Session 5 - Haemostasis, Thrombosis and Embolism

Question 1

Define haemostasis

Answer 1

Haemostasis: Consequence of tightly regulated processes that maintain blood in a fluid state in normal vessels yet also permit the rapid formation of a haemostatic clot at the site of a vascular injury

— Pathologic counterpart of haemostasis is thrombosis; this involves blood clot formation (thrombus) in an intact vessel

Question 2

Three steps of haemostasis

Answer 2

-> Vessel wall damage

1. The severed or damaged artery contracts. This contraction isn’t enough to stop the bleeding but does assist in reducing the pressure ‘downstream’ of the site of injury and control the bleeding. Contraction will not occur in veins due to a lack of muscular tissue in the vessels, however this is not an issue due to the lower pressure of venous blood flow.
2. A primary haemostatic plug of activated platelets will form at the site of injury. This plug will stick to the injured vessel and the tissue outside of it. Once more, this won’t stop the bleeding completely but will help to control the bleeding. The primary plug will form seconds to minutes following the vessel severing. PLATELET PLUG
3. A secondary haemostatic plug forms as fibrin filaments stabilise the fragile platelet plug to form a blood clot. This will take around 30 minutes to form in a healthy individual. CLOT

3 components: Vascular wall, platelets and coagulation cascade

Question 3

Platelets

Answer 3

Platelets are cell fragments which have no nucleus and are involved in the process of blood clotting. There are approximately 150-400 x10⁹ cells per litre of blood. Platelets are formed in the bone marrow by the process of budding from the cytoplasm of megakaryocytes.

Question 4

1. Primary haemostasis
   - medication link

Answer 4

1. Activation of platelets:
   Platelets can be activated in numerous ways:

• Contact with collagen surfaces - collagen is found in the blood vessel wall below the endothelium and is therefore exposed when the blood vessel is damaged and can activate platelets.
• ADP – released by activated platelets and red blood cells. This causes an amplified platelet response.
• Thromboxane A2 – a powerful platelet aggregator that is released by activated platelets in order to amplify the platelet response.
• Thrombin – activated in the clotting cascade and recruits more platelets.

2. Adhesion to the endothelial basement membrane:
   Once activated, platelets will adhere to von Willebrand factor (vWF), which is concentrated on the subendothelial basement membrane of blood vessels.
   [The platelets adhere to vWF and vWF acts as a bridge between the platelet surface and collagen in the vessel wall]
3. Aggregation of platelets:
   One adhered to the endothelial basement membrane, they will then proceed to aggregate with other platelets, growing the haemostatic plug.
4. Secretion:
   Once activated, the platelet will secrete factors recruiting more platelets (secretes ADP, thromboxane A2, Ca²⁺ etc -> positive feedback).

MEDICATION LINK: The drug aspirin irreversibly deactivates cyclooxygenase, an enzyme responsible for thromboxane A2 production. Therefore, long-term low-dose aspirin is used as secondary prevention against myocardial infarction and stroke (i.e. aspirin makes it more difficult for blood clots to form).

Question 5

2. Secondary haemostasis

Answer 5

The Clotting Cascade
For blood to clot, fibrin is needed which are threads of protein that can cross-link to form a fibrin mesh. This mesh catches platelets and holds the blood clot together to strengthen the platelet plug. The formation of fibrin is achieved via the clotting cascade, which is shown in the diagram below.

Coagulation can occur via two pathways which produce the same result. The two pathways use some different clotting factors shown in the Venn diagram. This means that timings of these pathways can be measured when investigating coagulation disorders to narrow down the clotting factors affected. The two pathways and their measurement are explained below:

• Intrinsic pathway (measured by Activated partial thromboplastin time - APTT) – Triggered by the exposure of collagen after a blood vessel is damaged.
• Extrinsic pathway (measured by Prothrombin Time - PT) – Triggered by the release of thromboplastin from damaged endothelial cells. It can be released in response to breaking of the blood vessel wall, hypoxia, sepsis, malignancy and inflammation.

PT can also be expressed by the international normalised ratio (INR) which compares the PT to a normal value in the form of a ratio. For example, an INR of 1 is normal, and blood with an INR of 3 will clot more slowly, for example if a patient is on warfarin.

Bleeding Time (BT):
There is an additional measurement which can be done to investigate clotting which is measuring how long the patient bleeds for after a break to the skin. A longer **bleeding time** (**BT**) suggests a problem with **platelet aggregation**.

TCT (Thrombin clotting time):
Measure of the final step of the pathway, the conversion of fibrinogen to fibrin via the action of thrombin

Question 6

Explain:

• APTT
• PT
• INR
• BT
• TCT

Answer 6

• APTT:
  Intrinsic pathway (measured by Activated partial thromboplastin time - APTT) – Triggered by the exposure of collagen after a blood vessel is damaged.
• PT:
  Extrinsic pathway (measured by Prothrombin Time - PT) – Triggered by the release of thromboplastin from damaged endothelial cells. It can be released in response to breaking of the blood vessel wall, hypoxia, sepsis, malignancy and inflammation.
• INR:
  PT can also be expressed by the international normalised ratio (INR) which compares the PT to a normal value in the form of a ratio. For example, an INR of 1 is normal, and blood with an INR of 3 will clot more slowly, for example if a patient is on warfarin.
• Bleeding Time (BT):
  There is an additional measurement which can be done to investigate clotting which is measuring how long the patient bleeds for after a break to the skin. A longer bleeding time (BT) suggests a problem with platelet aggregation.
• TCT (Thrombin clotting time):
  Measure of the final step of the pathway, the conversion of fibrinogen to fibrin via the action of thrombin

Question 7

Factors opposing clotting - Stopping the clotting process getting out of control

• Natural anticoagulants
• Fibronolysis (and drug link)

Answer 7

Natural anti-coagulants:
These factors include dilution of clotting factors by blood flow and natural anticoagulants. The main natural anticoagulants in the blood are antithrombin III, protein S and protein C. Natural anticoagulants don’t destroy clots (that deed is performed by the enzyme plasmin in fibrinolysis); rather they oppose the formation of fibrin in the first place to control the clotting cascade.

Fibrinolysis: (pic below)
The endothelium secretes a substance known as plasminogen which is converted to plasmin by tissue plasminogen activator (tPA). The activity of tPA is increased by the presence of fibrin. Plasmin is the major enzyme of the fibrinolytic system and destroys clots by degrading fibrin. Fibrin degradation/split products released in this process also inhibit clotting. This system ensures that clots do not persist beyond the time for which they are needed.
– Drug link: tPA
tPA can be used therapeutically in dissolving thrombi and thromboembolisms, however, a side effect of this can be undesired bleeding. An additional fibrinolytic agent which can be used therapeutically is streptokinase, however this is obtained from streptococci, meaning that it is antigenic and can only be used once

Question 8

When clotting goes wrong
- DIC

Answer 8

Disseminated intravascular coagulation is a pathological activation of the coagulation cascade. In DIC, clotting activators enter the systemic circulation and many microthrombi (small pathological clots in the vessels) are formed throughout the circulation. During this process many coagulation factors and platelets are consumed.

DIC:
When enough of these clotting factors have been used, haemorrhage can begin to occur as the body has no more platelets or clotting factors to form clots when they’re actually needed. The affected patient is at risk of bleeding, neurological impairment, gangrene, renal failure, respiratory distress and a wide variety of other complications. Red blood cells are often damaged in DIC as they are sheared by flowing past the microthrombi, and between this and haemorrhage the patient can develop anaemia.

Triggers for DIC can include malignancy, massive trauma (e.g. burns), infections, massive haemorrhage, and transfusion. Although in the short term DIC can be managed by transfusing the patient with clotting factors and platelets, in order to truly treat the condition, the underlying cause must be treated first

Question 9

When clotting goes wrong - Haemophilia
- two types

Answer 9

There are two main types of haemophilia, both of which are congenital disorders of clotting factors. These two types are haemophilia A and haemophilia B (also known as Christmas disease). Platelets are unaffected in haemophilia, however, as there are inadequate amounts of clotting factors, less fibrin is produced.

Both varieties of haemophilia are X-linked recessive and are therefore much more common in males than in females. Haemophilia A and haemophilia B are clinically indistinguishable from one another, with both having normal bleeding time and PT but prolonged APTT (factor VIII and IX are used in the intrinsic pathway causing the APTT to be prolonged, but neither factor is involved in the extrinsic pathway so PT is normal).

Haemophilia A is either a deficiency of the amount of factor VIII (8), or reduced factor VIII activity It is the most common hereditary disorder associated with serious bleeding.

Individuals with mild haemophilia A may experience prolonged bleeding after major trauma or surgery, and after venepuncture. In more severe cases, patients can present with spontaneous bleeding into joints (haemarthrosis), intramuscular haemorrhage and, in severe cases, intracranial haemorrhage.

Mild or moderate symptoms of haemophilia A are seen with 6-50% of normal factor VIII function or amount, while the severe form of the disease is seen with under 1% of factor VIII.

Patients with haemophilia A can be managed through infusion of recombinant factor VIII.

Haemophilia B is a deficiency in factor IX (9), and presents with similar symptoms to haemophilia A, but with an increased risk of bruising and experiencing mucosal bleeds. It can be treated through an infusion of recombinant factor IX.

Question 10

When clotting goes wrong - Von Willebrand Disease

Answer 10

Von Willebrand disease is the most common inherited bleeding disorder and can range in nature from asymptomatic to severe. Von Willebrand factor is either deficient or abnormal in this condition.

The function of this protein includes protecting factor VIII (8) from premature destruction and assisting in the formation of the platelet plug. Both bleeding time and APTT can be affected by this condition.

Features include bleeding following minor trauma or surgery, epistaxis and menorrhagia. Bleeding into muscles and joints is rare.

Management is similar to that of haemophilia A as patients can be given recombinant factor VIII infusions

Question 11

When clotting goes wrong - Thrombocytopenia

Answer 11

Thrombocytopenia is a condition in which the platelet count is less than 100 x10⁹/L. In this condition the bleeding time is affected, but the PT and APTT remain unaffected, as these are an indicator of clotting cascade function.

Patients with thrombocytopaenia can present with petechiae, which are small red or purple spots under the skin; these occur due to spontaneous bleeding from capillaries.

Thrombocytopenia has a variety of causes such as autoimmune thrombocytopenic purpura, DIC, liver failure or drugs such as heparin.

à see MEH notes

Vessel Wall Abnormalities
Sometimes vessel wall themselves are a problem.
- Easy bruising
- Spontaneous bleeding from small vessels
- Mainly skin, but can affect mucous membranes
—- Congenital: e.g. Connective tissue disorders like Ehlers Danlos syndrome
—- Acquired e.g. senile purpura, steroids, infections, scurvy

Question 12

When clotting goes wrong - Thrombophilia

Answer 12

Thrombophilia can due to be congenital or acquired defects of haemostasis that lead to an increased chance of thrombosis (pathological blood clotting).

Examples of thrombophilias include factor V Leiden thrombophilia, antithrombin deficiency, protein S or protein C deficiency, and antiphospholipid syndrome.

Question 13

Thrombosis and Embolism

Thrombus:

• Define thrombus
• Define thrombosis
• Virchow’s triad

Answer 13

• Thrombus = A thrombus is the name given to a solid mass which is formed from the constituents of the blood within the heart or vessels during life.
• Thrombosis = Thrombosis is the process of forming this thrombus and it occurs when the normal haemostatic mechanisms are turned on inappropriately.
• Virchow’s Triad = There are three main factors which will increase the likelihood of thrombosis occurring. Collectively these are known as Virchow’s Triad of thrombosis and consist of the following:
• Changes in the vessel wall (endothelial damage)
  
  • E.g. After myocardial infarction, surgery, chemical damage or ruptured atherosclerotic plaque
• Changes in the blood flow (slow or turbulent flow)
  
  • E.g. Due to long periods of immobility, varicose veins or venous obstruction, narrowing, low blood pressure
• Changes in the blood components (hypercoagulability) - ‘sticky blood’
  E.g. Due to cancer, thrombophilia, inflammatory disease, smoking or pregnancy

Thrombosis occurs via the same mechanisms as in haemostasis, i.e. platelets are activated and bound together by fibrin, which goes on to trap red blood cells to form a clot.

Question 14

Structure of Thrombi

• arterial thrombus
• venous thrombus

Answer 14

• Thrombi are laminated structures (they are made up of layers), and the lines of lamination are known as lines of Zahn. This is a consequence of the way thrombi are formed.

As platelets are the smallest constituents of blood, they tend to be pushed by the force of blood flow to the walls of the blood vessel and so are more concentrated along the endothelium. The platelets are therefore more likely to catch in any crevices, such as behind the valves of veins or abnormalities in the vessel wall. These caught platelets can form an aggregate and stick to the wall of the vessel. As blood continues to flow, additional platelets can join the aggregate, forming a white layer of platelets. Red blood cells then get caught in the aggregate, creating a red layer, followed by another white layer of platelets and the process repeats. These continual additions will result in the laminar form of the thrombus.

Lines of Zahn are more visible in arterial thrombi as blood flows directly over the surface of the forming thrombi in arteries, whereas there is a degree of shielding by the valves in veins. Clots formed post-mortem have no lines of Zahn as there is no blood flow.

Structure of thrombi - attached

Question 15

Outcome of thrombosis

Answer 15

Outcomes of Thrombosis L-PORE

There are several outcomes which can arise from thrombus formation:

• Lysis (breakdown) of the thrombus.
  
  • This is the most favourable outcome. Blood flow is re-established completely in this case. It is most likely when thrombi are small.
• Propagation
  
  • The thrombus will progressively spread. It can become bigger, and form new thrombi downstream of the original.
    (Distally in arteries, proximally in veins)
• Organisation
  
  • Fibroblasts act on the thrombus and scar tissue forms where the thrombus is. In this case the lumen of the blood vessel will remain partially occluded due to the scar.
• Recanalisation
  
  • One or more channels can be formed through the thrombus like tunnels, so blood flow is re-established but usually incompletely.
• Embolism
  
  • Part of the thrombus breaks off and travels through the bloodstream to lodge at a distant site.

Question 16

Embolism

• Define embolism
• What is a thromboembolism?
• What is a DVT?
• What is a pulomonary embolism?
• What can cause a stroke?

Answer 16

• Embolism = is the blockage of a blood vessel by solid, liquid or gas at a site distant from its origin.
• Thromboembolism = 90% of emboli are thromboemboli (originating from a thrombus)
• Most thromboemboli arise in the deep veins of the legs - called deep vein thrombosis (DVT). THROMBOSIS -> THROMBOEMBOLISM (MOST COMMON TYPE OF EMBOLISM)
• Pulmonary embolism = Emboli carried in the veins will most often travel to the right side of the heart and enter the pulmonary circulation to block an artery in the lungs. This is called a pulmonary embolism.
• Stroke = In arteries, the blood flows from larger vessels to smaller vessels, so emboli carried in the arteries will lodge in smaller arteries downstream. Emboli from systemic arteries or the left side of the heart can impact themselves in the systemic circulation. If this occurs in the brain it can cause a stroke.

Question 17

Embolism - other types of embolism?

Answer 17

• Solid - Fat and bone marrow embolism
  
  • Can occur after breaking a bone, especially a large one such as the femur. The fat or bone marrow form an insoluble embolism which travels through the blood and blocks a vessel.
  • Tumour cells (disseminated malignancy)
• Gas embolism
  
  • Air embolism can occur following physical trauma: veins can draw in air from the external environment. Air is transported to the right side of the heart and create a frothy mass that blocks circulation.
  • Injections, and removal of lines (medical equipment)
  • ‘The bends’ are another process of forming a gas embolism. If a diver surfaces from deep water too quickly, the sudden drop in pressure can cause dissolved gases in the blood to come out of solution and form bubbles in the circulation.
• Amniotic fluid embolism
  
  • A rare but dangerous complication of labour and C-section where amniotic fluid enters the maternal circulation through a tear in the amniotic membranes.
• Talcum embolism
  
  • Microscopic particles can enter the circulation of IV drug users.

Question 18

Sudden occlusion of an artery

• examples of how this can happen
• signs and symptoms of occlusion of an artery

Answer 18

Examples of how this can happen include:
An abdominal aortic aneurysm (AAA), atherosclerosis, or clot formation in the left atrium in atrial fibrillation. In each of these examples one or more elements of Virchow’s triad is affected.
Sudden occlusion of an artery is very serious and needs to be treated quickly to avoid damage to the limb or organ.

Signs and symptoms: Six P’s
The signs and symptoms of occlusion of an artery the a limb are the six P’s, and result from the lack of blood perfusing the tissues:

• Pain
• Paralysis
• Paraesthesia
• Pallor
• Perishingly cold to touch
• Pulseless

Question 19

Clinical effects of arterial thrombosis on the tissue

Answer 19

Question 20

Clinical effects of venous thrombosis on the tissue

• List
• DVT
• Suspect a DVT.. score to use

Answer 20

• List (below)

- Deep Vein Thrombosis:
Thrombus formation in veins occurs slower than in arteries, giving time for the body to mount an acute inflammatory response. Therefore, the signs of thrombus formation in veins are the five cardinal signs of inflammation: rubor, calor, tumor, dolor and loss of function.
The most common site for thrombus formation is in the deep veins of the legs. This is commonly due to immobility. As the calf muscles contract during movement, the venous blood is pushed back towards the heart. Therefore, by not moving, for example in a long-haul flight, the blood in the veins doesn’t flow as much, causing stasis of blood in the legs. This is one of Virchow’s triad, and a clot may develop. This is called a deep vein thrombosis (DVT). The five signs of inflammation may be seen in the calf.
Other risk factors for DVT: Oral contraceptives, post-operative e.g. immobile, pregnancy and post-partum (as thicker blood), severe burns, cardiac failures, disseminated cancer, bed-rest
If the DVT embolises, the fragment will travel through the venous system towards the right side of the heart and enter the pulmonary circulation. It will then lodge in an artery, causing a pulmonary embolism

- Score DVT: Suspect a DVT - WELL’S SCORE
• Well’s score (2 or more = s/c heparin and ultrasound scan leg) • D- dimers in blood (fibrinogen degradation product) • Anticoagulant therapy

Question 21

• Pulmonary thromboembolism

Answer 21

• haemodynamic effect -> right sided heart failure
• hypoxhypoxia effect

Question 22

Methods to prevent thrombosis and thromboembolis?

• not drug related methods
• drug related methods
• other treatments

Answer 22

• Identify high risk patients
• Heparin (anti-coagulant)
• Mobilise early (after surgery), limit bed rest
• Aspirin (anti-platelet) – increases bleeding time
• Leg compression during surgery e.g. TED stockings
• Warfarin – affects Factor II, VII, IX, X, Protein C and Protein S
• Rivaroxaban/Apixaban – directly inhibits factor Xa
• Heparin induces Antithrombin III which inhibits Va
• Clot bursters/thrombolysis – if haemodynamically unstable – Streptokinase, recombinant tPa

Other types of treatment:

• Embolectomy (e.g. remove the thrombus for acute limb ischaemia)
• Mechanical interventions (e.g. devices in the left atria to stop thrombus formation)