What causes a pulmonary embolism?
Usually a pulmonary embolism is caused by a blood clot travelling up from one of the deep veins in your legs. This kind of clot is called a deep vein thrombosis (DVT).
In many cases, the clot occurs because of a change such as pregnancy or recent surgery. But in about 40% of cases, your health care professional may be unable to find the cause of a pulmonary embolism. It can happen for no obvious reason.
This can be called ‘unprovoked’.
When you’ve been inactive for a long time, your chance of getting a pulmonary embolism increases.
This is because when you’re inactive, blood tends to collect in the lower parts of your body, particularly in your lower legs. This isn’t usually a problem because when you start to move, your blood flow increases and blood begins to move more evenly round your body. But if you’re immobile for a long time, the flow of blood around your body can slow a lot. This can be:
- after an operation or a serious limb injury
- after long periods of bed rest
- during a long-haul flight or a long train or car journey lasting more than 6 hours
Around half of all people with a pulmonary embolism get it while they’re in hospital.
Less commonly, you might have a condition that causes your blood to clot more easily than normal, such as cancer, or cancer treatments such as chemotherapy and radiotherapy.
Other factors that increase your risk of developing a pulmonary embolism include:
- being overweight
- pregnancy – your risk is increased for up to 6 weeks after giving birth
- taking some forms of hormone-based contraception or hormone replacement therapy (HRT). Your chances of developing a blood clot are very small if you’re taking the contraceptive pill or HRT, and your health care professional will consider your individual risk before they prescribe them.