Haemostasis

Question 1

Define normal haemostasis

Answer 1

Blood is maintained in a fluid, clot-free state and is poised to induce a rapid, localised plug at the site of vascular injury.

Question 2

What are the stages involved in haemostasis?

Answer 2

1. After vascular injury, local neurohumoural factors induce a transient vasoconstriction
2. Platelets adhere to exposed ECM via von Willebrands factor and are activated. Undergo shape change and release granules (thromboxane A2, ADP)
3. Causes further platelet aggregation, forms primary haemostatic plug
4. Local activation of coagulation cascade causes fibrin polymerisation - cements platelets into secondary haemostatic plug
5. Counter regulatory mechanisms limit plug to site of injury
6. Intact endothelial cells secrete factors that inhibit platelet adherence and blood clotting

Question 3

What is thrombocytopenia and how does it present clinically?

Answer 3

• Thrombocytopenia - decreased platelets in peripheral circulation
• Clinical signs - epistaxis, echhymoses, petechiae, haematuria, haematochezia, hyphaema, malaena

Question 4

What are the possible causes of thrombocytopenia?

Answer 4

• Increased destruction - primary (idiopathic) or secondary (immune mediated destruction)
• Decreased production e.g. BM disorder
• Increased consumption e.g. DIC, thrombosis
• Increased sequestration e.g. hypersplenism

Question 5

How is thrombocytopenia evaluated in the lab?

Answer 5

• CBC
• Blood smear examination (platelet aggregates)
• Manual platelet count (if below sensitivity of machine)

Question 6

What are the clinical signs of a platelet function disorder?

Answer 6

• Mucosal bleeding, haematuria, malaena etc.
• Petechiae
• Ecchymoses
• Normal platelet numbers on count

Question 7

What are the characteristics of von Willebrands disease?

Answer 7

• Caused by quantitative and functional deficiencies in vWF
• 3 types: partial quantitative disorder; absence of vWF; loss of large molecular weight multimers

Question 8

What is thrombocytosis and what are the 2 forms?

Answer 8

• Thrombocytosis - concentration of platelets in the blood is higher than the normal reference interval
• Reactive thrombocytosis - increased production (inflammation, blood loss, non-haemic neoplasia)
• Haemic neoplasia - primary essential thrombocythaemia; acute megakaryotic leukaemia

Question 9

How do clotting disorders manifest clinically?

Answer 9

Haematomas, GI and UT bleeding, haemoarthrosis.

Disorders can be aquired (vit K deficiency, hepatic disease) or hereditary (X-linked deficiency haemophilia)

Question 10

How are clotting disorders evaluated?

Answer 10

In vitro clotting tests - test for specific activators that are required for clotting cascade (tissue factor, contact activator)

Question 11

What is DIC?

Answer 11

Disseminated intravascular coagulation - acquired syndrome, intravascular coagulation with loss of localisation resulting from different causes. Can originate from, and damage, microvasculature, can produce organ dysfunction

Question 12

How is DIC initiated?

Answer 12

• Massive tissue trauma causes exposure to huge amounts of tissue factor
• Some conditions can induce TF expression
• Aberrant expression of TF by intravascular cells