked: indefinite anticoagulation
- For distal DVT without persisting risk, may be able to stop after 6 weeks
Use DOACS
22
Q
Strong RF for recurrent VTE
Factors that have little or no effect on recurrence
A
STRONG RF
- Unprovoked VTE
- Prior VTE
- PE or proximal DVT
- Persistent RF, eg: active cancer, APLS
- Antithrombin,, protein C/S deficiency
LITTLE/NO EFFECT
- Factor V Leiden or prothrombin gene hetrozygosity
- Residual thrombus on imaging
23
Q
VTE risk caused by hormones
A
- Highest post partum and then pregnancy
- Progesterone only (mirena, implanon, minipill): no significant risk except depot provera.
- Combined OCP higher risk
24
Q
How do you treat antiphospholipid syndrome?
A
Warfarin
25
Heparin induced thrombocytopenia
- Is a profoundly thrombotic state
- Iatrogenic disorder mediated by IgG antibodies that bind PF4-heparin complexes (platelet factor 4)
- These antibodies cause a hypercoagulable state by activating platelets and procoagulant microparticles
- One-third to one-half of patients with HIT develop venous, arterial or microvascular thrombosis
- Unfractionated heparin (UFH) is associated with a 10-fold increased risk of HIT compared to LMWH
DIAGNOSIS:
• It is diagnosed via the 4Ts score which assesses the pretest probability of HIT by looking at:
○ The degree of Thrombocytopenia: platelet count >50% and nadir >20x10^9
○ The Timing relative to heparin exposure (onset: 5-14 days)
○ The presence of Thrombosis - new VTE, skin necrosis, adrenal haemorrhage
○ Other causes of thrombocytopenia
○ A total score of 8 can be achieved where pre-test probabilities for HIT are:
- 0-3 points: low probability
- 4-5 points: intermediate probability
- 6-8 points: high probability
Diagnosis
- Clinical scenario, HIT pre-test probability score
- Immunoassay to detect HIT antibody that binds PF4
- Functional assay: serotonin release assay, heparin induced platelet aggregation
- The PF4 ELISA is a screening test
- The serotonin release assay is the gold standard test for confirmation of HIT.
Tx:
- Stop heparin
- Danaparoid: heparinoid, to monitor check anti Xa, can worsen renal fx, half-life of 24 hours
- Fondaparinux: factor Xa inhibitor – similar to LMWH
- Bivalirudin: direct thrombin factor IIa inhibitor, check APTT/PT
Direct thrombin inhibitor (lepirudin, argatroban) or Xa inhibitor (danaparoid)
- Must avoid warfarin (increase d thrombosis due to reduce protein C levels)
26
Pathophysiology of VITT - vaccine induced thrombocytopenia and thrombosis
• Adverse effect of certain adenoviral vector COVID-19 vaccines- Astra Zeneca. Johnson and Johnson/Janssen
VITT resembles HIT (heparin-induced thrombocytopenia)
• High levels of anti-PF4 antibodies
• Platelet-activating immune complexes (FcɣRIIa-dependent)
• Subsequent platelet aggregation and clot formation
VITT differs from HIT
• VITT occurs without exposure to heparin
• Pattern of platelet reactivity, does not demonstrate typical heparin-dependence, as seen with HIT
27
When to suspect VITT?
- D dimer > 5 x upper limit of normal AND
| - Platelets < 150 x 10^ 9
28
Compare platelet vs clotting factor bleeding
PLATELET TYPE
- Site of bleeding: skin, mucous membranes (epistaxis, gums, vaginal, gastrointestinal)
- Skin cut bleeds: yes, prolonged
- Petechiae: common
- Ecchymoses: small/superficial
- Hemarthrosis/muscle bleeding: extremely rare
- Bleeding after surgery: immediate, usually mild
CLOTTING FACTOR
- Site of Bleeding: deep in soft tissues (joints, muscles)
- Skin cuts bleed: no, minimal
- Petechiae: No, rare
- Ecchymoses: large, deep (haematomas)
- Haemarthrosis/muscle bleeding: common
- Bleeding after surgery: may be delayed (1-2 days), often severe
29
Types of Haemophilia
| Relationship of bleeding severity to clotting factor
Haemophillia A - factor VIII
Haemophillia B - factor IX
Haemophillia C - factor XI (no correlation with factor level)
A and B are both X linked RECESSIVE inheritance although can be acquired due to formation of antibodies. C can occur in both sexes but mostly Ashkenazi jews.
- Severe: <1% of normal, spontaneous bleed in joints + muscle
- Moderate: 1-5%, occasional spontaneous bleeding, prolonged bleeding with minor trauma/surgery
- Mild: 5-40%, spontaneous bleeding rare, severe bleeding with major surgery
Factor XI Deficiency
- Autosomal recessive
- Mild bleeding disorder, usually manifests after trauma/surgery
- Bleeding risk correlates poorly with FXI levels
- Consider replacement for moderate/high risk surgery in women who have not undergone significant hemostatic challenge
30
Which condition can be associated with severe haemophilia A?
Turners Syndrome
31
What is a prophylaxis medication that be used for haemophili A?
EMICIZUMAB
Bridges activated Factor IX and Factor X to restore function of missing FVIIIa.
FITUSIRAN
- Small interfering RNA which prevents production of antithrombin in liver --> rebalance haemostasis = effective in both Haemophilia A and B
32
Function of VWF
Function
- Promote normal platelet adhesion and aggregation
- VWF bridges platelets to subendothelium at sites of vascular injury
- Stabilising unactivated factor VIII in the plasma
- Increases half life by x5 fold
Binding to platelets
- Binding to platelets requires initial activation or alteration in the structure of VWF so that the binding sites in the A1 domain can engage the platelet receptor GP1b-IX-V complex on platelet surface
33
Inheritance patterns of VWD
- Most cases of VWD are transmitted as an AUTOSOMAL DOMINANT trait; this includes types 1 and 2B, and most types 2A and 2M.
- In contrast, type 2N and type 3 VWD are AUTOSOMAL RECESSIVE
Note: Haemophilia A and B are X-linked
34
Features of VWD
- Autosomal inheritance mostly dominant
Presentation
- Bruising + epistaxis
- Menorrhagia
- Post partum haemorrhage
- Bleeding after minor dental surgery
- Joint/soft tissue bleeding uncommon
35
What can increase VWF levels?
- Advancing age
- Non-O blood group
- Lewis blood group
- Adrenaline/Exercise
- Inflammation
- Hormones - pregnancy, OCP
36
Types of VWD
- Inherited deficiency (QUANTITATIVE) or dysfunction (QUALITATIVE) of vWF
- Gene located on chromsome 12
- Low FVIII and high APTT
Type 1 - partial quantitative deficiency, mild-moderate bleeding
Low levels, but normal function
Type 2 - qualitative issue, normal levels but abnormal function. cant form a multimer or cant bind to glycoprotein receptor or cant bind to factor 8
- 2A: selective deficiency HMW multimers - decreased VWF-dependent platelet adhesion
- 2B: selective deficiency HMW multimers, increased affinity of VWF for platelet GP1b
- 2M: no loss of HMW multimers but dysfunctional VWF
- 2N: decreased binding affinity for FVIII
Type 2 uncommon, variable -usually moderate bleed
2A - reduced large vWF, 2B mutant vWF (aggregation occurs without injury), 2M - reduced affinity to platelets, 2N- abnormal vWF (reduced affinity to factor 8)
Type 3 - severe disease, complete deficiency of vw factor which also causes VIII depletion (AR inheritance, whereas haemophillia A is X linked) - high bleeding
37
Treatment of VWD
- Desmopressin = stimulation of endothelial cells = release of endogenous vWF and FVIII- not effective in type 3,2B
- VWF concentrates
- Tranexamic acid - impair fibrinolysis by inhibiting plasmin generation
CI to TXA: DIC, renal tract bleeding
Adjunct
- Manage menorrhagia: OCP, endometrial ablation
- Optimise iron
- Avoid AP + AC
38
What is the most common cause of acquired protein C resistance?
Factor V leiden
- Most common inherited thrombophilia, autosomal dominant
- Point mutation in the G1691A factor V gee which leads to an amino acid substitution that renders Factor V resistant to inactivation by activated protein C
- Factor V leiden accounts for > 90% of activated protein C resistance, other causes less common, APLS, pregnancy, cancer
- Diagnosed by PCR testing of factor V gene
- Presence can be detected through an activated protein C resistant assay
- Heterozygosity for factor V leiden only a mild RF for VTE recurrence
- Homozygosity stronger +/- arterial thrombosis
39
What are the major platelet adhesion receptors?
(a) Integrin a2b3/glycoprotein IIb/IIIa
- Responsible for STABLE PLATELET ADHESION AND AGGREGATION
- Major adhesive ligand if fibrinogen/fibrin
- GP IIb/IIIa antagonists: abciximab, tirofiban
(b) GP Ib/V IX complex
- Second most abundant receptor
- Major adhesive ligand is vWF
- Caplacizumab: GP Ib/V IX antagonist
(c) G protein coupled receptors - where soluble agonists act to stimulate platelets
- PAR 1 and 4 receptors - where thrombin acts
- P2Y12 receptors - where ADP acts, target for clopidogrel, prasugrel and ticagrelor
40
How does a platelet plug form?
1. INJURY to endothelium leads to exposure of circulating blood to subendothelial elements
2. PLATELET ACTIVATION: platelet stimuli include collagen (binds to GP1a/IIb and GPV1 platelet receptor), thrombin (PAR1 and PAR4 receptors on platelets), ADP (P2Y1 and P2Y12 receptors) and epinephrin
3. PLATELET ADHESION
4. PLATELET AGGREGATION
PLATELET SECRETION
41
Where are procoagulants synthesised?
- All procoagulants are synthesised in the liver except vWF
- vWF synthesised in megakaryocytes and endotheial cells
- Factor VIII: endothelial cells in liver and other tissue
vWF stabilises factor VIII
42
Causes of hypofibrinogenemia
(A) CONGENITAL
- Quantitative: afibrinogenemia, hypofibrinogenemia
- Qualitative: dysfibrinogenemia
(B) ACQUIRED
- Liver disease
- DIC
- HLH
- Anti fibrinogen antibodies
- Autoimmune conditions - SLE/RA/UC/MM
Prolonged PT/APTT
Low fibrinogen
43
Protein C and S deficiency
- Protein C deficiency - inactivates factor Va and VIIIa
| - Protein S is cofactor of protein C
44
What is HLH
- Hemophagocytic lymphohistiocytosis (HLH): a life-threatening hematologic disorder involving pancytopenia and severe inflammation due to increased activity of cytotoxic T cells and macrophages from cytokine production
- May be triggered by infection, malignancy or rheumatological disease
Diagnosis
(a) Molecular diagnosis consistent with HLH: pathologic mutations of PRF1,UNC13D, Munc18-2, Rab27a, STX11, SH2D1A, or BIRC4
or
(b) Five of these eight criteria:
- Fever
- Splenomegaly
- Cytopenia of two or more cell lines (i.e. Hb <90 g/L, platelets <100 x 10E9/L, neutrophils <1 x 10E3/L)
- Either elevated triglycerides or low fibrinogen
- Histopathological evidence of hemophagocytosis (on either bone marrow, spleen or lymph node biopsy)
- Serum ferritin > 500 mcg/mL (often much higher in HLH: ferritin > 3000 ng/mL as concerning for HLH and ferritin > 10 000 as highly suspicious (96% specific))
- Low or absent NK cell activity (by flow cytometry, if available)
- Soluble CD25 > 2400 U/mL
Mx
- Treatment of the underlying cause
- Chemotherapy may be required in severe cases (e.g. etoposide, dexamethasone, and cyclosporine)
- Platelet transfusion as indicated
- Supportive care: pneumocystis jiroveci prophylaxis, fungal prophylaxis, intravenous immunoglobulin supplementation, and neutropenic precautions
45
MOA of heparin (unfractionated heparin) vs LMWH (clexane)
Heparin - factor II and Xa
| LMWH - factor Xa
46
What does thrombin (IIa) activate?
```
5, 8 , 11, 13, Protein C
Factor V
Factor VIII
Factor XI
Converts fibrinogen to fibrin and activates Factor XIII
Protein C + thrombomodulin
```
47
What are vitamin K dependent proteins?
Prothrombin
FVII
FIX
protein C, protein S
