Hemostasis

Question 1

Coagulation cascade diagram

Answer 1

Question 2

Mechanisms in Medicine Coagulation Cascade Diagram

Answer 2

Question 3

___ is the primary initiator of coagulation in vivo.

Answer 3

The extrinsic pathway is the primary initiator of coagulation in vivo.

Question 4

Active complexes of the coagulation cascade

Answer 4

Question 5

The many roles of vWF

Answer 5

• Stable binding partner of serum Factor VIII
• Binding sites for collagen and a glycoprotein complex on platelets
• Capable of ‘unwinding’ under sheer stress of bloodflow to be exposed and cut by ADAMTS13

Question 6

Thrombin activates platelets by ___.

Answer 6

Thrombin activates platelets by cleaving the thrombin receptor on their surface.

Question 7

Activated platelet image

Answer 7

Question 8

Things that happen when platelets are activated (by thrombin, or ATP, or whatever)

Answer 8

• Morphology change
• Release arachadonic acid from membrane and metabolize to TxA₂
• Release of dense granule contents (including more ADP)
• Release of alpha granule contents (containing proteins that contribute to hemostasis - platelet factor IV, thrombospondin, fibrinogen, and factor V)
• Expose P-selectin (normally housed within alpha granules, part of degranulation)
• Bring phosphatidylserine from the internal leaflet to the external leaflet

Question 9

Zymogens of the coagulation cascade are synthesized in ___.

Answer 9

Zymogens of the coagulation cascade are synthesized in the liver

Question 10

Basic structure and cell biology of coagulation cascade zymogens

Answer 10

• Signal peptide at N terminus
• Serine protease active site at C terminus
• Undergo crucial modifications at Golgi prior to release, including vitamin K-dependent carboxylation

Question 11

General role of protein cofactors in the coagulation cascade

Answer 11

Bind to platelet and endothelial cell membranes, where they serve as docking sites for zymogens and contribute importantly to the amplification that is essential for the rapid formation of the fibrin clot.

Question 12

Coagulation in a glass tube

Answer 12

Question 13

___ is not very important for coagulation ex-vivo, such as in a glass tube, but is crucial for in-vivo coagulation.

Answer 13

Factor IX is not very important for coagulation ex-vivo, such as in a glass tube, but is crucial for in-vivo coagulation.

Question 14

Coagulation in humans

Answer 14

Question 15

fibrinopeptides A and B

Answer 15

Released from fibrinogen when it is cleaved by thrombin to form fibrin. Measurement can be useful in evaluating patients with disseminated intravascular coagulation.

Question 16

Factor XIIIa

Answer 16

Cross-links fibrin with covalent bonds

Question 17

Tissue factor pathway inhibitor (TFPI)

Answer 17

Synthesized in endothelial cells and is present in plasma and platelets. blocks the extrinsic coagulation pathway after its initial activation by binding first to factor Xa, then jointly binding to and inactivating VIIa/TF complexes. This shuts down the extrinsic pathway after only small amounts of Xa and thrombin are generated.

This small amount of Xa and thrombin are sufficient to activate factor VIII and factor V, enabling the intrinsic pathway to provide a burst of procoagulant activity.

Question 18

Antithrombin

Answer 18

Most important plasma protease inhibitor in regulating hemostasis. Binds to heparin or other glycoproteins on endothelial cells and here undergoes a conformational change to reveal a site that serves as a serine protease inhibitor.

It inactivates not only thrombin but also factors VIIa, IXa, Xa, and XIa as they are carried in the blood away from the developing thrombus.

Question 19

Protein C and Protein S

Answer 19

Work together. When activated by thrombin bound to thrombomodulin on the surface of normal endothelial cells, innactivate factors Va and VIIIa.

Undergo vitamin K–dependent carboxylation and share other structural features with factors VII, IX, X, and prothrombin

Question 20

Platelet count

Answer 20

Platelets per microliter

Question 21

Bleeding time

Answer 21

Make a controlled superficial incision on the forearm and monitoring the time that elapses before the bleeding stops.

As the straight vertical and diagonal lines show, in those with normal platelet function, the bleeding time is not prolonged unless the platelet count is less than 100,000 per microliter.

Question 22

Requirements for aggregation vs agglutination

Answer 22

Aggregation requires calcium mobilization and the activation of the fibrinogen receptor, whereas agglu­tination merely depends on passive cross-linking of platelets mediated by bivalent binding of vWF to GPIb complexes and, therefore, occurs even with “dead” but intact platelets that have been fixed with formalin.

Question 23

Ristocetin

Answer 23

Antibiotic that facilitates vWF in plasma binding to normal platelets and inducing platelet agglutination

Question 24

In-vitro platelet-rich plasma tests

Answer 24

Question 25

In contrast to platelet function tests, ___ are routine tests done as a measure of numerous disease processes.

Answer 25

In contrast to platelet function tests, coagulation cascade function tests are routine tests done as a measure of numerous disease processes.

Question 26

Prothrombin Time (PT)

Answer 26

Assesses the extrinsic pathway of coagulation. Performed by adding recombinant tissue factor to the patient’s plasma, which normally clots after about 12 seconds.

A prolongation in the PT means that the patient has a functional deficiency of one or more of the clotting factors in the extrinsic pathway, specifically factors VII, X, V, prothrombin, or fibrinogen.

Question 27

Partial thromboplasatin time (PTT)

Answer 27

Assesses the intrinsic pathway of coagulation. Kaolin, a porous diatomaceous earth, is added to plasma, providing an artificial negatively charged contact surface that initiates the cascade by activating factor XII.

Question 28

Thrombin Time

Answer 28

Time required for a clot to form after the addition of exogenous thrombin to plasma. Assesses the amount and functionality of fibrinogen as well as the presence of thrombin inhibitors such as the anticoagulant heparin or breakdown products of fibrinogen and fibrin (fibrin degradation products) that interfere with fibrin formation.

Question 29

Interpreting PT, PTT, and TT together

Answer 29

Question 30

Once clotting deficiency has been narrowed down to a specific window, a functional diagnostic test may be done by. . .

Answer 30

. . . mixing patient plasma with standard plasma deficient in one particular clotting factor.

Question 31

In almost all of the inherited bleeding disorder, ___.

Answer 31

In almost all of the inherited bleeding disorder, only a single deficiency will be found

Question 32

If a patient has multiple functional deficiencies in the enzymes of coagulation, . . .

Answer 32

. . . it is likely to be an acquired condition due to liver disease, vitamin K deficiency, or disseminated intravascular coagulation.

Question 33

Vitamin K antagonists

Answer 33

Prevent carboxylation of factors VII, IX, X, and prothrombin, making them ineffective at promoting hemostasis.

The most commonly used vitamin K antagonist is warfarin. Patients treated with warfarin must be monitored closely with regular measurements of the PT to ensure that the degree of anticoagulation is within a therapeutic range. Warfin also has many food and drug interactions, which can be difficult for patients to manage.

Since the true mechanism of vitamin K antagonists is creating a functional deficiency in these enzymes, plasma transfusion may restore coagulability in an emergency.

Question 34

Heparin and heparin-like drugs

Answer 34

Polysaccharides, particularly heparin, activate the serine protease inhibitor antithrombin thereby inhibiting several steps in the coagulation cascade both in vivo and in vitro.

Heparin must be administered parenterally, but patients do not need to be monitored as closely as those on warfarin.

Unfractionated heparin occasionally causes a potentially serious complication, heparin-induced thrombocytopenia

Question 35

Heparin-induced thrombocytopenia

Answer 35

Autoantibody against a complex of the drug and platelet factor IV released from alpha granules.

This immune response activates platelets, commonly causing thrombocytopenia and, in some patients, life-threatening venous and/or arterial thrombosis

Question 36

Fibrinolytic drugs

Answer 36

Streptokinase and urokinase

More recently, recombinant tissue plasminogen activator has been shown to have superior efficacy

These therapies carry with them the risk of hemorrhage.

Question 37

Direct inhibitors

Answer 37

An advantage of these drugs is that they appear to have few interactions with other drugs or with foods and thus maintain patients in a therapeutic range of anticoagulation more consistently than does warfarin.

Argatroban, a high affinity thrombin inhibitor, is a major member of this family. Recently, an antibody reversal agent has been approved to mitigate bleeding due to dabigatran, a different direct thrombin inhibitor.

Direct oral factor Xa inhibitors also have been developed, including rivaroxaban, apixaban, and edoxaban, with reversal drugs on the way.

Question 38

X-V complex

Answer 38

Made by both the intrinsic and extrinsic pathways

Converts prothrombin to thrombin

Question 39

Thrombin

Answer 39

Among other things, converts the soluble molecule fibrinogen into a solid fibrin clot.

Question 40

Factors of the common pathway

Answer 40

Question 41

The extrinsic pathway utilizes ___ to activate factor X.

Answer 41

The extrinsic pathway (aka the PT pathway) utilizes factor VII to activate factor X.

Question 42

Intrinsic pathway order

Answer 42

Order of the intrinsic pathway (the PTT pathway)

Twelve

Eleven

Nine

Eight

Ten

Question 43

Clotting deficiencies - what matters clinically?

Answer 43

Question 44

Interpreting the results of a mixing study

Answer 44

• If a substantial correction is noted after mixing patient and standard plasma, a deficiency is present
• If there is no or little correction, an inhibitor is present

Question 45

Bleeding symtpoms in neonates

Answer 45

Question 46

Bleeding symptom tree

Answer 46

Question 47

Typical vWF panel

Answer 47

1. Antigen test (quantify vWF)
2. Functional test (usually ristocetin cofactor assay)
3. Factor VIII activity

Question 48

How do you go about treating patients with acquired inhibition of clotting factors?

Answer 48

For low titer antibody, consider increasing dosage of factor.

For high titer antibody:

1. Substitutes if available (Emcizumab)
2. Porcine
3. Recombinant Factor VIIa
4. Purified prothrombin complex concentrate
5. Induce tolerance through exposure therapy

Question 49

Patients with ___ are more likely to develop alloantibodies against clotting factors.

Answer 49

Patients with severe-grade hemophilia (<1% reference) are more likely to develop alloantibodies against clotting factors.

Question 50

What percentage baseline activity is considered to be rescued in a mixing study?

Answer 50

>40% reference

Question 51

What do you do if a hemophiliac patient comes with an acute bleed?

Answer 51

1. If the factor deficiency is known, supplement with stock of that factor
2. Emcizumab if Hemophilia A
3. Fresh frozen plasma

Question 52

What is the relationship between primary and secondary hemostasis?

Answer 52

The phospholipid surface created by primary hemostasis provides the negatively-charged platform for activation of secondary hemostasis

Question 53

The most common mutation in factor VIII is ___.

The most common mutation in factor IX is ___.

Answer 53

The most common mutation in factor VIII is an inversion.

The most common mutation in factor IX is a missense mutation.

Thus, factor VIII mutations tend to be more ablative and are more likely to develop alloantibodies against factor VIII.