Can long flights cause blood clots (DVT), and what reduces the risk?

What is DVT, and how can it affect the lungs?

A deep vein thrombosis, or DVT, is a blood clot in one of the deep veins, most often in the leg. The NHS says the main worry is that part of the clot can travel to the lungs. This is called a pulmonary embolism, and the NHS lists it among the reasons to call 999.

Some clots cause no symptoms at all, which is why a DVT can be hard to spot.

Why are long journeys linked to blood clots?

Blood flow in the leg veins slows when the legs stay still. The NHS lists being immobile, such as sitting or lying down for long periods, among the things that raise the risk of DVT. The NHS also says that on journeys of four hours or more by plane, train or car, there are things that can lower the chance of DVT.

TravelHealthPro, the UK national travel health service, has a factsheet on venous thromboembolism, the term covering DVT and pulmonary embolism together.

Who is at higher risk?

TravelHealthPro lists a previous DVT or pulmonary embolism, a family history of clots, a clotting disorder, recent surgery or injury, cancer, pregnancy and being overweight as factors that raise the risk of travel-related clots. It says travellers at increased risk are advised to seek advice from their health care provider.

The NHS DVT page also lists cancer, hormone replacement therapy and the oestrogen-based contraceptive pill among the things that raise the chance of DVT.

What can lower the risk on a long journey?

NHS 111 Wales travel advice describes walking around when it is possible, and doing calf and ankle exercises while seated. It also describes drinking water, avoiding alcohol and wearing loose, comfortable clothes.

On stockings, the sources differ. NHS 111 Wales says passengers who wear compression stockings on flights of four hours or more can significantly reduce their risk of DVT and leg swelling, and that ill-fitting ones could raise the risk. TravelHealthPro advises travellers at increased risk to consider properly fitted below-knee graduated compression socks. An NHS cancer alliance information sheet for GPs is more cautious: it says routine use of stockings or anticoagulants for all travellers is not recommended, and that stockings are for those at the highest risk.

What are the symptoms, and who should be contacted?

The NHS describes a throbbing pain and swelling in one leg, usually in the calf or thigh. The skin over the area can look red, blue or darkened, and the veins near the surface can be swollen.

For suspected DVT, the NHS says to ask for an urgent GP appointment or get help from NHS 111. The signs of a pulmonary embolism include sudden difficulty breathing, chest pain that is worse when breathing in, and coughing up blood. The NHS says to call 999 or go to A&E for sudden difficulty breathing, chest or upper back pain, a very fast heartbeat or passing out. It advises against driving yourself to A&E, and says to ask someone to drive or call 999 for an ambulance.

How is DVT treated?

Treatment usually involves anticoagulants, often called blood thinners. Their main role is to keep the clot from growing while the body gradually breaks it down. Hospital leaflets note that anticoagulants increase the risk of bleeding, so people taking them are told to report unusual bleeding.

Doctors choose the medicine and how long treatment lasts for each person, and the leaflets advise talking to a doctor before stopping it.