5. Embolism

Question 1

What is embolism?

Answer 1

Sudden blocking of an artery by an embolus which has been brought to its site of lodgement by the current of blood.

Question 2

What is an embolus?

Answer 2

Solid, liquid or gas that is carried by the blood and is large enough to become impacted in a vascular lumen.

Question 3

What is a thromboemboli?

Answer 3

Emboli that arise from thrombi. Most common type of emboli and are often multiple when they occur.

Question 4

Gives examples of types of emboli.

Answer 4

1. thromboemboli
2. body fat or bone marrow
3. atheromatous plaque material
4. tumour fragments
5. parasites
6. bubbles of air or other gases
7. amniotic fluid
8. medical equipment
9. bits of brain or liver after trauma

Question 5

Why can embolisation not occur in veins?

Answer 5

Blood flow is from smaller to larger vessels

Question 6

Where does material carried by blood in veins embolise?

Answer 6

Go through the right heart and embolise in pulmonary arteries

Question 7

Where does material from the left heart or aorta embolise?

Answer 7

Anywhere in the systemic circulation (as blood flow in arteries is from large to small arteries) but esp. in lower limbs.

Question 8

Where do most pulmonary emboli arise from?

Answer 8

Approx. 80% arise from thrombi in the deep veins of the thigh and the popliteal vein (rather than the smaller veins of the calf).

Question 9

What are the clinical effects of pulmonary emboli?

Answer 9

• Most are small and clinically silent. But multiple small pulmonary emboli can cause pulmonary hypertension.
• Large pulmonary emboli (resulting in >60% occlusion of the pulmonary circulation) will cause:
  
  • sudden death
  • cor pulmonale (right-sided heart failure)
  • cardiovascular collapse

Question 10

What is a saddle embolus?

Answer 10

Large emboli that become lodged astride the birfurcation of an artery, thus blocking both branches.

Classically occur at the bifurcation of the pulmonary arteries and result in sudden death.

Question 11

Where do thromboemboli in systemic arteries arise from and embolise to?

Answer 11

• Arise from left heart, aneurysms and thrombi on ulcerated atherosclerosis.
• Embolise to lower extremities, brain, intestines, kidneys, spleen and arms.

Question 12

Give 3 reasons why thrombi are often seen in the left heart.

Answer 12

1. Infarcts commonly affect the LV. Thrombi can then form on the necrotic endothelium in the ventricular cavity. As the heart is beating, these often embolise.
2. Atrial fibrillation results in decreased atrial contraction, dilatation of the LA, stagnation of blood in the LA and hence thrombus formation.
3. Vegetations are more common on valves of left side of heart.

Question 13

What is the most common cause of systemic thromboemboli?

Answer 13

80% are from cardiac mural thrombi (in association with an MI or atrial fibrillation)

Question 14

What are paradoxical emboli?

Answer 14

RARE thromboemboli that form in the systemic veins but embolise to the systemic arteries.

Question 15

How can paradoxical emboli bypass the lungs?

Answer 15

Manage to bypass the lungs in 1 of 2 ways:

1. Small emboli are able to pass through the arterio-venous anastamoses in the pulmonary circulation (these anastamoses are 20-40x the diameter of a capillary).
2. Larger emboli can only enter the systemic circulation by passing through defects in the interventricular septum or a patent foramen ovale during coughing, lifting or straining (which increases the pressure in the right side of the heart to greater than that in the left, pushing the thrombus through the defect).

Question 16

What is an atheroembolus and why do these occur?

Answer 16

Release of atheroma (gruel-like necrotic material present in atherosclerotic plaques) into the blood when a plaque breaks open.

Can happen spontaneously or during surgery or catherisation for coronary artery disease.

Question 17

Which organ do atheroemboli often affect?

Answer 17

Intestines (present with abdominal pain)

Question 18

What are transient ischaemic attacks and how are these caused?

Answer 18

TIAs = episodes of neurological dysfunction that appear suddenly, last minutes to hours and then disappear.

Are the result of microscopic emboli, usually atheroemboli, to the brain.
Atheroembolus usually comes from the carotid arteries but sometimes arises in left heart.

As the emboli are very small, they they break up quickly before any lasting damage is done.

Question 19

Why do fat and bone marrow emboli occur?

Answer 19

Usually complication of bone fractures but fat emboli can also occur after liposuction.

Question 20

How does a bone fracture lead to fat and bone marrow embolism?

Answer 20

i) Bone fracture causes injured bone marrow fat cells to break up and release oil droplets.
ii) Oil droplets coalesce over a period of days and a are then sucked into gaping venules that have been torn by fracture.
iii) Symptoms of fat embolism (respiratory distress and neurological symptoms) thus seen 1-3 days after fracture.

Question 21

What is the fat and bone marrow embolism occurence rate and mortality rate in fracture patients?

Answer 21

• Occur in 5-10% of patients with pelvic or long bone fractures.
• Mortality rate in 10-15% of patients.

Question 22

What are the clinical consequences of fat and bone marrow emboli?

Answer 22

• Emboli that lodge in the lungs cause respiratory symptoms - respiratory distress.
• Some droplets however pass through the lungs (via the arterio-venous anastamoses in pulmonary circulation) and into organs such as the brain, kidneys and skin where they cause symptoms such as agitation, coma, renal failure and a petechial rash.

Question 23

What are the possible causes for air emboli?

Answer 23

1. There is negative pressure in the veins of the chest and head during inspiration in the upright position - these veins can draw in air after, e.g. trauma of neck and chest.
2. Can occur in labour as air can enter the uterus and be forced into open veins during uterine contraction.

Question 24

What is a fatal amount of air in embolism?

Answer 24

~100 mls

Question 25

What are the consequences of air emboli?

Answer 25

Air is transported to the right heart where bubbles gather as a frothy mass that stops circulation.

Question 26

What causes 'the bends'?

Answer 26

i) Whilst a diver is breathing air underwater (where surrounding pressure is higher than on land), increased amounts of gases become dissolved in blood and body tissues. ii) If diver surfaces too quickly, sudden depressurisation results in dissolved gases coming out of solution and being released into the body as bubbles. iii) Bubbles distort the tissues (v. painful) and act as emboli in blood.

Question 27

Which gas is particularly important in the bends and why?

Answer 27

Nitrogen - is fat soluble so when it comes out of solution, it produces persistent bubbles and focal ischaemia in the lipid-rich tissues, such as CNS, where it was previously dissolved. Also forms bubbles in skeletal muscle and joints (v. painful) and in lung tissue.

Question 28

What is the bends called when lung tissue is affected?

Answer 28

the chokes

Question 29

How is the bends treated?

Answer 29

Prompt recompression in a special recompression chamber to force the gas back into solution. P can then undergo slow recompression.

Question 30

What is amniotic fluid embolism and when does it occur? What is the mortality rate?

Answer 30

Complication of labour and caesarean section when amniotic fluid enters the maternal circulation through a tear in the amniotic membranes. Occurs in approx. 1/50,000 deliveries. 20-40% mortality rate.

Question 31

What are the symptoms of amniotic fluid embolism?

Answer 31

- sudden respiratory distress - hypotension - seizures - loss of consciousness - DIC (as amniotic fluid contains prothrombotic substances)

Question 32

Which type of embolism can occur in IV drug abusers? What does this cause?

Answer 32

- Microscopic foreign bodies with which drugs have been 'cut', e.g. talcum, are found in lungs. - Can produce a marked foreign body reaction and pulmonary symptoms.

Question 33

How is general prophylaxis of thromboembolic disease achieved after an operation?

Answer 33

Either by preventing venous stasis or by preventing hypercoagulability. i) Stasis prevention: - early mobilisation after operation/illness - during and after operation, legs can be elevated - increase venous return via compression stockings, calf muscle stimulation and passive calf muscle exercises ii) Hypercoagulability prevention using anticoagulants.

Question 34

Which anticoagulants can be used in the prophylaxis and/or treatment of thrombosis?

Answer 34

1. aspirin - used in certain patients to reduce risk of MI and stroke. Also recommended as prophylactic against DVT in patients on long haul flights 2. heparin - prophylaxis and treatment of thrombosis 3. warfarin - prophylaxis and treatment of thrombosis

Question 35

How does aspirin prevent thrombosis?

Answer 35

Irreversibly acetylates an enzyme of prostaglandin synthesis (cyclooygenase)... platelets can't produce thromboxane A2 which is a powerful platelet aggregator... inhibits formation of platelet plus and prolongs bleeding time.

Question 36

How does heparin prevent thrombosis?

Answer 36

Forms irreversible complexes with antithrombin III resulting in its activation.

Question 37

How does warfarin prevent thrombosis?

Answer 37

Oral medication that interferes with vitK metabolism.

Question 38

Apart from anticoagulants, how can pulmonary emboli be prevented?

Answer 38

Filters - umbrella-shaped filter in inferior vena cava

Question 39

What is Trousseau's syndrome?

Answer 39

- Increased risk of thromboembolism in disseminated cancer. - Occurs as tumours cause inflammation and tissue injury, causing procoagulative factor release. - Some also produce and release mucin, also a procoagulant.

Question 40

Why can pulmonary embolism cause death?

Answer 40

Respiratory compromise - there is a non-perfused but ventilated area of lung, i.e. a ventilation-perfusion mismatch which results in hypoxaemia.