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What is embolism?

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


What is an embolus?

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


What is a thromboemboli?

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


Gives examples of types of emboli.

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


Why can embolisation not occur in veins?

Blood flow is from smaller to larger vessels


Where does material carried by blood in veins embolise?

Go through the right heart and embolise in pulmonary arteries


Where does material from the left heart or aorta embolise?

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


Where do most pulmonary emboli arise from?

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


What are the clinical effects of pulmonary emboli?

- 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


What is a saddle embolus?

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.


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

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


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

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.


What is the most common cause of systemic thromboemboli?

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


What are paradoxical emboli?

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


How can paradoxical emboli bypass the lungs?

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).


What is an atheroembolus and why do these occur?

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.


Which organ do atheroemboli often affect?

Intestines (present with abdominal pain)


What are transient ischaemic attacks and how are these caused?

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.


Why do fat and bone marrow emboli occur?

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


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

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.


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

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


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

- 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.


What are the possible causes for air emboli?

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.


What is a fatal amount of air in embolism?

~100 mls


What are the consequences of air emboli?

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


What causes 'the bends'?

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.


Which gas is particularly important in the bends and why?

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.


What is the bends called when lung tissue is affected?

the chokes


How is the bends treated?

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


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

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.