Hemostasis

Question 1

How long do most platelets last in circulation? How long does it take for a platelet plug to develop?

Answer 1

5-10 days (species-specific)

5 minutes

Question 2

What causes the beginning of platelet plug development? What are the 3 major stages?

Answer 2

disruption of vascular endothelium results in collagen exposure

platelets are attracted to collagen and undergo:

1. adhesion and shape changes
2. secrete attractive factors
3. aggregation

Question 3

What does exposure of subendothelium and platelet adhesion depend on?

Answer 3

von Willebrand factor

Question 4

What is the coagulation cascade?

Answer 4

series of reactions ultimately leading to cross-linked fibrin, trapping of RBCs, plasma, and platelets within the platelet plug and formation of a durable fibrin clot

Question 5

How is excessive clot formation prevented within the bloodstream?

Answer 5

thrombin not absorbed into fibrin in inactivated by anti-thrombin

• heparin can bind to anti-thrombin and make it more potent, preventing coagulation

Question 6

What are signs of hemostatic disease?

Answer 6

• petechiae
• ecchymoses
• membrane bleeding

Question 7

When is spontaneous bleeding a concern in dogs and cats?

Answer 7

<30,000/uL platelets

Question 8

What does buccal mucosal bleeding times measure? What is considered normal?

Answer 8

time it takes for bleeding to completely stop

less than 4 mins

Question 9

What are the 2 most common tests used to evaluate the intrinsic pathway?

Answer 9

1. activating blotting time (ACT) - measures time required for fibrin clot formation in fresh whole blood by activating the clotting cascade - tests intrinsic (prekallikrein, kininogen, XII, XI, IX, VIII) and common (X, V, II, I) pathways
2. activated partial thromboplastin time (PTT) - tests intrinsic and common pathways, more sensitive

Question 10

How is the extrinsic pathway evaluated?

Answer 10

prothrombin time (PT) - checks function of factors X, V, II, and I of the common pathway and factor VII of the extrinsic pathway

Question 11

What is von Willebrand’s disease? How are affected patients prepared prior to surgery?

Answer 11

low concentration of von Willebrand’s factor leading to platelet dysfunction - most common in Dobermans and transmitted as a autosomal dominant trait with variable penetrance

• DDAVP 30 mins prior to induction and recheck buccal mucosal time before surgery
• pre-treat with cryoprecipitate of FFP

Question 12

What are the 2 major X-linked coagulation factor deficiencies most common in veterinary medicine?

Answer 12

1. Hemophilia A - Factor VIII deficiency, ACT and PPT elevated
2. Hemophilia B - Factor IX deficiency, ACT and PPT elevated

Question 13

What factors are affected by vitamin K deficiencies? What tests are affected?

Answer 13

• II
• VII
• IX
• X
• proteins C and D

PT and PTT elevated –> T1/2 of Factor VII is 8 hours, which is when PT will be elevated (clinical bleeding is seen in 1-2 days)

Question 14

How is vitamin K deficiency treated?

Answer 14

• vitamin K for 4 weeks
• recheck clotting times after finishing therapy

Question 15

What factors take part in the common pathway?

Answer 15

II, V, I, X

(2x5x1 = 10)

Question 16

What factors take part in the intrinsic pathway?

Answer 16

XII, XI, IX, VIII

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