Bleeding & Hemostasis

Question 1

components of the hemostatic system

Answer 1

• circulating blood (contains platelets and coagulation factors)
• endothelium
• subendothelial matrices

Question 2

how is hemostasis regulated

Answer 2

balance of pro and antithrombotic properties
- prevents inappropriate thrombosis
- allows rapid hemostasis if injured

Question 3

steps of hemostasis

Answer 3

1. vasoconstriction
2. primary hemostasis
3. secondary hemostasis
4. fibrinolysis

Question 4

primary hemostasis

Answer 4

formation of a platelet plug

involves:
- platelets
- vWF
- endothelium

Question 5

resting endothelial cell properties

Answer 5

• inactive
• have an ANTIcoagulant surface to prevent platelet adherence
• no expression of tissue factor or exposure of collagen
• normal vWF levels

Question 6

anticoagulant endothelial cell membrane

Answer 6

when inactive - endothelium has a neutral outer phospholipid membrane and a procoagulant inner membrane (negatively charged)

maintained by flippase and floppase enzymes

Question 7

how does endothelium change when vascular injury occurs

Answer 7

• activated
• scramblase enzyme flips the phospholipids so that the outer membrane becomes procoagulant
• facilitates adhesion of circulating platelets

Question 8

platelets

Answer 8

anuclear, discoid shaped fragments of megakaryocytes

primary effectors of hemostasis - secretes granules that facilitate hemostasis

SHORT lifespan (7-10 days)

Question 9

steps of platelet activation

Answer 9

1. initiation (adhesion)
2. extension (activation)
3. stabilization (aggregation)

Question 10

platelet initiation/adhesion

Answer 10

1. vascular injury causes exposure of collagen + vWF
2. circulating platelets tether to vWF and collagen
3. platelets roll along endothelium
4. adhesion of a platelet monolayer over injured endothelium –> forms platelet plug

Question 11

platelet extension/activation

Answer 11

1. additional platelets adhere to platelet plug
2. platelets get activated by adhesion and soluble agonists
3. changes platelet from low to high affinity state

high affinity platelets
- increased granule release
- procoagulant membrane formation
- integrin activation (aIIbB3)

Question 12

platelet stabilization/aggregation

Answer 12

1. aIIbB binds to fibrinogen to initiate aggregation
2. allows fibrinogen, fibrin, and vWF to bind to aIIbB3
3. initiates outside-in signaling leading to:
   - clustering of integrins
   - actin remodeling
   - clot retraction
4. stabilizes platelet plug

Question 13

secondary hemostasis

Answer 13

formation of a stable fibrin clot

involves:
- coagulation factors

Question 14

coagulation factors

Answer 14

factors that circulate as inactive zymogens

requires activation via coagulation cascade

some may require activation by vitamin K

Question 15

what is used as the docking site for the assembly of coagulation complexes

Answer 15

pro-coagulant phospholipid membrane on cell surface

Question 16

coagulation cascade

Answer 16

series of zymogen cleavage into active clotting factors

allows for signal amplification and multiple points of regulation

Question 17

extrinsic pathway

Answer 17

initiated by endothelial injury

1. injury exposes tissue factor (III)
2. tissue factor binds to factor VII
3. factor VII activates factor X

Question 18

intrinsic pathway

Answer 18

1. factor XII activates factor XI
2. factor XI activates factor IX
3. factor IX activates factor VIII
4. factor VIII activates factor X

Question 19

common pathway

Answer 19

1. activated factor X (from intrinsic and extrinsic), V, and C cleave prothrombin (factor II) into thrombin
2. thrombin cleaves fibrinogen (factor I) into fibrin
3. thrombin activates factor XIII
4. factor XIII binds active C to create fibrin cross links that stabilize the clot

Question 20

how does coagulation in vivo typically work

Answer 20

mostly via the extrinsic pathway

tissue factor (III) + factor VII are the main initiators of clotting in vivo

Question 21

thrombin

Answer 21

pro-inflammatory:
- activates platelets
- activates TF V, IX, XI, XIII
- cleaves fibrinogen into fibrin
- crosslinks fibrin

has some anticoagulant and fibrinolytic properties

Question 22

fibrinolysis

Answer 22

breakdown and removal of clot

goal is to reestablish blood flow after vascular healing

HIGHLY regulated balance of proteolytic and inhibitory proteins

Question 23

plasminogen/plasmin

Answer 23

1. plasminogen gets cleaved into plasmin by activators
2. plasmin cleaves fibrin to form fibrin degradation products (FDPs) and D-dimers

Question 24

FDPs vs D-dimers

Answer 24

FDPs: breakdown product of all fibrin
D-dimers: breakdown product of cross-linked fibrin only

Question 25

what are primary hemostatic disorders

Answer 25

abnormalities in PLATELET number or function can also be caused by vWF or endothelial abnormalities

Question 26

what are secondary hemostatic disorders

Answer 26

abnormalities in COAGULATION FACTORS can be acquired or congenital

Question 27

hyperfibrinolysis

Answer 27

RARE excessive or rapid clot breakdown

Question 28

clinical signs of primary hemostatic disorders

Answer 28

SMALL body bleeds - petechiae/ecchymosis - epistaxis - gingival bleeding - hyphema - hematuria - melena - CNS/pulmonary hemorrhage

Question 29

clinical signs of secondary hemostatic disorders

Answer 29

LARGE body bleeds - hemoabdomen - hemothorax - hemoptysis - hemarthrosis - hematoma - SQ/IM bleeding - CNS/pulmonary hemorrhage

Question 30

causes of primary hemostatic disease

Answer 30

1. thrombocytopenia 2. von willebrand's disease

Question 31

causes of thrombocytopenia

Answer 31

1. destruction of platelets - IMTP 2. decreased production 3. increased consumption 4. sequestration diagnosed on platelet count

Question 32

von willebrand's disease

Answer 32

inherited vWF disorder type 1: decreased vWF type 2: decreased & abnormal vWF type 3: marked reduced or absent vWF diagnosed on vWF assay

Question 33

causes of acquired secondary hemostatic disease

Answer 33

- anticoagulant rodenticide toxicity - liver failure - heat stroke - DIC - shock - severe hypothermia - hemodilution - acidemia - drug ingestion

Question 34

causes of congenital secondary hemostatic disease

Answer 34

hemophilia A: factor 8 deficiency (males only, GSDs) hemophilia B: factor 9 deficiency hageman factor deficiency: factor 12 deficiency (NO clinical bleeding but has prolonged aPTT)

Question 35

primary hemostatic disorder diagnostics

Answer 35

1. platelet count - CBC + manual slide review 2. vWF assay 3. buccal mucosal bleeding time 4. platelet function testing

Question 36

secondary hemostatic disorder diagnostics

Answer 36

1. PT/aPTT 2. ACT 3. viscoelastic testing 4. single factor analysis/assay

Question 37

hyperfibrinolysis diagnostics

Answer 37

1. viscoelastic testing 2. FDPs or D-dimers

Question 38

platelet count

Answer 38

CBC + manual review check for clumping - causes artificially low platelet count on CBC count on high power (100x) with oil immersion 1 platelet per high power field = 15,000 platelets

Question 39

vWF assay

Answer 39

measures concentration and function of vWF antigen assay - type 1 vs type 3 collagen binding - type 2

Question 40

buccal mucosal bleeding time (BMBT)

Answer 40

highly subjective test of primary hemostatic disorder dogs: 3-4 min cats: <2 min only do once thrombocytopenia has been ruled out

Question 41

PT

Answer 41

prothrombin time tests extrinsic & common pathways

Question 42

aPTT

Answer 42

partial thromboplastin time tests intrinsic & common pathways

Question 43

ACT

Answer 43

activated clotting time tests intrinsic & common pathways less sensitive than aPTT

Question 44

viscoelastic testing (TEG)

Answer 44

visual representation of hemostasis measures: 1. time to form initial fibrin clot 2. time to reach certain clot firmness 3. rate of fibrin accumulation and cross linking 4. clot strength 5. rate of fibrinolysis