Flashcards in Haemostasis Deck (32):
A result of tightly regulated processes, which accomplishes two important functions:
1. Maintain blood in fluid, clot free state.
2. Poised to induce rapid, localised plug at site of vascular injury.
NORMAL HAEMOSTATIC PROCESS
1. Reflex vasoconstriction- seen after injury, in response to local neurohumoral factors. Transient.
2. Primary haemostasis- platelets adhere to exposed ECM via vW factor, change shape (flatten) and release granules (ADP, thromboxane A2). This attracts further platelets -> aggregation forms primary haemostatic plug.
3. Secondary haemostasis- local activation of coagulation cascade results in fibrin polymerisation (thrombin converts fibrinogen to fibrin) and formation of definitive secondary haemostatic plug.
4. Tertiary haemostasis- counter-regulatory methods limit haemostasis to site of injury. Release of t-PA (tissue type plasminogen activator) causes fibrinolysis at non affected areas- activates plasmin to nibble clot and ensure it is correct size.
Thrombomodulin is released and interferes with coagulation cascade to prevent clotting at wrong site.
Intact endothelial cells serve primarily to inhibit platelet adherence and blood clotting.
Injury or activation of endothelial cells results in a procoagulant phenotype that augments clot formation.
PLATELET PHOSPHOLIPID MEMBRANE
The phospholipid membrane STABILISES coagulation factors.
Coagulation factors are activated.
Platelets play a central role in (primary) haemostasis.
After injury, they come in to contact with the ECM and undergo three general mechanisms:
1. ADHESION AND SHAPE CHANGE (FLATTENING)
2. GRANULE SECRETION
ADP released in step 2 stimulates primary haemostatic plug formation.
Fibrin deposition stabilises and anchors the aggregated platelets.
Platelets expose phospholipid complexes that are important in triggering the intrinsic coagulation pathway.
Injured or activated endothelial cells expose Tissue Factor, which triggers the extrinsic coagulation cascade.
A SERIES OF ENZYMATIC CONVERSIONS THAT TURNS INACTIVE PROENZYMES IN TO ACTIVE ENZYMES.
-Thrombin converts plasma fibrinogen to insoluble fibrin
-Cascade is traditionally divided in to- intrinsic, extrinsic and common pathway.
Most important in life- tissue injury triggers tissue factor activation.
FACTOR VII is the start of the pathway- converts tissue factor to tissue factor VIIa.
Begins with factor XII
Less important than intrinsic pathway- shown by cats with factor XII deficiency- they are clinically normal. Factor VII (intrinsic pathway) deficiency would have clinical effects.
The fibrinolytic system is activated with the coagulation cascade.
Part of tertiary haemostasis
Generates PLASMIN- BREAKS DOWN FIBRIN, by interfering with it's polymerisation.
Fibrin split products are produced- D-DIMERS, FIBRIN DEGRADATION PRODUCTS- these are measured to test if tertiary system is working.
Free plasmin rapidly complexes to alpha2-plasmin inhibitor, inactivating it.
A PLATELET DISORDER of PRIMARY haemostasis.
DECREASED circulating platelets in the peripheral circulation.
Clinical presentation- epistaxis, ecchymoses, petechiae, haematuria, haematochezia, hyphaemia, melaena- all bleeding problems.
Can see no clinical signs, or may also present with signs related to underlying disease.
Haemorrhage SOLELY caused by thrombocytopaenia doesn't normally occur until platelet count is <50 x10^9 per litre.
LAB EVALUATION OF THROMBOCYTOPAENIA
Complete blood count
Smear examination- check for platelet aggregates, morphology and size.
Manual platelet count- if numbers fall below sensitivity of automated machine.
NORMAL= 200-500 x10^9 platelets per litre.
CAUSES OF THROMBOCYTOPAENIA
-INCREASED DESTRUCTION of platelets- primary (idiopathic) or secondary immune mediated destruction
-DECREASED PRODUCTION eg. marrow disorder
-INCREASED CONSUMPTION eg. DIC, thrombosis.
-INCREASED SEQUESTRATION eg. hypersplensim (rare)
PLATELET FUNCTION DISORDERS
Clinical signs- may vary, include mucosal bleeding, haematuria, petechiae, ecchymoses etc.
PLATELET NUMBERS ARE NORMAL- the platelets just aren't functioning properly.
-> PLATELET FUNCTION TEST- buccal mucosal bleeding time- should be less than 4 minutes in normal, healthy small animal.
PLATELET FUNCTION TEST
Bleeding device nicks buccal mucosa, which should take less than 4 minutes to stop in the normal, healthy small animal.
NOT USED IN THROMBOCYTOPAENIA- ensure platelets are normal first (platelet count should be normal on CBC, check morphology etc. on blood smear)
VON WILLEBRAND FACTOR
Produced in endothelial cells and megakaryocytes.
Present in subendothelial matrix of normal blood vessels, and in alpha granules of platelets.
Subendothelial vW factor is exposed after vessel injury, causing adhesion of platelets, primarily via glycoprotein 1b platelet receptor.
Circulating vWF and vWF from platelet alpha granules can bind exposed subendothelial matrix, further contributing to platelet adhesion and activation.
Synthesised in liver and kidney.
Associates with vW factor to form a complex in the circulation.
Takes part in the coagulation cascade as a cofactor in the activation of factor X in the surface of activated platelets.
VON WILLEBRAND'S DISEASE
Caused by quantitative and functional deficiencies in vWF.
1. PARTIAL QUANTITATIVE DISORDER
2. LOSS OF LARGE MOLECULAR WEIGHT MULTIMERS
vW disease is the most common inherited disorder of haemostasis in dogs.
Type 1 is most commonly seen in dogs.
Often found post-operatively, after sterilisation, in young dogs.
LAB EVALUATION OF VON WILLEBRAND'S DISEASE
-Buccal Mucosal Bleeding Time (prolonged, not specific for vW disease- could be any platelet disorder)
-Quantitative assay- ELISA for vW Ag most common. Reported as a percentage of normal- less than 50% generally indicates vWF deficiency.
-Functional assays- Ristocetin cofactor assay, vWF collagen binding assay.
BLOOD PLATELET CONCENTRATION ABOVE REFERENCE INTERVAL.
-REACTIVE THROMBOCYTOSIS- increased platelet production due to inflammation, iron deficiency, blood loss, nonhaemic neoplasia, redistribution (physiologic- splenic contraction due to exercise, adrenaline. Seen in horses, dogs. Transient)
-HAEMIC NEOPLASIA- primary essential thrombocythemia- mature platelets released.
-acute megakaryocytic leukaemia- immature lymphocytes released.
SECONDARY CLOTTING DISORDERS
Bleeding manifests as haematomas, GI and urinary tract bleeding, haemarthrosis.
Bleeding is more severe than in primary disorders.
Can be acquired (vitamin K deficiency, severe hepatic disease, DIC)...
Or hereditary (X-linked deficiency haemophilia A (factor VIII) and B (factor IX), factor VII deficiency in beagles)
LAB EVALUATION OF SECONDARY CLOTTING DISORDERS
-In vitro clotting tests used to evaluate clotting mechanism
-Require specific activators for the clotting cascade- Tissue Factor for extrinsic pathway, contact activator for intrinsic pathway, exogenous source of phospholipids and calcium.
-Tests are insensitive- clotting factor levels have to be considerably lowered before tests will detect a change.
-Clot formation is timed with results reported in seconds
-Performed on citrated plasma (green blood tube)
HEREDITARY SECONDARY CLOTTING DISORDERS
Most common: Haemophilia A (factor VIII)- dogs, cats, horses
Also see Haemophilia B (factor IX).
X-linked traits- males are affected. Affected animals present clinically with haemarthrosis, haematomas.
DIAGNOSIS- based in coagulation assays. Both cause prolonged intrinsic/common pathway tests.
Specific diagnosis can be achieved by testing with specific factor deficient plasma.
ACQUIRED SECONDARY CLOTTING DISORDERS
-Vitamin K deficiency- eg. from biliary obstruction, infiltrative bowel disease- poor nutrient absorption.
-Vitamin K antagonism- eg. coumadin (warfarin) toxicity, second generation anticoagulant rodenticides.
-Affects factors II, VII, IX and X
-In vitro tests- prolonged prothrombin test and activated partial thromboplastin time.
Thrombin clotting time (TCT) is normal.
Fat soluble, so requires bile acids for digestion.
This is why bile duct obstruction can cause an acquired secondary clotting disorder.
Vitamin K is required to recycle clotting factors II, VII, IX and X.
PROTHROMBIN TEST (PT)
Measures function of extrinsic pathway.
ACTIVATED PARTIAL PROTHROMBIN TEST (aPTT)
Measures function of intrinsic pathway
THROMBIN CLOTTING TIME (TCT)
Measures activity of common pathway.
Fibrinogen is required for the formation of the fibrin plug.
It is measured by the modified Claus method (thrombin initiated clotting rate)
-> Rate of clot formation in diluted citrate plasma with addition of high concentration thrombin solution.
Adding thrombin means lack of thrombin cannot limit rate.
Fibrinogen is a POSITIVE ACUTE PHASE PROTEIN- it increases with inflammation.
Useful test in cattle and horses- inflammation can't always be detected on CBC
2.Abnormal blood flow
DISSEMINATED INTRAVASCULAR COAGULATION
"An acquired syndrome characterised by the intravascular activation of coagulation and loss of localisation resulting from different causes. It can originate from and cause damage to the microvasculature, which if sufficiently severe can produce organ dysfunction"
DIC is ALWAYS a secondary phenomenon.
CAUSES OF DIC
-Primary disease conditions- neoplasia, systemic infection, endotoxaemia, sepsis.
Initiation of DIC- massive tissue trauma causes exposure of huge amounts of Tissue Factor.
Some conditions can also induce Tissue Factor expression.
Aberrant expression of Tissue Factor can be seen from intravascular cells (monocytes, tumour cells).
PATHOPHYSIOLOGY OF DIC
-> Release of Tissue Factor
-> Widespread microvascular thrombosis
-> Microangiopathic haemolytic anaemia (many schistocytes may be seen)
-> Ischaemic tissue damage and BLEEDING DISORDER