Hemostasis and approach to the bleeding patient Flashcards
- Discuss events occurring during hemostasis, comparing primary and secondary hemostasis.
Adhesion, activation, and aggregation of platelets to form a platelet plug constitute the first events in formation of a clot (primary hemostasis).
The platelet plug is stabilized by formation of a fibrin network generated through the coagulation cascade (secondary hemostasis).
Optimal numbers and function of platelets are key to cessation of bleeding from small vascular injuries. Disorders of platelet number or function can lead to bleeding from the skin, mucous membranes, brain, or other sites.
What is the precursor of platelets?
Where are most of the platelets?
What is the main growth factor and maturation factor for platelet?
Megakaryocyte in bone marrow: precursor for platelets derived from hematopoietic stem cells
- Up to 2000 platelets
- Bud off, circulate 7-10 days
- Spleen: 1/3 of platelets
- TPO: thrombopoietin main growth and maturation factor for megakaryocytes
- Diagram the structure of a mature platelet and show the location of: dense granules, alpha granules, glycoprotein Ib, glycoprotein IIb/IIIa, and phospholipids.
What granules contain ATP, ADP, serotonin, and calcium?
What granules contain coagulating factors?
- List three functions of platelets.
Wha do intact endotheliam cells secrete to prevent coagulation?
Platelets play several important roles in hemostasis, including (1)adhesion to the vascular subendothelium at sites of injury to begin the hemostatic process, (2)activation of intracellular signaling pathways leading to cytoskeletal changes and release of intracellular granules to enhance platelet plug formation, (3) aggregation to form the platelet plug, and support of thrombin generation by providing a phospholipid surface for the coagulation cascade to take place.
While these different platelet functions are discussed separately, the process is a continuous and dynamic interaction of vessel, platelet, and plasma components.
The endothelial cells of intact vessels prevent blood coagulation by secretion of a heparinlike molecule and through expression of thrombomodulin, which when bound to thrombin activates protein C and S.
Intact endothelial cells prevent platelet aggregation by the secretion of nitric oxide and prostacyclin, inhibitors of platelet activation.
- Construct a simple diagram that depicts the process of platelet adhesion. Include in the drawing subendothelial collagen, von Willebrand factor, and glycoprotein Ib. Explain why platelet adhesion to blood vessels does not occur under normal circumstances.
With vessel injury, subendothelial components are exposed.
1- Circulating von Willebrand factor (vWF) adheres to the damaged, exposed subendothelium.
2- Under conditions of high shear flow, circulating platelets then contact the exposed subendothelium in a rolling fashion and adhere by interaction between glycoprotein Ib (GP1b) on the platelet surface and vWF.
3- With exposure to soluble agonists such as thrombin, ADP, epinephrine, and thromboxane A2, or to adhesive proteins in the subendothelial matrix such as collagen and vWF, the platelet integrin GPIIb‐IIIa (αIIbβ3) increases its affinity for vWF, leading to tighter binding.
GPVI also interacts directly with collagen in the subendothelium.
-Numerous ligands in the subendothelium, such as collagen, laminin, and fibronectin, also interact with β1 integrins on the platelet surface. All of these interactions lead to firm adherence of the platelet to the subendothelial surface.
Platelet activation
Soluble agonists interact with G protein receptors on the surface of platelets and lead to release of intreacellular_______.
What effect does ____ have?
With platelet activation membrane reorganiation also occurs, with switching of the phospholipid _______ from the inner to the outer membrane leaflet.
With adherence to the injured vessel wall, platelets undergo shape change through cytoskeletal activation, becoming more spherical with extended pseudopods and spreading over the exposed subendothelium.
The contents of platelet granules are released. Soluble agonists, including thrombin, thromboxane A2, epinephrine, and ADP, interact with their respective G protein coupled platelet membrane receptors,
leading to intracellular signaling and calcium mobilization. Calcium activates phospholipase A2, which releases arachidonic acid from phospholipids.
Cyclooxygenase (COX‐1) then converts arachidonic acid to prostaglandin H2, which in turn is converted to thromboxane A2 by thromboxane synthetase. Thromboxane A2, along with other agonists, is released, acting to further amplify platelet activation. With platelet activation, membrane reorganization also occurs, with switching of the phospholipid phosphatidylserine from the inner to the outer membrane leaflet, making it available to interact with clotting factors that then lead to thrombin generation.
More on platelet activation:
How do activated plates activate other plates?
Includes:
(a) secretion of intracellular alpha and dense granule contents from platelet that includes ADP in dense granules.
(b) Interaction of the extracellular soluble agonists thrombin, thromboxane A2, and ADP with their respective G protein coupled platelet membrane receptors, thromboxane receptor, and P2Y12.
(c) Signaling following activation of these receptors
(d) Signaling following calcium mobilization internally and calcium influx from outside the platelet
(e) Interaction of membrane skeletal protein talin and its counterpart kindlin with GPIIbIIIa.
–> New platelets stick to activated platelets and are themselves activated through release of compounds that further amplify platelet activation. These include:
(a) Products of oxidation of arachidonic acid by the cyclooxygenase pathway that includes thromboxane A2 (this pathway is blocked by aspirin)
(b) ADP released from dense granules
Information about granules (do not need to know):
3 different types of granules
- lysosomes (for intracellular protein degradation)
- dense granules (delta)
- alpha granules
Electrone Dense granules contain non-protein platelet AGONISTS: calcium, serotonin and non-metabolic pool of ADP and ATP. There are 3-8 dense granules per platelet. Calcium content makes them appear dense under e-microscope. Release of contents promote platelet activation.
Alpha granules are the predominant granules, about 50 per platelet. Broad variety of proteins: 1. adhesive proteins (vWF, fibrinogen, fibronectin, GPIIb/IIIa) 2. procoagulant factors (factors 5, 11, 13) 3. anticoagulant factors (Prot S) 4. anti-lytic factors (PAI-1) 5. anti-heparin s (PF4) 6. growth-promoting factors (PDGFR, TGF-beta)
- Construct a simple diagram that shows the process of platelet aggregation, including the release reaction (ADP), thromboxane synthesis, ADP and thromboxane receptors, glycoprotein IIb/IIIa, and fibrinogen.
What binds to fibrinogen to make it act as a bridge to lace platelets together into aggregates?
Wha are some agonist agents from platelet aggregation?
With platelet adhesion and with binding of soluble agonists to receptors to amplify platelet activation, GPIIb‐IIIa is converted to a high‐affinity state where it can bind fibrinogen and vWF.
Binding of the membrane protein talin to GPIIb‐IIIa is the last step to mediate the change from a low‐affinity to a high‐affinity state.
GPIIb‐IIIa can then bind fibrinogen, which acts as a bridge to lace platelets together into an aggregate. Thombin generated through activation of the coagulation cascade then converts fibrinogen to fibrin to stabilize the platelet plug
Summary of platelet plug formation:
- List and describe three mechanisms that could lead to thrombocytopenia.
Thrombocytopenia (a low platelet count) can be due to decreased platelet production, increased platelet destruction or consumption, or sequestration of platelets in the spleen.
1. Decreased production of platelets
2. Increased destruction of platelets
3. Distribution disorders (Increased sequestration of platelets due to splenomegaly)
- —-> Dilution (massive transfusion)
However first step: look at smear (rule out Pseudo-thrombocytopenia
- List and describe three mechanisms that could lead to thrombocytopenia.
Thrombocytopenia due to decreased platelet production can occur with primary bone marrow disorders such as aplastic anemia, myelodysplasia, and leukemia. It can also occur with bone marrow invasion by metastatic cancer, myelofibrosis, or infections such as tuberculosis. Toxins such as chemotherapeutic drugs, chemicals, and exposure to radiation can injure the bone marrow and lead to thrombocytopenia. And, severe nutritional disorders such as B12 or folate deficiency can affect megakaryopoiesis. Finally, rare congenital disorders can lead to a decreased platelet count.
Sample case:
Low platelets in the setting of pancytopenia:
- In the elderly think of Myelodysplastic syndrome (high MCV) and other hemat. malignancies (non-Hodgkin’s Lymphoma)
- Nutritional deficiencies (B12/Folate)
Sample case:
Infection
- Very common cause of low platelets
- Can be seen in bacterial and viral infections
- Viral: HIV, hepatitis C
Sample case:
Sample case:
- She undergoes further work-up: no obvious causes (medications, infections)
- BM BX negative for malignancy, megakaryocytes present at increased numbers
- She is diagnosed with ITP (immune thrombocytopenic purpura), aka
idiopathic thrombocytopenic purpura
=platelets are coated with anti-platelet antibodies leading to premature removal of platelets by the spleen
What are some receptor agonist?
- Describe the molecular defect, typical clinical course, and general approach to treatment for a patient with Von Willebrand Disease.
in what form is it inherited?
It is a QUALITATIVE platelet disorder
- Most common congenital bleeding disorder
- Usually autosomal dominant
- Abnormality in platelet/endothelial interaction
- Clinical problems:
- Mucosal bleeding
- Nose bleeds
- GI bleeds
- Menorrhagia
- Bleeding after surgery if no correction
Pathophysiology:
-Von Willebrand protein is extremely large protein in a series of forms with 2 functions
1ST FUNCTION:
-Adhere platelets to exposed collagen that acts as scaffolding for vWF at the site of a wound or vessel disruption
2nd FUNCTION:
- Carry Factor VIII
- Without this protein, Factor VIII has a VERY short half-life
- Describe the molecular defect, typical clinical course, and general approach to treatment for a patient with Von Willebrand Disease.
Type 1: Partial quantitative vWF deficiency. 70-80% cases
Type 2: Qualitative defects – either decreased (2A) or increased (2B) adhesion to platelets with latter accelerating vWF clearance. 15-30% cases
Type 3: Almost complete absence vWF. Rare
- List important questions to ask when obtaining a bleeding history in a patient with excessive bleeding.
- List important questions to ask when obtaining a bleeding history in a patient with excessive bleeding.
- List important questions to ask when obtaining a bleeding history in a patient with excessive bleeding.
- List important laboratory studies to obtain when evaluating a patient with excessive bleeding.
Platelet function screen
Platelet function analyzer/PFA
- Replaces the bleeding time as a test of platelet function
- PFA-100; ordered as “platelet function screen”
- Measures the time it takes for blood to block membrane coated with either collagen/epinephrine or collagen/ADP
- abnormal results will differentiate between vWD/intrinsic platelet defect, and aspirin effect
What are some PFA-100 pitfalls?
