Deep Vein
Thrombosis

The risk for DVT while flying is increased because of the reduced cabin pressure at high altitudes, which causes fluid to pass from the blood vessels into the surrounding tissue, causing thickening of the blood. Normal movement of the calf muscle when walking helps pump blood from the legs to the heart, but with the loss of fluid while sitting for long periods, the blood can thicken, solidify and form a clot in the deep veins of the leg.

In most cases, the clots do not dislodge from the leg and merely cause pain, and they may also be symptomless.

Other collected data suggest the lower oxygen level experienced during flight can lead to an increased tendency for blood coagulation.

One recent UK study concluded that, “symptomless DVT might occur in up to 10% of long haul airline travellers”, but Dr Jack Hirsch, a clot expert at McMaster University in Hamilton, Ontario, suggested in the Lancet that this might be overstating the risk and that the study methods had their shortcomings.

Over one billion people travel by air each year. Professor Caprini estimates that the death rate during flight is one in 3.25 million passengers, with VTE of the lung accounting for 18% of deaths. “Based on reported cases, it may be concluded that VTE is very rare. However, it may develop during or immediately after a flight or even days later. Increased air travel has made the problem more important.”

Risk factors for developing DVT, and VTE, include:

• blood clotting disorders
• varicose veins
• cancer
• heart failure
• pregnancy
• using oestrogen-containing medications
• recent surgery or trauma
• smoking
• being overweight (body mass index > 27)
• age over 40 years

If you are planning a long trip you could mention this to your doctor at your next visit. Your doctor can then assess any risk factors you may have.

Even though the link between clots and long trips is still being investigated, there are many things long distance travellers can do to minimise any potential risk.

Steps to help prevent DVT on long-haul airline flights and during long car, train or bus journeys include:

• do ankle and knee exercises every half hour while seated
• walk down the aisles regularly, or if travelling by car, frequently stop, get out and take a walk
• wriggle toes frequently
• drink plenty of water (one litre every five hours)
• go easy on alcohol and caffeine
• take aspirin (if advised by your doctor)
• wear knee-high compression stockings (if advised by your doctor).