Pulmonary Thromboembolism

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Pulmonary thromboembolism is an extremely common preventable condition.
The most common preventable cause of death in hospital patients is pulmonary thromboembolismز The vast majority of cases are caused by emboli arising from thrombosis of deep leg veins (calf, popliteal, femoral and iliac veins).
Diagnosis is notoriously difficult as symptoms and signs are non-specific; most cases are silent, resolving without detection.

The two main consequences of embolization to the pulmonary arterial tree are an increase in pulmonary arterial pressure (which puts a strain on the right side of the heart) and ischaemia of the lung,
with ventilated areas not being perfused by blood.
If 60% of the pulmonary vasculature is suddenly blocked, the heart cannot pump blood through the lungs. There is cardiovascular collapse, with electromechanical dissociation of the heart as it continues to
beat but develops no output. This pattern of blockage is known as massive pulmonary embolism. Causing rapid death, it accounts for about 5% of all cases of pulmonary thromboembolism .

Major pulmonary embolism occurs when there is blockage of middle-sized pulmonary arteries. Patients commonly experience breathlessness.
Infarction of lung develops in only about 10% of such cases. It can lead to haemoptysis and, if adjacent to the pleura, pleuritic chest pain.
It is not uncommon for patients to develop a subsequent massive thromboembolism if untreated.

In about 85% of all cases of pulmonary thromboembolism there is blockage of small peripheral vessels by small emboli (minor pulmonary embolism).
Patients may be asymptomatic or may experience breathlessness and pleuritic chest pain as a result of small infarcts.
A very small number of patients develop recurrent minor pulmonary embolism. There is blockage of many small peripheral arteries over a period of many months by recurrent small emboli.
This leads to obliteration of the vascular bed and right heart strain, causing pulmonary hypertension.
The main pulmonary artery contains a mass of red thrombus, a thromboembolus.
This originated by embolization from one of the femoral veins.

Embolism of material other than thrombus is less common.
Although thromboembolism is common, other material can form emboli in unusual circumstances. For example, cholesterol embolism is caused by fragments of fat derived from atheroma in major arteries, and fat and bone marrow embolism usually form as a result of severe trauma to bones, with fat and marrow from the medullary cavity entering the venous system.
Tumour cell embolism is an important method of spread of malignant tumour.
Air embolus is usually due to accidental pumping of air into the venous system during intravenous injection or transfusion, and bubbles of gas (nitrogen embolus) may form in the bloodstream in deep sea divers with decompression sickness .

Amniotic fluid embolism occurs in about 1:70,000 deliveries; amniotic fluid enters veins during delivery causing extensive intravascular coagulation.
Therapeutic embolization is being used increasingly to treat inoperable congenital vascular malformations (particularly of the brain)by interventional radiology. Wire, gelfoam, glue and balloons are
all used deliberately to block vessels.
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