Session 5 - Thrombosis and Embolism

Question 1

What is a thrombus?

Answer 1

A thrombus is a solid mass formed from the constituents of the blood within the heart or vessels during life.

Question 2

When does thrombosis occur?

Answer 2

This is the process by which a thrombus forms. It occurs when normal haemostatic mechanisms are turned on inappropriately.

Question 3

What is Virchow’s triad?

Answer 3

Virchow was the the first person to name the process of thrombosis and he said that it depended on 3 things.

1. change in the vascular wall (endothelial damage)
2. change in the blood flow (slow or turbulent)
3. Changes in the blood (hypercoagulability)

Question 4

Where are arterial and cardiac thrombi often seen?

Answer 4

At a site of endothelial injury or turbulence.

Question 5

Where are venous thrombi often seen?

Answer 5

At a location where there is stasis

Question 6

How many factors of the Virchows triad usually have to occur for a thrombus to form?

Answer 6

2 out of the 3.

Question 7

Why do platelets tend to flow nearer to the inner surface of the vessels?

Answer 7

This is because they are smaller and therefore as part of the laminar flow they flow on the outside.

Question 8

What are lines of Zahn and how do they form?

Answer 8

lines of Zahn is the name of the pattern formed in an arterial thrombus. There are alternating layers of platelets and red blood cells combined with fibrin.

• Initially platelets will bind to the endothelium and aggregate, especially if there is endothelial injury or the blood flow is slow.
• Fibrinogen binds the platelets together and fibrin grows out of the platelet layer. The fibrin traps the red blood cells.
• The surface of the red layer is thrombogenic and platelets stick to the exposed fibrin. A second white layer of platelets forms and the process continues.

Question 9

What are some of the most common clinical effects of thrombosis?

Answer 9

• Occlusion of an artery at the site of the thrombus resulting in iscahemia and infarction.
• Embolism at a distant artery. eg - pulmonary embolism and cerebrovascular accident.
• Congestion and oedema in venous bed resulting in pain and skin ulceration
• Repeated miscarriages due to thrombosis of uteroplacental vasculature

Question 10

What is an embolus?

Answer 10

Sudden blocking of an artery by a thrombus or foreigh material that has been brought to its site of lodgement by the blood current - it can be a solid, liquid or gas that is large enough to lodge and impact at a secondary site.

Question 11

Provide some examples of emboli material (Name at least 5)

Answer 11

• body fat
• bone marrow
• atheromatous plaques
• tumour fragments
• parasites
• bubbles of air or other gases
• debris injected intravenously
• amniotic fluid
• medical equipment
• fragments of brain or liver after trauma.

Question 12

How do emboli occur in the veins?

Answer 12

Trick question. Veins cannot emobilise. This is because the blood flows from smaller vessels to larger vessels and therefore the blood will go through the heart and usually lodge in the pulmonary arteries.

Question 13

What happens to emboli originating in the left heart or aorta?

Answer 13

The emboli will end up anywhere in the systemic circulation - especially in the lower limbs.

Question 14

Where do most pulmonary emboli originate from?

Answer 14

Deep veins of the thigh and the popliteal vein.

Question 15

What can small pulmonary emboli lead to?

Answer 15

They are quite often clinically silent however can lead to pulmonary hypertension.

Question 16

How would you define a large pulmonary emboli?

Answer 16

This is an emboli which results in >60% occlusion of the pulmonary circulation causing sudden death, right-sided heart failure or cardiovascular collapse.

Question 17

What is a saddle embolus?

Answer 17

This is a large emboli that becomes lodged at the site of a bifurcation of an artery thus blocking both branches. This classically occurs at the bifurcation of pulmonary arteries and can result in sudden death.

Question 18

Why are thrombi often seen on the left hand side of the heart?

Answer 18

• Atrial fibrillaiton resulting in dilatation of the left atrium and stagnation of blood resulting in thrombus formation
• Vegetations are more common on the valves of the left hand side of the heart
• Infarcts commonly affect the left ventricle. Thrombi then form on the affected necrotic endothelium of the ventricular cavity.

Question 19

What is the name of an embolus that forms in the systemic veins and embolises in the systemic arteries?

Answer 19

A paradoxical embolus.

Question 20

What can cause a paradoxical embolus?

Answer 20

1. Passing through the arterial venous anastamoses in the pulmonary circulation - they are 20-40 times the diameter of a capillary.
2. septal defects
3. passing through a patent foramen ovale during coughing, lifting or straining.

Question 21

What location(s) do an emboli derived from an atheroma usually affect?

Answer 21

• usually affects the intestine. Patients present with abdominal pain.
• Can affect the brain resulting in transient ischaemic attacks

Question 22

What is the most common original location of an emboli resulting in a transient ischaemic attack?

Answer 22

carotid artery atheroemboli

can be a result of a thromboemboli from the left heart.

Question 23

What differentiates between a transient ischaemic attack and a stroke pathologically?

Answer 23

A stroke is a complete occlusion of brain matter with no resolution.

A transient ischaemic attack is a result of a microemboli and due to its small size it is broken down within a matter of hours and therefore this is why the patient only has temporary loss of neurological function.

Question 24

Provide two examples of how a gas emboli could occur?

Answer 24

• Divers resurfacing too quickly and the change in pressure causes the nitrogen to become gas in the blood and cause multiple gas emboli.
• Trauma to the neck and chest - air drawn in upon inspiration.
• During childbirth

Question 25

What are some of the signs of an amniotic fluid embolus?

Answer 25

• Respiratory distress
• Hypotension
• Loss of consciousness
• Seizures
• DIC - as amniotic fluid contains prothrombotic substances

Question 26

What is a talcum emboli? What do they usually cause?

Answer 26

This is a substance which is used to ‘cut’ drugs and therefore is found in the lungs of intravenous drug users.

Can produce a foreign body reaction and pulmonary symptoms.

Question 27

Name some examples of general prophylaxis for thromboembolic disease?

Answer 27

• mobilise early after an operation or illness
• Elevating legs to improve venous return
• Compression stockings
• calf muscle stimulation
• Passive calf muscle exercises
• Anticoagulants

Question 28

How does heparin work pharmacologically?

Answer 28

Low molecular weight heparin forms an irreversible complex with antithrombin III resulting in its activation and therefore having anti-coagulative effects.

Question 29

How does warfarin work pharmacologically?

Answer 29

Warfarin interferes with vitamin K metabolism and therefore the clotting cascade. It is specifically titrated to each patient depending on their PT test results and specifically the INR ratio (which is the patients PT result in relation to a normal control).

Question 30

Why should you be careful with a patient who is on warfarin when prescribing antibiotics?

Answer 30

• Antibiotics can inhibit CYP450 which will prolong the amount of time that the warfarin is in the system therefore enhancing its effect
• The broad spectrum antibiotics will kill the flora in the gut that usually inhibits vitamin K

Both of these factors will downregulte the amount of vitamin K in the body and enhance bleeding.

Question 31

What is the Leiden mutation?

Answer 31

This is a mutation of factor V that is present in up to 15% of the population and leads to hypercoagulability - one of the causes of thrombophilia.

Question 32

what is the process through which a thrombus undergoes organisation and blood vessels run through the thrombus?

Answer 32

recanalisation. The blood flow is usually re-established but not completely. One or more channels are formed through the thrombus.

Question 33

What is propogation?

Answer 33

This is the process that occurs in the veins and arteries. It is defined as the progressive spread of the thrombus distally in the arteries (away from the heart) and proximally in the veins (towards the heart).

Question 34

How would you define organisation in thrombosis?

Answer 34

This is where the body tries to repair damage by ingrowth of fibroblasts and capillaries (the formation of tissue similar to granulation tissue) - however the lumen remains obstructed.