Haemostasis Flashcards
Explain the difference between primary and secondary haemostasis.
Primary - vascular contraction, platelet adhesion and aggregate plug forms.
Secondary - stabilises the clot and maintains vasoconstriction.
What occurs during primary haemostasis?
Formation of the platelet plug:
- Vascular endothelial damage
- Decreased factors preventing P adhesion
- Subendothelial ligands exposed - collagen
- Release of vWf
- Release of P-selectin
- Platelets activated, attracted and loosely aggregated
- Local vasoconstriction
What part of the platelet adhere to vWf?
Surface glycoproteins
What substance, exposed by endothelial damage, causes platelet activation?
Collagen
Outline the functions of platelets.
(x5)
- Adhesion via GPI to vWF or collagen
- Aggregation via GPIIbIIIa through fibrinogen or vWF bridges
- Secretion, activated PLT release Thromboxan, serotonin, factorV, ADP, ATP, plasminogen
- Procoagulant surface phospholipids involved in coagulation (PF3)
- Clot retraction
Draw and label a platelet.
- Peripheral Zone
- Glycocalyx
- Plasma membrane
- Lipids
- Receptors
- Surface Connected Canalicular System
- Dense Tubular System
- Sol-Gel Zone
- Cytoskeleton
- Microtubules
- Microfilaments
- Organelle Zone
- Cytoplasmic Content
- PLT Granules
- Alpha
- Dense
- Lysosomes
Describe the features and function of von Willebrand factor.
- Large multimeric plasma protein produced by endothelial cells
- Megakaryocytes and PLT also contain vWf
- Involved in platelet adhesion (via GPIb) and aggregation (via GPIIb)
- Secreted serve as carrier of factor VIII
What is a Weibel–Palade body?
Storage granules of endothelial cells of blood vessels (and heart), they contain vWf and P-selectin.
What occurs during secondary haemostasis?
Formation of the definitive clot
- Exposure of subendothelial structures and platelet activation initiates thrombin generation
- Activation of coagulation cascade
- Irreversible platelet aggregation
- Formation of fibrin clot
- Thrombin converts soluble fibrinogen to insoluble fibrin
- Fibrin crosslinkage forms mesh
What are the end products of the clotting cascade?
Thrombin and fibrin
Draw the “Y” and label the extrinsic, intrinsic and common pathways.
What factors initiate the intrinsic pathway?
-
CONTACT with negatively charged surfaces
- E.g. collagen (subendothelial)
- Glass in vitro
How are the activate versions of clotting factors represented?
“a” added to the end
Outline the intrinsic cascade.
- XIIa activates XI
- XIa activates IX (requires Ca++)
- FIXa + VIIIa = Tenase activates FX (requires Ca++ & PL)
REMEMBER: 12, 11, 9, 8
What is the initiating factor of the extrinsic pathway?
- TISSUE FACTOR (TF)/ III
- Released by damaged tissue
- Aka thromboplastin
- Activated monos, macs
- Possibly endothelial cells
Which complex of the extrinsic pathway is able to activate factor X?
TF - tissue factor
VIIa complex
What role does Vitamin K have in the activation of clotting factors?
Which clotting factors are considered to be Vitamin K dependent?
VitK (NAD(P)H dehydrogenase) is a cofactor for carboxilation of glutamate residues on proteins. This reaction is necessary to allow binding of CFs to Ca which induces conformational changes and enables binding to phospholipid membranes.
II, VII, IX, X - 2,7,9,10
Which clotting factors are produced in the hepatocytes?
II, VII, IX, X, XI, XII,XIII
2,7,9,10,11,12,13
Name and describe the place of origin of the non-enzymatic clotting factors.
TF (III) – transmembrane cellular receptor
Fibrinogen (I), V, VIII – hepatocytes
Where is von Willebrand factor produced?
endothelial cells +/- platelets
Which pathway of haemostasis do each of these lab tests measure?
- ACT
- PTT
- PT
- TT
- Activating clotting time - Intrinsic
- Partial thromboplastin time - Intrinsic
- Prothrombin time - Extrinsic
- Thrombin clotting time - Common
Describe three anticoagulants used in analysis of secondary haemostasis.
Citrate - blue - forms a reversible ionic bond with calcium.
Heparin - green - forms an ionic bond with Calcium. Not used in testing as it causes platelet clumping and is overridden by the coagulation system.
EDTA - purple - a chelator of Ca++ but not is not generally used.
Why should blood samples be chilled on storage?
Slows down coagulation
How is PTT performed?
PTT is prolonged if what? Is this test a sensitive one?
- Citrated plasma + reagents à activated XII
- Measure time to clot
- Prolonged if:
- Factors <30% normal
- Not very sensitive!
How is ACT performed?
What is a normal value for ACT? Is it sensitive or insensitive?
- Procedure:
- Prewarmed (37C) 2ml tube with contact activators
- Add 2ml WHOLE blood
- Record time to definite evidence of a clot
- Normal: 60-120s
- Abnormal if: <5-10% of factor present
- VERY insensitive - Severe thrombocytopenia (<10000/uL)
How is the Prothrombin time performed?
Abnormal value? Sensitivity?
- Performance
- Citrated plasma + fCa2+ + reagent > activation FVII
- Measure time to clot
- Abnormal if factors <30% normal. Not very sensitive.
How is TT performed? Outline two situations in which TT may be prolonged.
- Performance:
- Citrated plasma + thrombin
- Measure time to clot
- Prolonged if:
- Hypofibrinogenemia
- Inhibitor of thrombin present (e.g. heparin, FDPs)
Outline which tests can be used to assess the different coagulation pathways of secondary haemostasis.
- Intrinsic
- TT, ACT
- Extrinsic
- PT
- Common
- PTT, ACT, PT
- Fibrinogen > Fibrin
- TT
Name the 4 main natural anticoagulants + others.
- Intact endothelium
- Antithrombin III (ATIII)
- Protein C
- Tissue Factor pathway coagulation inhibitor (TFPI)
- Others – α2-macroglobulin, protein S, protein Z, α1-proteinase
How does Antithrombin III work?
Where is it produce?
Name two substances which can be used to enhance ATIII action.
- Primarily binds and inactivates THROMBIN (IIa)
- Also binds XIIa, XIa, IXa, Xa
- Produced by liver
- Action markedly enhanced by
- HEPARIN (exogenous or endogenous from mast cells)
- or heparan sulfate (endothelial cells)
Reduced levels of ATIII can be found in which clinical situations?
(x3)
- DIC
- Failure of hepatic synthesis
- Loss of ATIII via GIT or glomerular protein loss
- ATIII is similar size to albumin
Outline the mechanism of action, site of production of the natural anticoagulants protein C and TFPI.
- Protein C
- Hepatocytes, Vit K dependent (like II, VII, IX, X)
- Thrombin > activated protein C (APC)
- Inactivates Va and VIIIa (accelerators of coagulation cascade)
- Tissue Pathway Factor coagulation Inhibitor (TFPI)
- Hepatocytes, endothelial cells, monos/macs
- Inhibits TF-FVIIa by complexing and inactivating
Lysis of platelet-fibrin network
Tertiary haemostasis
How does tertiary haemostasis work?
What factor inhibits this process?
Plasminogen is activated by tissue plasminogen activator (T-PA) to form Plasmin which degrades fibrin to FDPs (including D-dimers)
Plasminogen activator inhibitor (PAI) released by endothelium.
What is the difference between FDP and D-dimers? In which situations are their values altered?
- FDPs
- Fibrinogen, fibrin and cross-linked fibrin breakdown
- Semiquantitative (<1:5, 1:5-1:20, >1:20)
- D-dimers
- Cross-linked fibrin breakdown
- <0.2ug/mL
- Increased in
- DIC/PTE, Hemorrhage, Inflammation, Post-surgery, Hepatic/renal dysfunction
What are the healthy and significant values of FDP blood levels?
Healthy = \< 10ug/ml Significant = \>40ug/ml
Normal level of Fibrinogen in dogs and cats
1.5-3.0g/l
What labratory tests make up a complete clotting profile? (x6)
- Clotting profile
- Coagulation times
- PT and APTT
- Fibrinogen
- FDPs/Ddimers
- Specific factor assays
- vWf
- Antiplatelet antibodies
- Coagulation times
Describe the steps of haemostasis (x7)
- Vessel injury
- Exposure of subendothelial collagen
- Release of Tissue Factor
- Activation of platelets and coagulations pathways
- Platelet adhesion and aggregation
- Release of prothrombotic products
- Fibrin clot stabilizes platelet plug
Outline the causes and clinical signs of primary haemostasis defects.
- Causes
- Platelet disorders
- Vasculopathies
- Relatively uncommon
- Clinical signs - SMALL HOLES
- Ooze from small wounds
- Petechial and ecchymotic haemorrhages
- Bleeding from mucous membranes
- Epistaxis
- Melaena
- Underlying disease
Outline the causes and clinical signs of secondary haemostasis defects.
- Causes
- Defect/deficiency of coagulation factors
- Clinical signs - LARGE HOLES
- Haematomas
- Large ecchymotic haemorrhages
- Haemarthrosis
- Intracavitatory haemorrhage
- Rebleeding after apparently normal haemostasis
- Clinical signs of underlying disease
Which hereditary diseases of the intrinsic pathway are found in which breeds of dog and cats?
(x3/4 + factors involved)
What clinical signs are seen with each?
- Hemophilia A (VIII), hemophilia B (IX)
- A more common than B
- Males – X-linked, GSDs
- Cavitary bleeding – hemarthroses/post-sx
- Factor XI deficiency
- UNCOMMON
- Delayed hemorrhage post trauma/sx
- Kerry Blue Terriers
- Factor XII deficiency
- No clinical signs
- Cats
Aquired defects of the intrinsic pathway are caused by what?
Hepatic disease and heparin
- Normal platelet count
- Normal PTT
- Prolonged PT
- Normal FDPs
This clotting profile is indicative of defects to which coagulation pathway, which factors is effected?
Extrinsic
Factor VII
Which type of hereditary defect of coagulation is found in Beagles?
What clinical signs are seen?
Extrinsic pathway defect
- VII deficiency
- Mild hemorrhage often associated with surgery
- Normal platelets
- Prolonged PT and PTT
- Normal FDPs
This clotting profile is indicative of defects of which part of the coagulation pathway? Which factors are involved?
- INTRINSIC
- EXTRINSIC
- X, V, II
- Decreased platelets
- Prolonged PT and PTT
- Increased FDPs
This coagulation profile is indicative of defects of which areas of Haemostasis?
- Primary
- Secondary
- Intrinsic
- Extrinsic
- Common
- Fibrinolysis
Haematology of a blood sample of a patient with disseminated intravascular coagulation shows what characteristic signs?
- Consumption of platelets and coagulation proteins
- Thrombocytopenia
- Prolonged PT, PTT, TT
- Hypofibrinogenemia
- Decreased ATIII
- Blood flows past clots > schistocytes and/or keratocytes
- Fibrinolysis usually also activated
- Increased FDPs and D-dimers
- FDPs can act as anticoagulants themselves
Describe the aetiology of DIC.
What conditions cause it?
- Widespread activation of coagulation
- Usually started via TF release from damaged/neoplastic tissue or endotoxin-stimulated endothelium
- Conditions that are known to cause it
- Neoplasia (esp. hemangiosarcoma)
- Tissue necrosis e.g. pancreatitis, ischemic bowel
- Severe inflammation +/- endotoxin +/- sepsis
- Heatstroke
What catastrophic consequences can occur with DIC?
- Microthrombi in organs > ischemia and necrosis
- multiorgan failure
- Severe bleeding tendency
What inherited coagulation defect causes prolonged PT without PTT prolongation?
- Hereditary factor XI deficiency
- Hereditary thrombin (Factor II) deficiency
- Hereditary factor VII deficiency
- Hereditary factor XII deficiency
3
What is the other name for an inherited deficiency of factor VIII?
- Hemophilia A
- Hereditary thrombin deficiency
- Hemophilia B
- Hereditary afibrinogenemia
1
What factors are affected by rodenticide poisoning?
- II, III, IV, V
- IV, VII, VIII, IX
- II, VII, IX, X
- I, V, VIII, XIII
3 - vitamin k dependent factors
What type of erythrocyte is commonly seen in animals with fulminant DIC?
- Spherocyte
- Schistocyte
- Target cell (codocyte)
- Elliptocyte
2
RVVT
- Russell’s Viper Venom Test
- Not widely offered
- Direct activation of X
- COMMON pathway
PIVKA
- Proteins Induced by Vitamin K Antagonism Test
- Humans: specifically measure decarboxylated II, VII, IX, X
- Veterinary: Modified PT = Thrombotest PT
- factor V and I are added
- No benefit over a PT !!
Other haemostasis tests
(x3)
- Thromboelastography
- Global haemostasis
- Hypercoagulability
