Haemostasis

Question 1

NORMAL HAEMOSTASIS

Answer 1

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.

Question 2

NORMAL HAEMOSTATIC PROCESS

Answer 2

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.

Question 3

PLATELET PHOSPHOLIPID MEMBRANE

Answer 3

The phospholipid membrane STABILISES coagulation factors.

Coagulation factors are activated.

Question 4

PLATELETS

Answer 4

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
3. AGGREGATION

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.

Question 5

COAGULATION

Answer 5

A SERIES OF ENZYMATIC CONVERSIONS THAT TURNS INACTIVE PROENZYMES IN TO ACTIVE ENZYMES.

• Produces thrombin
• Thrombin converts plasma fibrinogen to insoluble fibrin
• Cascade is traditionally divided in to- intrinsic, extrinsic and common pathway.

Question 6

EXTRINSIC PATHWAY

Answer 6

Most important in life- tissue injury triggers tissue factor activation.
‘Triggering cascade’
FACTOR VII is the start of the pathway- converts tissue factor to tissue factor VIIa.

Question 7

INTRINSIC PATHWAY

Answer 7

Begins with factor XII
‘Amplifying cascade’
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.

Question 8

FIBRINOLYTIC SYSTEM

Answer 8

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.
CONTROLLED system.

Question 9

THROMBOCYTOPAENIA

Answer 9

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.

Question 10

LAB EVALUATION OF THROMBOCYTOPAENIA

Answer 10

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.

Question 11

CAUSES OF THROMBOCYTOPAENIA

Answer 11

• 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)

Question 12

PLATELET FUNCTION DISORDERS

Answer 12

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.

Question 13

PLATELET FUNCTION TEST

Answer 13

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)

Question 14

VON WILLEBRAND FACTOR

Answer 14

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.

Question 15

FACTOR VIII

Answer 15

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.

Question 16

VON WILLEBRAND’S DISEASE

Answer 16

Caused by quantitative and functional deficiencies in vWF.
Three types:
1. PARTIAL QUANTITATIVE DISORDER
2. LOSS OF LARGE MOLECULAR WEIGHT MULTIMERS
3. ABSENCE
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.

Question 17

LAB EVALUATION OF VON WILLEBRAND’S DISEASE

Answer 17

• 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.

Question 18

THROMBOCYTOSIS

Answer 18

BLOOD PLATELET CONCENTRATION ABOVE REFERENCE INTERVAL.
Causes:
-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.

Question 19

SECONDARY CLOTTING DISORDERS

Answer 19

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)

Question 20

LAB EVALUATION OF SECONDARY CLOTTING DISORDERS

Answer 20

• 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)

Question 21

HEREDITARY SECONDARY CLOTTING DISORDERS

Answer 21

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.

Question 22

ACQUIRED SECONDARY CLOTTING DISORDERS

Answer 22

-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.

Question 23

VITAMIN K

Answer 23

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.

Question 24

PROTHROMBIN TEST (PT)

Answer 24

Measures function of extrinsic pathway.

Question 25

ACTIVATED PARTIAL PROTHROMBIN TEST (aPTT)

Answer 25

Measures function of intrinsic pathway

Question 26

THROMBIN CLOTTING TIME (TCT)

Answer 26

Measures activity of common pathway.

Question 27

FIBRINOGEN MEASUREMENT

Answer 27

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

Question 28

VIRCHOW’S TRIAD

Answer 28

1. Endothelial injury
2. Abnormal blood flow
3. Hypercoagulability
   - > THROMBOSIS

Question 29

DISSEMINATED INTRAVASCULAR COAGULATION

Answer 29

“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.

Question 30

CAUSES OF DIC

Answer 30

-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).

Question 31

PATHOPHYSIOLOGY OF DIC

Answer 31

Sepsis/neoplasia/endotoxaemia etc

• > Release of Tissue Factor
• > Widespread microvascular thrombosis
• > Microangiopathic haemolytic anaemia (many schistocytes may be seen)
• > Ischaemic tissue damage and BLEEDING DISORDER

Question 32

DIAGNOSIS OF DIC

Answer 32

CBC- thrombocytopaenia (low platelet count)
Clotting times- aPTT and PT are prolonged
Hypofibrinogenaemia
Elevated D dimers (fibrin split products)- diagnose with D dimer latex agglutination assay.
Once DIC has been diagnosed, THE UNDERLYING PROBLEM CAUSING IT HAS TO BE ADDRESSED.