Pulmonary embolism - Causes of pulmonary embolism

A pulmonary embolism occurs when the artery that carries blood to the lungs (pulmonary artery) becomes blocked.

The blockage is usually a blood clot, although it can be a fat droplet, an air bubble or amniotic fluid (fluid that surrounds unborn babies).

If the blockage is caused by a blood clot, it probably will have come from one of the deep veins in your legs and is known as deep vein thrombosis (DVT).

Three of the main reasons why blood clots develop are described below.

Inactivity

If you are inactive, blood tends to collect in the lower parts of your body, particularly in your lower legs. This is not usually a problem because when you start to move, your blood flow increases and blood begins to move evenly around your body.

However, if you are immobile for a long period of time, the flow of blood around your body can slow down considerably. You are likely to be immobile:

after a debilitating illness, such as a stroke
after an injury or operation
when travelling on a long journey by plane, train or car

If your blood flow slows down because of a prolonged period of inactivity, your risk of a blood clot forming increases.

Blood vessel damage

If a blood vessel is damaged, the inside of the blood vessel can become narrowed or blocked. This can result in a blood clot forming.

Blood vessels can be damaged by injuries such as broken bones or severe muscle damage. If a blood vessel is damaged during surgery, a blood clot may develop, particularly in operations that are carried out on the lower half of your body.

Conditions such as vasculitis (inflammation of the wall of a blood vessel) and some types of medication, such as chemotherapy medication, can also lead to blood vessel damage.

Blood that clots too easily

Your risk of developing a pulmonary embolism is increased if you have a condition that causes your blood to clot more easily than normal.

Conditions that increase the likelihood of your blood clotting include:

cancer – cancer treatments, such as chemotherapy and radiotherapy, can also make your blood clot more easily
heart failure
thrombophilia – an inherited condition that makes your blood more prone to clotting
Hughes syndrome – a condition where the blood becomes abnormally sticky, increasing its tendency to clot

Other factors

Other factors that increase your risk of developing a pulmonary embolism include:

your age – people aged 60 or over have an increased risk
having a previous blood clot
having a family member who has had a blood clot
being overweight or obese
being pregnant – your risk is also higher for up to six weeks after giving birth
smoking
taking the combined oral contraceptive pill or hormone replacement therapy (HRT)

Your chances of developing a blood clot are very small if you are taking the pill or HRT, and your GP will usually assess your individual risk before prescribing either medication.