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 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 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 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 takeoff.
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 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 (Lancet, November 11, 2000, pp. 1657-58).
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 risk.
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):
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.
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 operations.
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.
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. 1989).
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.
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.
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.
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
NOTE: Flight hose (socks) specifically designed for air travel are available at major airports and pharmacies.