Week 3/4 - D - Haemostasis - Primary (collagen, v.W.F, T.X.A.2, A.D.P) / Secondary, Anti-platelets/coagulants, thrombolytic

Question 1

What is haemostasis?

Answer 1

Haemostasis is the stopping of blood flow - ie stopping of blood loss (haemorrhage) from a damaged vessel and the maintenance of vascular patency

Question 2

What are the components of the normal haemostatic system?

Answer 2

Normally there is Formation of the platelet plug - primary haemostasis Formation of the fibrin clot - secondary haemostasis Fibrinolysis Anti-coagulant defence Once a clot is formed, we start breaking it down almost immediately

Question 3

What is primary haemostasis and why does it occur? What is secondary haemostasis and why does it occur? What is fibrinolysis and why does it occur?

Answer 3

Primary haemostasis is the formation of a platelet plug which stops small bleeds such as paper cuts Larger bleeds require a larger plug to be strong enough and therefore secondary haemostasis is required –> fibrin clot is formed - a solid fibrin mesh on the surface of the platelet plug Fibrinolysis occurs once bleeding has stopped in order to maintain vascular patency

Question 4

PRIMARY HAEMOSTASIS When there is damage to the endothelium eg due to rupture of a plaque, how does this result in platelet adhesion?

Answer 4

When there is damage to the endothelium, this triggers the exposure of collagen and vWF (von Willebrand Factor) to which the platelets bind

Question 5

What is the platelet receptor to which vWF binds?

Answer 5

vWF binds to the glycoprotein Ib receptors (Gp Ib receptors are the platelet vWF receptor)

Question 6

Once the platelets have adhered to the exposed collagen/vWF, they become activate and release various chemicals causing platelet aggregation What is released to cause further platelet aggregation? How are these 2 substances released? What mediates 5HT release and what does this cause?

Answer 6

Further platelet aggregation is brought about via TXA2 and ADP (adenosine diphosphate) The activated platelets synthesise thrombooxin A2 (TXA2) form arachdonic acid mediated by the enzyme cyclo-oxygenase-1 (COX-1) TXA2 mediates release of both 5-HT (serotonin) causing vasoconstriction and ADP which further increases platelet aggregation

Question 7

TXA2 binds to platelet GPCR TXA2 receptors What platelet receptors does ADP bind to? What is the receptors that allows platelets to bind to one another known as? - the platelets bind via a fibrinogen molecule

Answer 7

ADP binds to GPCR P2Y12 receptors Platelet receptors GPIIb/IIIA binds fibrinogen which allows for the aggregation of the platelets into a soft plug

Question 8

SECONDARY HAEMOSTASIS Secondary haemostasis is the formation of the fibrin clot providing stability for the platelet plug How does the platelet plug start off the secondary haemostasis?

Answer 8

Once the platelet plug is formed, the platelets release phospholipid and calcium The phospholipid binds to the surface of the platelet and the calcium binds to the negatively charged aspect of the phospholipid As calcium is positively charged, this allows for the binding of the nagtively charged clotting factors –> COAGULATION CASCADE can begin

Question 9

What are the three different factors in the coagulation cascade that result in fibrin clot formation? What are the three main steps of the coagulation cascade?

Answer 9

Pathways * Extrinsic pathway - requires the release of TF from the damage tissue * Intrinsic pathway - components are already in blood * Common pathway - basically the stage at which both intrinsic and extrinsic pathways come together Main steps of coagulation cascade –> * initial, * amplification, * propagation

Question 10

Main steps of the coagulation cascade are initiation (initial phase), amplification and propagation Initiation What is released in response to a damaged endothelium? What does this cause? What pathway is this?

Answer 10

Tissue factor is released from the damaged endothelium The tissue factor binds to factor VIIa from the plasma forming a TF/VIIa complex Initiation is part of the extrinsic pathway

Question 11

What does the TF/VIIa complex activate and what does this cause?

Answer 11

Tf/VIIa complex activates factors X to factor Xa, which in combination with factor Va converts prothrombin (factor II) to thrombin (factor IIa) The scene is now set for amplification

Question 12

Thrombin starts amplification by having a positive feedback mechanism What are the amplification effects that thrombin (factor IIa) has on the coagulation cascade? * How does it effect amplification?

Answer 12

Thrombin releases factor VIII from vWF to which it normally circulates with and activates it to VIIIa. It also cause the activation of factor IX to IXa (does this by activating XI to XIa) - Both IXa and VIIIa increase activation of factor X to Xa increasing thrombin. Thrombin also increases V to Va.

Question 13

Propagation is the final phase of the coagulation cascade, what happens in this phase?

Answer 13

Propogation is how the three pathways come to form the fibrin clot Factor XIa (activated by thrombin) activates factor IX to IXa. Factor IXa and VIIIa together, with TF/VIIa, activate factor X to Xa. Factor Xa activates prothrombin to thrombin. Thrombin causes fibrinogen to cleave into fibrin and this forms the cross-linked fibrin clot

Question 14

What is it that cross-links the fibrin to form a cross-linked fibrin network and therefore solid clot?

Answer 14

This would be factor XIIIa (activated by thrombin)

Question 15

What are the clotting factors in the: * Extrinsic pathway? * Intrinsic pathway * Common pathway?

Answer 15

Extrinsic pathway * Factor VII Intrinsc pathway * Factor XII, XI, IX, VIII Common pathway * Factor X, V, II, I, (and XIIIa)

Question 16

FIBRINOLYSIS We have now disussed the platelet formation and fibrin clot formation The third part of haemostasis is fibinolysis - the breakdown of the clot How does this occur? Where is the initial enzyme release from?

Answer 16

The vascular endothelium itself releases tissue plasminogen activator (tPA) tPA catalyses the conversion of plasminogen to plasmin Plasmin is the main enyme for clot breakdown converting fibrin to fibrin degradation products (can see this by elevated D-dimer levels)

Question 17

NATURALLY OCCURRING ANTI-COAGULANTS The final stage of haemostasis is the anti-coagulation which occurs once the endothelial damage has been plugged to stop continual clotting - done to maintain vascular patency What are the main naturally occurring anti-coagulants?

Answer 17

This would be protein C, protein S and serine protein inhibitors

Question 18

What is one of the main serine protein inhibitors and what does it inhibit?

Answer 18

One of the main serine protein inhibitors is anti-thrombin III (simply known as anti-thrombin) It inhibits the action of various coagulation factors mainly factors Xa and IIa (to a lesser extent IXa, IXa and XIIa)

Question 19

Once the clot has formed, a signal also goes out to thrombin changing its mechanism of action from promoting the amplification of coagulation via factors VIII/IXa. What does thrombin now do? Which anticoagulants does it promote the activity of?

Answer 19

Once the fibrin clot has formed and bleeding has ceased, the thrombin now binds to thrombomodulin, a protein on the endoethelial surface and this then activates Protein C and S to start the anticoagulation process

Question 20

What are the main factors that Protein C and Protein S exert there effect on?

Answer 20

Protein C and Protein S mainly inactivate factor VIIIa and factor Va (both of these work in converting factor X to Xa)

Question 21

Thrombosis – pathological haemostasis – a haematological plug in the absence of bleeding. Predisposing factors are Virchow’s triad What is Virchow’s triad?

Answer 21

* Endothelial injury - injury to the vessel eg due to atheromatous plaque rupture * Stasis of blood flow * Hypercoaguability of blood

Question 22

What is the difference between an arterial thrombus and venous thrombus * Colour? * Platelet or fibrin rich? * If it forms an embolus where does it go? * Primarily treated with what?

Answer 22

Arterial thrombus is a white thrombus * Platelet rich * Forms an embolus if detached from eg left heart or carotid - lodges in an artery in brain or other organ * Primarily treated with - antiplatelet drugs Venous thrombus is a red thrombus * Fibrin rich * Forms an embolus that usually dislodges in lung eg PE * Primarily treated with anti-coagulants

Question 23

Lets discuss anti-platelets Anti-platelet drugs are used mainly in the treatment of arterial thrombus * eg Aspirin * Clopidogrel * Prasgruel * Tricagrelor * Dipydridamole What is the mechanism of action of aspirin? When is usually used in cardiovascular patients?

Answer 23

Aspirin irreversibly blocks cycloxygenase enzyme (COX) in platelets, therefore preventing thromboxane A2 synthesis (needed for platelet aggregation directly and via increasing ADP release which also cause it) Usually used mainly for thromboprophylaxis in patients at high cardiovascular risk

Question 24

What are the side effects of aspirin?

Answer 24

Due to inhiting the COX enzyme This also inhibits specific prostoglandin formation Therefore can cause gastric ulcers/bleeding, bronchoconstriction, bronchospasm

Question 25

How does clopidogrel and prasgruel work?

Answer 25

Clopidogrel and prasgruel inhibit platelet aggregation also through a different mechanism * They irreversibly block the ADP receptors * Therefore can be known as ADP receptor antagonists or P2Y12 receptor antagonists (this is the name of the ADP platelet receptor)

Question 26

What is the difference in the mechanism of action between tricagrelor and clopiogrel/prasgruel?

Answer 26

BOTH INHIBIT PLATELET AGGREGATION Tricagrelor reversibly inhibits the ADP receptor * reversible ADP receptor antagonist (aka reversible P2Y12 recetpor antagonist) Clopidogrel and prasgruel are irreversible inhibitors of the P2Y12 receptor Tricagrelor has been shown to have superior outcomes in various studies

Question 27

How does dipyridamole work?

Answer 27

Dipyridamole is a phosphodiesterase inhibitor Phosphodiesterase increase cAMP which is a ‘second’ messenger in platelet activation and dipyridamole inhibits this - phosphodisterase inhibitor

Question 28

Used widely in the prevention and treatment of venous thrombosis and embolism * deep vein thrombosis (DVT) * prevention of post-operative thrombosis * patients with artificial heart valves * atrial fibrillation Warfarin, NOACs, Heparin * What is the mechanism of action of warfarin? * Route of administration?

Answer 28

Warfarin is a vitamin K anatagonist Vitamin K is needed for the production of factors II, VII, XI and X (as well as protein C and S) It is administered orally

Question 29

Warfarin level monitoring is done by motoring the INR levels What is the target INR? What is given to reverse warfarin effects and when? (NICE GUIDLEINES/OXFORD handbood/BNF) What is phytomenadione?

Answer 29

Target INR is 2-3 (phytomenadione = vit K1) * INR high but less than 5 –> potentially omit dose * INR 5-8 but no bleed –> omit warfarin dose, restart when less than 5 * INR 5-8 with minor bleeding –> omit warfarin, probs give Vit K IV, restart when less than 5 * INR >8 with no or minor bleeding –> omit warfarin, give vitamin K IV, restart when less than 5 * Major bleeding - STOP AWARFARIN - give vit K + prothrombin complex concentrate (concentrate of factos II, VII, IX, X) (fresh frozen plasma if unavailable)

Question 30

How does unfractioned heparin and LMWH work? Give examples of LMWH? What can be given to reverse the effects of heparin?

Answer 30

Unfractioned heparin binds to antithrombin III increasing its ability to inhibit factor Xa and IIa (thrombin) LMWH (eg dalterparin or enoxaparin) binds to anti-thrombin III only increasing its ability to inhibit factor Xa Protamine sulphate can reverse heparin effect if severe bleed (only works in LMWH)

Question 31

Which type of heparin is prescribed in patietns with renal failure and why?

Answer 31

Elimination of unfractioned heparin is zero order, whereas that of LMWHs is first order Elimination of LMWHs is via renal excretion, hence unfractioned heparin (which has alternative elimination pathways) is preferred in renal failure

Question 32

What is the mechanism of action of fondaparinux?

Answer 32

Fondaparinux like LMWH also activates anti-thrombin III which in turn potentiates the inhibition of coagulation factor Xa

Question 33

Newer orally active agents that act as direct inhibitors are becoming increasingly common Give drug examples of the different types? What is an advantage? What is a disadvantage?

Answer 33

Factor Xa inhibitors - eg rivoroxaban, apixaban, edoxaban Factor IIa (thrombin) inhibitors eg dabigatran Advantages - regular routine monitoring is not required Disadvantage - no specific agent is available to reduce haemorrhage in overdose

Question 34

Fibrinolytics are used principally to reopen occluded arteries in acute myocardial infarction (M.I.), or stroke – less frequently life-threatening venous thrombosis or pulmonary embolism Name different types? Route of administration?

Answer 34

Streptokinase Alteplase Given IV

Question 35

Streptokinase is not an enyme but a protein extracted from cultures of streptococci How does alteplase work? What can be given to reverse the major side effect that is haemorrhage?

Answer 35

Alteplase is a recombinant tPA As such, it converted plasminogen to plasmin to cause breakdown of fibrin The major adverse effect of fibrinolytics is haemorrhage that may be controlled by oral tranexamic acid which inhibits plasminogen activation