Coagulopathies 1&2 Flashcards
Define haemostasis
The interaction b/w BVs, PLTs and coagulation factors that normally maintains blood in a fluid state and allows for formation of PLT plugs and clots when vessels are injured
4 components of haemostasis
- endothelium
- PLT
- coagulation factors
- fibrinolytic factors
Result of abnormal haemostasis
haemorrhage or thrombosis
Outline endothelial cells in haemostasis
- flattened cells that line BVs
- have pro- and anticoagulant properties
- normally are anticoagulant
- act as barrier to subendothelial collagen which is procaogulant
Describe von Willebrand’s factor (vWF)
- produced by endothelium and PLTs
- stored in Weibel Palade bodies
- released early in haemostatic process
- responsible for PLT adhesion to collagen
Describe PLTs
- small, discoid, anuclear cells found in circulation
- 3-5 micrometers, pale basophilic, small red granules
- derived from cytoplasm of megakaryocytes in BM (thrombopoiesis)
- mediated by thrombopoietin
- circulate for 5-9d (most spp)
Describe PLT structure
- OM contains Rs for adhesion and aggregation
- contain cytoskeleton with actin and myosin that allows for shape change
- contain membrane bound granules (alpha granules are red and contain vWF, fibrinogen an factors 5 and 8.. Dense granules contain ADP and Ca)
Describe PLT surface receptors
- Gps associated with PLT membrane
- GP1b binds vWF
- GP2b3a binds fibrinogen on adjacent PLTs and allows PLTs to aggregate
- defects in Rs lead to abnormal PLT function and clot formation
Stages of haemostasis
- PRIMARY: formation of primary PLT plug
- SECONDARY: activation of coagulation cascade and generation of insoluble fibrin which stabilises the PLT plug
- FIBRINOLYSIS: breakdown of fibrin and PLT plug
Steps in primary haemostasis
- damage to endothelium and exposure of subendothelial collagen
- vWF released from damaged endothelium
- PLT adhesion occurs
- PLTs bind to collagen via R GP1b and vWF from endothelium
- once PLTs have adhered to collagen, they undergo shape change and become spherical with filipodia
- additional receptors for vWF (GP1b) and fibrinogen (GP2b3a) are exposed
Describe PLT aggregation
- PLTs bind fibrinogen via GP2b3a
- this occurs b/w adjacent PLTs thereby forming a clump/aggregate of PLTs
- fibrinogen is generated from the coagulation cascade, it is released from PLTs and found in plasma
Describe PLT secretion
- aggregating PLTs rapidly degranulate
- release of ADP, fibrinogen, vWF
- Thromboxane A2 also released from PLT membrane
- these all increase PLT adhesion and aggregation
- PLTs also release factors 5 and 8 which are involved in coagulation
Describe secondary haemostasis
- involves activation of coagulation cascade
- soluble enzymes (serine proteases) found in circulation
- each step of coagulation cascade converts one of these factors from an inactive stage (proenzyme) to its active state
- each step amplifies the system
- end result is the formation of insoluble fibrin
- fibrin stabilises the primary PLT plug
Outline the coagulation cascade
- divided into intrinsic, extrinsic, common pathways
- done to facilitate lab tests
- division doesn’t exist in vivo
Describe the extrinsic system (initiation)
- most important in vivo
- tissue factor (TF) released from damaged tissue binds to and activates F8 in presence of Ca
- TF-F8 complex activates F5 of the common pathway and F9 of the intrinsic pathway
Describe the intrinsic pathway (amplification)
- F7 is activated by contact with a negatively charged surface (cofactor HMWK)
- activated F8 cleaves and activates F11 which in turn activates F9 (calcium required)
- activated F9 in turn activates F10 of the common pathway (calcium required)
Describe the common pathway
- starts with activation of factor 10
- activated F10 binds activated F5 and calcium on PLT surface
- this complex converts prothrombin (F2) to thrombin (F2a)
- thrombin converts fibrinogen (F1) to fibrin (F1a)
- fibrin crosslinked by activated F13
What are inhibitors of coagulation?
- Antithrombin 3: inhibits thrombin and activated F10
- Activity of AT3 increased by heparin from endothelium
- Protein C: inactivates factors 5 and 8
Outline fibrinolysis
- enzymatic breakdown of fibrin by plasmin
- plasmin is derived from plasminogen found in PLT membrane and plasma
- plasmin degrade to both fibrinogen and fibrin to produce Fibrin Degradation Products (FDPs)
Describe lab evaluation of PLTs by PLT concentration
- automated counts can be done on PLTs collected into EDTA as part of CBC
- good accuracy for all spp except cat/sheep/ goat d/t overlap b/w RBC and PLT size (i.e. small RBCs)
- PLT clumps also cause inaccurate counts d/t lack of even distribution and d/t overlap in size b/w clumps and RBCs (v. common in cats)
- can also estimate PLT count by SMEAR. Must be done in cats and CKCS as latter often thrombocytopaenic with giant PLTs which may be counted as RBCs d/t large size
Describe interpretation of PLT numbers
- upper reference limit (>1000*1069/L) are consistent with THROMBOCYTOSIS and may be associated with increased risk of thrombosis
What does buccal mucosal bleeding time assess?
- PLTs function (not number)
- measures length of time for PLT plug to form: evaluates primary haemostasis and PLT function
- use spring loaded cassette to make small incision into buccal mucosa and blood is blotted until bleeding stops
- v low sensitivity
- INCREASED: if thrombocytopaenia, vWF dz, disorders of PLT function. Will NOT be increased with coagulation deficiencies
Name 3 disorders of PLTs
- thrombcytopaenia
- thrombocytosis
- disorders of PLT function
What can PLT disorders lead to?
- haemorrhage if PLT numbers are decreased or function is impaired. typically seen as ecchymoses or petechiae
- can lead to increased risk of thrombosis if numbers increased
Thrombocytopaenia mechanisms
- INCREASED PLT DESTRUCTION OR CONSUMPTION: immune-mediated, haemorrhage, DIC, sequestration
- DECREASED PRODUCTION (BM problem)
- INFECTIOUS: numerous causes
Outline immune-mediated thrombocytopaenia (IMTP)
- commonest cause of thrombocytopaenia
- PLT numbers often v low
What is Evan’s syndrome?
concurrent immune-mediated thrombocytopaenia and anaemia
How are RBCs normally destroyed?
Majority destroyed in periphery, subset can be destroyed by megakaryocytes
Types of IMTP
- PRIMARY: Abs are produced against PLT antigens
- SECONDRAY: other immune dz (SLE), drugs/vaccine / injection, neoplasia, infectious
CS - IMTP
- profound thrombocytopaenia (always recheck #s, look for clumps on smear and clots in tube)
- evidence of petechial or ecchymotic haemorrhages
- Hx of bleeding from gums, mucosal surfaces, prolonged bleeding from wounds etc
Dx - IMTP
- difficult to confirm as dx of exclusion
- may see megakaryocyte hyerplasia (BM)
- BM exam can be done even if PLT #s v low - animals rarely bleed from this site
- Anti-platelet Ab - need large volumes of blood as PLT #s will be low
- response to tx
Causes - thrombocytopaenia
- INCREASED CONSUMPTION
- SEQUESTRATION
- DECREASED PRODUCTION
- INFECTION
Outline increased consumption as cause of thrombocytopaenia
- haemorrhage: #s shouldn’t be lower than 100*10^9/L
- DIC: #s may be v low
- usually with DIC there will be other signs of a coagulation defect
- check PT, PTT - prolonged
- check FDPs - decreased
Outline sequestration as a cause of thrombocytopaenia
Rare but may occur with splenomegaly or with large cavitated mass
Outline decreased production as a cause of thrombocytopaenia
- BM dz
- neoplasia
- drugs
Outline infectious causes of thrombocytopaenia
- number of causes possible including immune-mediated and decreased production: FeLV, BVD, Ehrlichia, Leishmania
List some disorders of PLT function
- Glanzmann’s thrombocytopaenia - defect in GP2b3a
- Canine thrombopathia - abnormal GP2b3a exposure and impaired degranulation
- Bovine thrombopathia - defect not known
Describe Glanzmann’s thrombocytopaenia
- defect in GP2b3a
- otterhounds and Great Pyrenees
- Quarter horse
- defective PLT aggegation and abnormal clot retraction
Describe canine thrombopathia
- abnormal GP2b3a exposure and impaired degranulation
- Basset hounds
Describe bovine thrombopathia
- defect not known
- simmentals
- mild to severe bleeding
3 broad causes of thrombocytosis
- physiological (tx)
- reactive (secondary)
- essential thrombocythemia
What can cause transient physiological thrombocytosis?
epinephrine induced splenic contraction
Describe reactive (secondary) thrombocytosis
- increased thrombopoeitin and possibly IL-6
- inflammation, haemorrhage, iron deficiency
Outline essential thrombocythemia
- PLT equivalent of leukaemia
- myeloproliferative disorder
- marked persistent increase in PLTs
- BM megakaryocytes increased and may have abnormal morphology
- function variable - may see petechiae and ecchymoses or thrombosis
- TPO levels normal or increased
Describe von Willebrand’s disease
- vWF is a plasma GP needed for PLT adherence to collagen and formation of primary haemostatic plug
- synthesised by endothelial cells, PLTs and megakaryocytes- circulates bound to F8 (protective function of F8)
- may see concurrent decrease in F8
- exists in small, medium and large multimers: large multimers are the most active in haemostasis
- common in dogs, rare in cats/horses
- 3 types
CS - von Willebrand’s disease
- mucosal bleeding (GIT, epistaxis, haematuria)
- bleeding may be absent
- no petechiae (differentiate it from other PLT disorders)
- see prolonged buccal mucosal bleeding time w/o thrombocytopaenia
- ## clotting times usually normal but PTT may be prolonged d/t decrease in F8
Type 1 von Willebrand’s disease
- ALL multimers present but at decreased concentrations
- variable severity of bleeding but not until concentration of vWF are
Type 2 von Willebrand’s disease
- qualitative abnormalities in vWF structure and function
- often disproportionate decrease in large multimers
- severe and uncommon
- seen in german shorthaired and wirehaired pointers, one horse case
- autosomal recessive
Type 3 von Willebrand’s disease
absence of all vWF multimers
Tests/Dx for von Willebrand’s disease
- measure levels of vWF Ag
- collect blood into EDTA or citrate (for latter, ensure blood: citrate ratio of 1:9)
- vWF levels will be decreased by clots in the sample and by hemolysis but are unaffected by lipemia
- separate plasma immediately, freeze and ship with ice
- ELISA: quantitative measurement of vWF using species specific Abs,
Interpretation of vWF ELISA
-
Tx - vWD
- transfusion to supply vWF
- CRYOPRECIPITATE best as concentration of vWF 5-10 times greater than plasma, give 1 IU/10kg
- PLASMA at 6-12ml/kg if cryoprecipitate not available
- whole blood if animal is anemic
- desmopressin (DDAVO) as preop prophylaxis for dogs with type 1, causes release of vWF from endothelium, give 1microg/kg SC 30 min prior to sx, also an IN preparation available
Outline sample collection for lab evaluation of coagulation
- citrated plasma (most tests)
- ratio of anticoagulant: blood should be 1:9 (fill to line), always check for clots
- don’t sample through herparinised catheters
- minimise trauma when collecting blood otherwise will activate PLTs and coagulation
- centrifuge to separate plasma within 1hr
- analyse within 4 hrs or freeze plasma
- always include a spp specific control
Describe the ACT test
- Activated Clotting Time
- evaluates intrinsic and common pathways
- collect 2ml whole blood into ACT tube containing diatomaceous earth
- incubate for 60s at 37 degrees and check for clots every 5-10s
- time to initial signs of clot is the ACT (s)
- interpretation similar to PTT but less sensitive
- will be prolonged with thrombocytopaenia
Describe the PTT test
= Partial Thromboplastin Time
- screening test for intrinsic and common pathways
- incubate citrated plasma with excess phospholipid, contact activator and calcium
- measure time to formation of clot
- lipemia, hemolysis, oxyglobin tx and icterus interfere with clot formation
Explain PTT interpretation
- prolonged PTT indicates defect in intrinsic factors (12, 11, 9 and 8) or common pathways (F10, 5, 2 and fibrinogen)
- factor activity must be
Describe the PT test
- prothrombin time
- screening for defects in extrinsic and common pathways
- incubate citrated plasma with tissue thromboplastin (TF) and calcium
- measure time to clot formation
Explain PT interpretation
- prolonged PT indicates defect in extrinsic (factor 7) or common pathways of coagulation
- Factor activity must be
Which coagulation tests measure intrinsic and common pathways?
PTT and ACT
Which coagulation test measures the extrinsic and common pathways?
PT
What are other coagulation tests?
- specific factor analysis
- to detect specific factor deficiencies
- usually done to detect hereditary deficiencies
- done by correcting PT and PTT of test plasma with normal plasma
What are tests of fibrinolysis?
= FDPs
- use latex agglutination test
- special test and kit requires specialised tubes so lab needs to be contacted
- done on serum (needs to be separated within 30 minutes)
- test immediately or freeze
- INCREASED FDPs: with DIC
- not specific
- may also increase with haemorrhage, jugular vein thrombosis (horse) and liver disease
- D-dimers: plasmin mediated degradation of cross-linked fibrin, detects fibrinolysis after fibrin cross-linking
Examples - disorders of coagulation
- acquired factor deficiencies
- main one is the result of vit K deficiency
- typically with rodenticide toxicity (coumarin, indanedione) or sweet clover ingestion (cattle)
- vit K dependent factors are factors 2, 7, 9 and 10.
Outline mechanism of vit K deficiency
- factors 2, 7, 9 and 10 are produced in liver
- are activated by vit K dependent carboxylase: this step requires reduced vit K
- production of reduced vit K requires action of vit K reductase
- vit K reductase inhibited by coumarin type rodenticides
- leads to lack of active factors 2, 7, 9 and 10 and a coagulopathy
- extrinsic, intrinsic and common pathways affected
- factor 7 has shortest half life so will decrease first
- PT is often prolonged 1st in early vit K deficiency
CS and tests for vit K deficiency
- CS: haemorrhage (thorax, abdomen),
- TEST: elevated PT and PTT
- PLT numbers and buccal mucosal bleeding time should be normal but mild thrombocytopaenia possible (d/t consumption associated with haemorrhage)
- FDPs may be elevated
- will see same results if animal has coagulopathy secondary to hepatic dz
Tx - vit K deficiency
- emetics, cathartics, activated charcoal if ingestion of rodenticide recent
- transfusions of WFB or FFP to replace coagulation factors
- may also need PRBCs to severe anaemia
- Vit K deficiency: use vit K1 orally or PN, can give loading dose SC followed by lower dose divided q 8hrs, same dose can be given orally with a fatty meal, may take 12 hours before vit K therapy will shorten PT and decrease bleeding
How long do you need to tx for vit K deficiency?
- Warfarin or other 1st generation rodenticide; 1 week
- 2nd/ 3rd generation rodeniticides used: need to tx for at least 3 wks, often up to 6 wks
- check PT 24-48 hrs after last dose, if prolonged reinstate tx for another 2 wks and recheck PT
Examples - hereditary defect of coagulation
- F7 deficiency
- Haemophilia A (F8 deficiency)
- Haemophilia B (F4 deficiency)
- F11 deficiency
- F12 deficiency
Tx - inherited coagulation defects
- transfusions of FWB or plasma to replace factor deficiency and red cells
- use of fresh or frozen plasma will give a small amount of factor
- cryoprecipitate: 10x more factor 8 than plasma
What is D.I.C.?
= disseminated intravascular coagulation
- mixed haemostatic defect
- results when excessive coagulation leads to widespread thrombosis
- haemorrhage eventually results as all coagulation factors are consumed
- not a primary event but secondary to other underlying disease (neoplasia, liver disease, I-M dz, infectious dz)
- acute or chronic
What are the haemostatic abnormalities with D.I.C.?
- thrombocytopaenia (almost always)
- prolonged PT and TT (commonly)
- elevated FDPs
- decreased fibrinogen
- decreased AT3
Tx - D.I.C.
- stop coagulation process
- heparin (different regimens)
- transfusion of whole blood, plasma or cryoprecipitate as source of AT3
- aspirin to stop PLT activation
- correct other unerlying abnormalities
Px - D.I.C.
Poor
Summarise the general approach to the bleeding patient
- PLT defect likely if haemorrhages are ecchymotic or petechiall
- if not - coagulation defect (consider age, if v young hereditary possible, hx of toxin exposure)
- Check CBC (PLT #)
- evaluate HCT and PLT #s
Summarise approach to bleeding patient if thrombocytopaenia is present
- check for clots, check smear, recheck #s
- assess degree of TP
- PLTs approx 100*10^9/L consider haemorrhage as a cause
- PLTs
What should you consider if PLT #s are WNL but haemorrhages or has ecchymosis/ petechiae?
- consider vWD (haemorrhages)
- consider PLT function defects (ecchymosis or petechiae)
- if possible check BMBT (buccal mucosa bleeding time)
- assay for vWF Ag
- check clotting function (PTT may be increased d/t concurrent decrease in factor 8)
What to do if PT and PTT are prolonged?
- consider vit K deficiency or DIC
What if PTT is prolonged alone?
consider defects of intrinsic pathways (haemophilia A and B)
What if PT is prolonged alone?
consider early vit K deficiency, liver dz, early DIC
