Deep Vein Thrombosis
in Air Travellers
by Maurice McKeown, BDS, PhD
(our New Zealand correspondent)
problem of deep vein thrombosis (DVT) after airline travel rises to public
consciousness from time to time as the popular press report stories about
healthy young travellers succumbing to the condition, or provide details
of high profile cases going to court.
What is deep vein thrombosis?
In deep vein thrombosis a blood clot forms in one of the veins deep
inside the leg. It can occur at any age but is much more common in older
people. It can be caused by a wide variety of factors other than air
travel. The clot may break away from its position and travel through the
heart to the lung where it may cause severe symptoms which can result in
What are the risks of developing DVT after air travel?
Are we all becoming more and more unjustifiably fearful in a stressful
world? My own perception is that the average airline passenger is much
more concerned that the plane will crash than the possibility that they
could develop a life-threatening medical condition. Yet the chances of the
former are miniscule compared to the latter. The airlines have done little
to help. Many have now put general flight-health advice on their websites
and placed information pamphlets on in-flight health in the pocket in
front of our seats, the ones with the exercise advice that almost never
mention the words 'deep vein thrombosis'. One suspects that their motive
is to forestall litigation rather than help their customers avoid the
condition. The airlines are clearly in denial but a growing body of
research is pointing to a major health problem.
There are a number of important questions to ask ourselves. How common
is the problem? How susceptible are we to developing the condition and
what can be done to minimise the risk?
We now have a rough estimate of the frequency of DVT and some
indication of which groups of individuals are in a higher risk category.
Unfortunately, the categories of individuals who are at greater risk are
quite long. One is left with the impression that the majority of
travellers on any particular flight fall into a high-risk category. High-
risk groups include older people (probably those over 60 years),
travellers with a variety of known medical conditions, pregnant women,
those who have recently had surgery and women taking oral contraceptives
or hormone replacement therapy.
The risk of developing DVT, which can of course occur quite
independently from air travel, is very strongly linked to age. It is very
uncommon in young people and very common in the elderly. If we consider
air travel in isolation, studies have shown that 3-5% of travellers
develop clots in veins. Some are, of course, asymptomatic or occur with
mild symptoms. Thus a precise statistical analysis of the incidence of the
condition is very difficult. In other words the traveller is unaware that
they have suffered a clot. In 2001 The Lancet published an analysis
estimating that 1 million cases of DVT related to air travel occur in the
US every year and that 100,000 of these cases result in death (Lancet,
September 8, 2001, p. 838).
However imprecise these figures may prove to be; they do suggest that
the risks posed by air travel may be dramatically greater than commonly
perceived. One analysis has concluded that frequent business travellers
have a 5% risk of contracting DVT in any one year; data which ought to
bring the matter to the attention of their companies' medical department,
not to mention their health insurers.
What can be done to minimise risk?
The popular press have dubbed the condition – "economy class syndrome",
in the belief that the cramped seating arrangements, particularly with
respect to legroom, are the prime cause. But is this really true? Recent
analysis of the frequency of the condition suggests that it is equally
prevalent in all classes of passengers and even aircrew. (The possible
exception being cabin attendants who move around all the time - if on
duty.) It seems that cramped conditions may not be the only precipitating
cause. It appears more likely that lack of movement is much more
important. On a recent round trip from New Zealand to Europe I formed the
distinct impression that diligent exercisers were in a very distinct
minority on board my flight.
People at risk
It is best to consider risk in the context of those who are apparently
healthy and in those who have a known medical problem. If the healthy
individual seeks advice from their doctor they may be told that they have
little to fear, particularly if they are young and healthy. They may be
advised that if they are really concerned they should take plenty of
in-flight exercise, drink lots of water and even take an aspirin before
The exercise advice appears to be excellent and it should of course
include a warning not to go to sleep! (Difficult on flights of 8 -14 hours
or more.) Perhaps the doctor might prescribe an amphetamine!! Advise on
regularly drinking of plain water may be misplaced.
Is it of value to drink large amounts of water?
There appears to be no evidence that it is. In fact a study in Japan by
Hamada et al, published in the Journal of the American Medical
Association found that subjects who drank one cup of water per hour
during a nine- hour flight experienced increased blood viscosity.
Interestingly the study found that those who drank an electrolyte fluid
(similar to a good quality sports drink) in the same manner, had no
increase in blood viscosity and no increase in urinary output. Hamada used
an electrolyte drink containing 110 mg (per 8 oz cup) of sodium and 30 mg
of potassium (JAMA, February 20, 2002, pp. 844-45).
Advice is also commonly given to avoid caffeinated beverages because of
their diuretic effect. Despite the fact that research has shown that
coffee and other caffeinated beverages do not increase dehydration.
(Armstrong: International Journal of Sport Nutrition and Exercise
Metabolism June 2002)
Travellers are also advised to avoid alcohol because of its dehydrating
effects. Yet red wine consumption is not particularly diuretic and is
known to be beneficial to the health of blood vessels and has the effect,
in moderation at least, of reducing the stickiness of blood platelets. A
Polish research group last year found that the resveratrol present in the
human diet (red wine carries significant amounts) may be an important
compound responsible for the reduction of platelet adhesion and changed
reactivity of blood platelets in the inflammatory process. (Olas et. al.
Thrombosis Research 15 August 2002)
It remains to be determined what effect a moderate amount of red wine
might have for the flying public. It is an impending research study which
ought to have no lack of volunteers.
Does aspirin prevent blood clots?
I have been unable to unearth any definitive research that says it
does. Medical opinion is divided. There is no doubt that it confers
certain benefits on the arterial side of the cardiovascular system since
it reduces the incidence of first heart attacks if taken daily for long
periods. It should be pointed out that the arterial side of the
cardiovascular system can also be affected by thrombosis, although to a
much lesser extent than the venous part of the system. Arterial clotting
has been closely linked to platelet adhesion. Aspirin and other natural
substances can help reduce this. Some medical researchers feel that
aspirin's beneficial influence in the venous system is minimal. A study of
300 high-risk passengers recorded a 4.8% incidence in the control group
and a 3.6% incidence in those taking aspirin. Clearly aspirin had some
value in this instance. (Belcaro et al: Angiology Vol 230, 2002).
Are only long haul passengers at risk?
A study by the UK-based Aviation Health Institute found that 17% of
flight-related DVT cases occurred in association with short flights. It
has also been demonstrated that the duration of travel is not linked to
the severity of the thrombosis suffered. (Parsi et. al. Australian and
New Zealand Journal of Phlebology June 2001).
The British Independent newspaper has just published interim findings
on the incidence of DVT in high-risk passengers as a result of a
short-haul flight (London-Rome), which is of less than 3 hours duration.
The authors released the preliminary results prior to journal publication
because of their potential significance. The authors found that 4.3% of
568 passengers developed clots, which were detected by ultra sound. Two of
the victims went on to suffer a pulmonary embolism. The lead researcher
Professor Gianni Belcaro, of G d'Annunzio University in Italy said that
their research suggested that most blood clots develop in the first two to
three hours of a journey and grow larger and more dangerous with time.
Unfortunately, we shall have to wait for the completion of the project and
publication of the final report in order to find out full details, such as
who was deemed to be at high risk.
What groups of healthy people are at risk?
It is now clear that aircraft cabin altitude, determined by the
pressure within, appears to be the key-precipitating factor for an
increased risk of blood clotting. Altitude, not immobility, is the primary
problem. The information below suggests that we are all at risk. The
degree of risk is determined primarily by our own physiological/genetic
A Norwegian study published by Bendz et al in The Lancet put 20
young men in a hypobaric chamber, which simulated usual aircraft cabin
Cabin pressures simulated an altitude of 5000-8000 feet in various
aircraft types; reducing oxygen pressure from 98 to 79 mmHg as calculated
for a Boeing 747. It has also been calculated that this can lead to 90%
saturation of haemoglobin with oxygen; a figure that may be reduced even
further by sleep and the effects of cramped conditions on respiratory
mobility. Other environmental factors, notably humidity levels are also
being investigated. Cabin humidity falls rapidly after take off. Its
potential effect on factors like dehydration is currently controversial.
In individuals with other respiratory problems, reduced oxygen saturation
can lead to a chain of events in their blood which favours clotting.
It was found in the Norwegian study that a substantial hour-by-hour
increase in blood clotting factors occurred in all of the healthy
subjects. There was a 2-8 fold increase in clotting factors. The
implication is that all flyers are subject to this increased risk,
suggesting that those who succumbed to DVT have a variety of risk factors
deriving from their own genetic and physiological make-up, and their
environmental circumstances (Lancet, November 11, 2000, pp.
Enhanced likelihood of coagulation has also been demonstrated by
Wolfgang Schobersberger et. al. in a study measuring coagulation factors
on an actual long-haul flight. The effects were observed in all test
subjects. He concluded, "Long-haul flights induce a certain activation of
the coagulation system. This activated coagulation could be a risk factor
for VTE during long-haul flights mainly when other risk factors are
present." (Thrombosis Research October 2002).
It has been established that people carrying the Factor V Leiden
variant are much more likely to suffer DVT than those without the
variation. Caucasians populations can have a 5% incidence of the gene
variant. The mutation does not appear to be present in Black or Asian
populations, although it is present to a limited extent in Afro-
Americans. Factor V Leiden increases the risk of venous thrombosis 3-8
fold for heterozygous (one bad gene inherited) and by 30-140 fold, for
homozygous individuals (two bad genes inherited). Risk is dramatically
increased beyond that if the individual is also suffering high blood
homocysteine levels. The Wellman clinic in London (UK) has developed a
series of tests to identify the Factor V Leiden variant and a number of
other genetic variants related to increased clotting risk. It is estimated
that the Factor V Leiden variation is responsible for 40% of all cases of
thrombosis. Interestingly, it increases the risk of DVT for men by 8-fold
and in women by 80-fold. Women therefore appear to be at much greater
Women taking oral contraceptives are also much more vulnerable to DVT,
although the risk for those on estrogen replacement therapy is higher
because they are older and face a greater base-line risk. Women who have
recently given birth are also at increased risk. There are those who
believe that pregnant women are at such increased risk that they should
not fly at all, since preventative anticoagulant therapy can have serious
consequences for the foetus. If they do fly it is advisable that they
scrupulously follow the preventative advise at the end of this article.
Conventional wisdom says that the young, fit and healthy have little to
be concerned about. Unfortunately, this may not be the case. In fact they
may be at much greater risk than the young and unfit! This applies
particularly to athletes, especially endurance athletes, who show a high
incidence of the condition according to some researchers. This may be
partly due to their very efficient cardiovascular systems pumping blood
around more slowly.
It has recently been reported that at least two international soccer
teams wear compression stockings on long haul flights, as a number of
studies have shown that wearers of compression stockings are dramatically
less susceptible to DVT than those not wearing the hose. One study has
shown a complete elimination of the risk in stocking wearers (Lancet,
May 12, 2001, pp. 1485-88). It has been claimed that 85% of flight DVT
victims fall into the athletic category.
Airhealth.org, an organization dedicated to the dissemination of
information concerning DVT estimates that 100,000 deaths due to air travel
related DVT occur in the US every year (Lancet, September 8, 2001, pp.
838). This would make the condition the 5th leading cause of death
overall after heart disease, cancer, stroke, and respiratory disease.
According to the US National Center for Health Statistics at the Center
for Disease Control and Prevention the following four are the most common
causes of death in the 20 to 44 year age group (National Vital
Statistics Report, Vol. 49, No. 11, Deaths: Leading Causes for 1999):
- Accidents - 34,540
- Cancer - 21,404
- Heart disease - 16,767
- Suicide - 13,727
Air travel related DVT, however, may well be the most important cause
of death among people in the 20 to 44 year age group. Airhealth.org
reports that 47% of the victims in their registry were in this age group
corresponding to 47,000 deaths out of the total 100,000 deaths (www.Airhealth.org).
A substantial number of people, commonly in older age groups, have a
variety of medical problems which require special attention e.g. those who
have recently had surgery*, those already taking anticoagulant medication,
people with cancer, heart disease or diabetes or a family history of DVT.
They all need medical advice about preventative measures, which might
include anti-coagulant therapy.
* Some authorities recommend that patients who have undergone surgery –
particularly orthopaedic surgery, should not fly for 90 days after their
What are the symptoms of DVT?
It should be pointed out that symptoms may arise many days after the
end of a flight. One symposium concluded that a two-week post-flight risk
period is likely. You may not therefore immediately associate any ill
effects with your trip. Most patients report symptoms within one week of
the end of their trip however.
- Sudden swelling in one lower leg. The left is much more commonly
affected. (A little swelling in both legs is very common in flight.)
- Cramp or tenderness in one lower leg. (Healthy athletes may mistake
this for cramp)
- A bruise or swelling behind the knee.
- Shortness of breath
- Rapid breathing
- Cramp in your side, painful breathing
- Chest pain, sometimes accompanied by shoulder pain
- Coughing up blood
- Fainting (often the first sign, especially in older people)
How can a diagnosis be made?
Leg clots can be readily diagnosed with ultrasound. A clot, which has
moved to the lung, is more difficult to diagnose. A measure of your blood
oxygen concentration usually taken with a simple attachment to your
finger, can help. If it is low for no obvious reason further testing is
required. It is sobering to note that one US research study has shown that
50% of DVT cases have no symptoms and 50% of those will progress to
pulmonary embolism. In the study forty seven percent of fatal pulmonary
embolisms were not diagnosed before death. (Zamula: FDA Consumer Nov.
If you have long legs you may want to choose an airline with the
largest seat pitch possible. (The seat pitch is the horizontal distance
between similar points on two seats situated one behind the other.)
Comparative seat pitch information for major airlines is available on the
Internet on various websites. For a general comparison of airlines see www.aviation-health.org For more
detailed information related to different aircraft types try www.simplyquick.com
It is probable, but not proven, that DVT is much more likely to occur
in people with specific risk factors, notably inherited genetic clotting
abnormalities. It has been estimated that up to one third of the
population have some degree of thrombophelia – an enhanced tendency to
form blood clots. Since most of us are unaware of these latent tendencies
it would seem prudent to take all reasonable precautions. Little is
usually said about nutritional supplements but there is good theoretical
reason to believe that they may help.
Likely helpful substances are:
Vitamin E (400-800 IU) daily for some days on either side of the
trip. Consult your doctor if already taking anti-coagulant medication.
Pycnogenol or Grape Seed Extract Both contain the active
bioflavonoid called proanthocyanidins (OPC's), which helps strengthen
capillaries, and enhances blood flow.
Vitamin B6 (in the form of pyridoxine hydrochloride), which has
been shown to reduce platelet stickiness. (www.yourhealthbase.com/database/a124m.htm
Pinokinase This is a new commercially available pill, which has
recently become available. It is an oral pro-fibrinolytic anticoagulant.
It is made from fermented extracts of pine bark and soybeans. A study just
published in Angiology journal shows that it was 100% effective in
preventing clots. Info at www.flighttabs.com
A small amount of aspirin (junior size) may help, if tolerated.
People eating diets very high in fruit and vegetables may have as much
salicylates in their blood as that provided by a pill according to a study
of Buddhist monks in Scotland. (Blacklock et.al. J of Clinical
Pathology 2001 Vol 54)
Resveratrol Available in the form of supplement capsules for
those who do not drink red wine.
Other nutrients of possible value are ginkgo biloba, niacin, vitamin C,
and vitamin B12.
Other important measures
- Wear properly fitting flight hose designed to be tight around the
ankle, pressure gradually reducing further up the calf. This is the
single most effective preventive measure.
- Flight stockings differ from ordinary hose. There have special
elastic threads that are utilized to apply a precise amount of pressure
to specific areas of the leg. They are available in a number of
different strengths that are prescribed for a variety of medical
conditions. Compression stockings exert maximum pressure at the ankles
and gradually reduce that pressure up the length of the stocking. The
pressure exerted at the ankle reduces by approximately 50% at the top of
the stocking. This compression is scientifically designed to force blood
from the secondary venous system near the skin into the primary deep
vein system. The value of the stocking has been demonstrated by a study
of flyers who wore flight hose on only one leg. This resulted in
negligible swelling in the stocking clad leg, while the leg without the
stocking experienced significant increases in volume. Thus flight hose
can also relieve simple swelling of the lower extremity.
- If you have varicose veins it may be necessary to wear stockings
above the knee. Please seek professional advice.
- Do leg flexing exercises while seated as studies have demonstrated
that a variety of foot flexion movements result in substantial increases
in blood flow in the leg.
- One authority recommends flexing of foot and calf muscles, while
seated, for two minutes every half hour. If you are an avid armchair
exerciser you might like to visit the Qantas airlines website. In the
in-flight health section you can look at pictures of recommended
exercises which ought not to injure the passenger seated next to you!
- Avoid sleeping if at all possible. If your jet lag is worse as a
result try melatonin. (A prescription drug in many countries) It works
miracles for me!
- Keep hydrated but do not drink pure water. Drink an
electrolyte-balanced solution, available in a good quality sports drink.
You may have to take some powder on board and add it to the water
provided, if your baggage limit is in jeopardy.
- Do not drink large amounts of dehydrating beverages.
- Wear loose comfortable clothes.
- Take suggested supplements before, during and after your trip.
- If you are in a high risk category talk to your doctor and above all
familiarise yourself with the symptoms of DVT, the diagnostic procedures
necessary and possibly learn a little about what treatments might be
- If the worst happens and you suspect that you have a clot seek
immediate medical attention.
NOTE: Flight hose (socks) specifically designed for air travel are
available at major airports and pharmacies or can be purchased online at