Dr Sarah Jarvis, MBE
Author: Dr Sarah Jarvis, MBE, General Practitioner (GP)

Sarah is the Clinical Director of the Patient Platform, an active medical writer, broadcaster, and is the resident doctor for BBC Radio 2.

What is a deep vein thrombosis?

A deep vein thrombosis or DVT is a blood clot that forms in a vein – most commonly in one of the veins in your lower leg. The deep veins in your legs aren’t the ones you can see just under the surface of the skin, and they’re different from varicose veins. The deep veins of your leg go from your feet through the muscles of your calves and thighs, carrying blood back to your heart.

DVT – the consequences

A DVT can cause severe pain, swelling, redness, and increased skin temperature over one limb. A complication called post-thrombotic syndrome can result in longer-term pain, swelling, skin discolouration, and sometimes skin ulcers. This is more likely if you don’t get prompt treatment, if the DVT forms in a vein in your thigh, and among older people. Although this can be debilitating, it’s never fatal.

If, on the other hand, the clot breaks off and travels to the lung, it can lodge there and cause a pulmonary embolus, which is a potentially life-threatening medical emergency. Symptoms include shortness of breath, stabbing chest pain which is worse when you breathe in, coughing up blood, palpitations, feeling faint, collapse, and occasionally cardiac arrest.

If you can’t move it

One of the reasons clots form more easily in your veins than in your arteries (which carry blood from the heart around to the body, supplying your organs with oxygen) is that the pressure in your veins is lower. Veins have thinner walls than arteries and can become squashed more easily. This can slow down the flow of blood through these veins, leading to clots forming.

Another factor that keeps blood flowing freely through the veins of your lower legs is movement. The contractions of muscles in your lower legs act as pumps, squeezing the veins to push the blood through. Because the veins in your legs are such a long way from your heart, they’re having to work against gravity if you’re standing or sitting with your legs down.

That means that if you’re not moving around, your calf muscle pumps aren’t helping with blood flow. In addition to the risk of DVT, fluid can leak into the surrounding tissues if it’s not leaving the legs efficiently – hence the tendency to develop swollen ankles if you’ve been sitting still for a long time.

The veins are much more likely to become squashed if they have pressure on them – and that’s usually because you’re not moving around. This combination of factors means that immobility is the number one cause of DVT. This includes:

  • Surgery under general anaesthetic, especially if the operation involves your pelvis or legs.
  • Being severely unwell and confined to bed for long periods (such as needing hospital or particularly intensive care unit admission).
  • Having a leg immobilised in a plaster cast.

Sticky business

If the lining of your vein is damaged, it’s easier for clots to form and stick to the walls. This could be the result of chemotherapy for cancer or a previous DVT. Some hormone treatments, including the combined oral contraceptive pill (often called ‘the pill’) and tablet forms of HRT can increase your risk of blood clots, although the risk is small in real terms. So too can inherited blood clotting disorders, called thrombophilia: if one or more close relatives has had a DVT, you’re more likely to get one as well.

Finally, having cancer appears to increase the tendency for blood to clot, so people with some cancers are more prone to DVT. So too does some cancer therapy.

Treating DVT

The mainstay of treatment for DVT is medication called anticoagulants, which make the blood less prone to clotting. In the past, the standard treatment was warfarin, which is highly effective but requires regular monitoring.

These days, you are more likely to be treated with one of the newer anticoagulant drugs, which are just as effective but which don’t need the same number of blood tests to monitor your dose. Before you start on treatment with one of these tablets, you may be given daily injections with heparin to reduce blood clotting.

The length of time you’ll need treatment depends on where the clot forms, whether you’ve had a previous clot, and whether there was an obvious reason for your DVT (such as being immobile for a long time). Your doctor will usually recommend you take anticoagulant treatment for at least 3 months.

While not strictly a treatment, stopping smoking can significantly reduce the risk of you having another DVT – there’s free support and treatment available on the NHS to help you quit.

When can I fly after a DVT?

After you’ve had a DVT, you should walk regularly to keep the blood flowing in the veins of your leg and keep your leg up when you’re sitting. Avoid sitting with your legs crossed, which can squash the veins and reduce blood flow.

It’s also very important to avoid dehydration – this can make the blood thicker and more likely to clot again. So keep up your intake of non-alcoholic fluids, and avoid excess alcohol, which can dehydrate you.        

Once you’re taking anticoagulant medicine, you’ll be advised to wait for at least 2 weeks before you take any air flights or car or train journeys longer than an hour or two.

Ideally, you might want to delay any flights for at least 4 weeks, just in case you have a complication while you’re in the air and can’t access emergency treatment. Always check with your doctor before you travel to make sure they’re happy that it’s safe.

How can I avoid another DVT if I’m travelling?

Any long journey where you’re immobile can increase the risk of DVT. The risk is lower, for instance, if you’re travelling by train and can get up and move around more easily. If you’re travelling by car, you should stop the car (safely) and get out and walk around every hour or two.

The risk is higher for air travel than ground travel – partly because you’re sitting for long periods, partly because aircraft seats tend to be more cramped than train or car seats, and partly due to changes in air pressure and oxygen levels in aircraft cabins. The longer the flight, the greater the risk – although in absolute terms, the risk is still small. Some studies suggest for every 4,656 long-haul flights, one person has a DVT.

If you’ve had a DVT already, you are more likely to suffer another, but there are several steps you can take to cut your risk. If you’ve been prescribed anticoagulant medicines, it’s essential you continue taking them until your doctor advises you to stop – these offer very good protection against another DVT.

In addition, you may want to consider wearing compression stockings, also known as thromboembolic deterrent (TED) stockings. They work by applying pressure from outside, helping to squeeze blood in your veins back up your legs. You should use graduated compression stockings, where the level of pressure changes at different points in your leg. At the ankle, they should be providing 15-30 mmHg of pressure.

Before you have these fitted, you should have the pressure inside the arteries in your legs checked using a simple probe on the skin – your hospital clinic can advise. Your pharmacist can give you full advice and sell you the correct stockings.

In addition, you can reduce your risk by:

  • Getting up and walking around the cabin regularly (choose an aisle seat to make this easier).
  • Placing your hand luggage in the rack above you, rather than under the seat in front of you, where it could restrict your leg movement.
  • Wearing loose, comfortable clothing.
  • Drinking plenty of non-alcoholic fluid (at least 1 glass – 250ml – every 2 hours) and avoiding alcohol, which can dehydrate you.
  • Doing regular calf muscle exercises – in-flight magazines usually have instructions for these.
  • Avoid sleeping tablets, which can lead to you being very immobile and putting more pressure on the veins in your legs.
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