The 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 travelers 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 death.
- 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 minimize 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 travelers on any particular flight fall into a high-risk category. High- risk groups include older people (probably those over 60 years), travelers 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 travelers 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 traveler 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 travelers 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 minimize 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 is 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 takeoff. The exercise advice appears to be excellent and it should of course include a warning not to go to sleep! It is 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.
- Alcohol consumption
Travelers are also advised to avoid alcohol because of its dehydrating effects.
- 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.
- 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.
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 make-up.
A Norwegian study published by Bendz et al in The Lancet put 20 young men in a hypobaric chamber, which simulated usual aircraft cabin altitude.
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.
(Source: yourhealthbase