Deep vein thrombosis

Also known as "DVT" for short, this condition develops in the leg veins and if left untreated can be fatal.

Deep vein thrombosis is where a blood clot forms in a deep vein in the lower leg, in particular the calf muscle, although it can develop in the veins in your arms.

But this tends to be rare.

The danger is if a section of that blood clot breaks off and blocks a blood vessel in the lung. This is known as a pulmonary embolism and if large enough, may cause the lungs to collapse and heart failure.

Causes of deep vein thrombosis

DVT is caused by a variety of factors which as similar to those which are responsible for varicose veins.

These include:

  • Long period of inactivity
  • Smoking
  • Overweight or obese
  • Age
  • Oral contraceptives/HRT
  • Pregnancy
  • Medical condition, e.g. rheumatoid arthritis
  • Hospital stay
  • Damaged blood vessels e.g. vasculitis

Long period of inactivity

A good example of this is travel, for instance, long haul flying.

In the media there has been a spate of deaths as a result of DVT which appeared to have occurred after a long haul flight. Whilst there may be other factors involved there does seem to be some evidence which shows that inactivity is a factor.

Sitting still in the same position for a long period of time such as a flight, car journey or on a coach trip can lead to DVT.

What happens is that when you sit in the one position for a period of time, the blood flow in your body slows down and collects in the lower half of your body. Normally, any movement ensures that blood is circulated around your body due to the activity of your leg muscles.

But if you are sat down and with your legs crossed then this blood pools in your lower legs. This can increase the risk of a blood clot.

This is why experts advise people to get up and walk around or move their feet whilst on a long haul flight to prevent this from happening.


Smoking causes a range of health problems, one of these being the formation of blood clots. If you smoke you are at increased risk of a condition called atherosclerosis – a hardening of the arteries caused by the build up of fatty deposits in the arteries.

If this occurs then blood clots will form in the artery causing a blockage.

Smokers are at greater risk of blood clots than non-smokers.

Overweight or obese

If you are overweight or obese then you are at increased risk of a blood clot. One of the reasons for this is that you may be less active than someone of a normal weight and this inactivity means poor circulation and the formation of a blood clot.

Another possibility is that obese people tend to produce higher levels of the hormone leptin. This hormone is produced by fat cells within the body but people with high levels of fat produce greater amounts of this hormone which increases the risk of blood clots.

Obesity also slows down the blood flow in the body which also increases this risk.


The risk of blood clots increases as we age. This is partly due to the fact that our arteries and veins weaken as part of the ageing process which increases the risk of conditions such as a blood clot.

There is also the fact that many older people are less active than when they were younger which causes the circulation to slow down, increasing the risk of thrombosis.

Oral contraceptives/HRT

Both of these contain a synthetic form of the female hormone oestrogen. This hormone performs a range of functions but causes the walls of the veins to relax which increases the risk of varicose veins.

If you take any of these then be aware that it can cause your blood to clot more easily which is a risk factor.


The body has a way of preventing excess blood loss during labour which is a tendency towards clotting. While this may be useful during childbirth it does increase the risk of a blood clot and conditions such as deep vein thrombosis.

Medical conditions

There are certain conditions which increase the risk of blood clots. These include hepatitis (liver disease), rheumatoid arthritis and heart disease.

Cancer treatment such as chemotherapy also increases this risk.

Plus some people have a genetic condition called thrombophilia which means that they are born with a tendency towards clotting.

Hospital stay

If you have stay in hospital for any period of time then this increases the risk of a blood clot. The main reason for this is that you are unlikely to be moving around and it is this inactivity which causes the circulation to slow down, hence the risk of a blood clot.

This is particularly the case if you have to undergo surgery which lasts for more than an hour and a half or are confined to a bed for several days.

Hospitals are aware of this risk and will give you compression stockings to wear and an injection of heparin - a drug which thins your blood. Both of these will reduce the risk of a blood clot.

You will also be encouraged to get up and move around as soon as possible.

Damaged blood vessels

This can occur after an accident or injury, e.g. broken leg or as a result of damage to your veins during surgery.

There are conditions such as vasculitis (inflammation of the wall of the vein) and varicose veins which also increase the risk of a blood clot.

Symptoms of deep vein thrombosis

These include:

  • Pain and swelling in the lower part of your leg, e.g. calf
  • Soreness
  • A dull ache
  • Warm skin
  • Redness of the skin

This usually occurs in the one leg although it can develop in both legs.

If the blood clot has become dislodged and has travelled to your lungs then you will experience chest pain, difficulty breathing and possible collapse.

This requires urgent medical treatment.

Diagnosing deep vein thrombosis

Your GP will examine you and ask you about your medical history before referring you for tests. These include an ultrasound scan, the D-dimer test and a venogram.

An ultrasound scan is often used in many treatments for varicose veins and uses a series of images transmitted via a probe which enables the doctor to check the condition of the veins.

In this case it enables him/her to see how quickly blood is flowing through the veins. If this is moving slowly or not at all then a blood clot is suspected.

The D-dimer test is a special form of blood test which checks to see how many pieces of a blood clot are in your veins. The more pieces there are the greater the likelihood of a blood clot.

A venogram involves the use of a special dye which is injected into your leg. An X-ray is carried out to chart the progress of the dye. If a gap shows up then this is an indicator of a blood clot.

Treatment for deep vein thrombosis

There are various forms of treatment available which include anticoagulants, compression stockings and elevation.


These are a group of medicines which prevent blood clots from forming. Many people assume that they thin the blood but what they actually do is to alter the conditions which are likely to cause a blood clot.   

Two examples of these medicines are heparin and warfarin.

Compression tights/stockings

Compression stockings are often recommended for people with varicose veins. They work by squeezing the lower half of the leg which forces the muscles to pump blood up towards the heart.

Your GP will prescribe these stockings for you. There are different types of stockings which are graded according to their level of compression.


Another treatment is elevation. This is also advised for varicose veins sufferers and is easy to do. It involves you raising your legs for a set amount of time each day.

This can be done either sat down or lying down. For example rest your feet on something which ensures that your feet are lying above the level of your hip. Alternately, raise the foot of your bed or place a pillow under your legs.

Both compression stockings and elevation are discussed further in our preventing varicose veins section.

Complications of deep vein thrombosis

DVT is unlikely to cause any further problems if treated sooner rather than later. But there is a risk of complications which include:

  • Pulmonary embolism
  • Limb ischaemia
  • Post thrombotic syndrome

Pulmonary embolism is the most serious complication which in some cases can be fatal. If a section of a blood clot becomes detached and then travels through the blood stream to the lungs it can cause a blockage. This blockage blocks off the blood supply to the lungs resulting in collapse.

Limb ischaemia is where blood is unable to flow through a part of your body such as your leg leading to ulcers, infection and possibly gangrene.

This is very rare.

Post thrombotic syndrome occurs if the valves within your veins become damaged which causes blood flow to collect in lower part of your body.

This can cause varicose veins or a condition called chronic venous insufficiency (CVI) where the skin becomes damaged as a result of this leading to skin ulcers and infection.

Guide to Varicose Veins

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