Deep Venous Thrombosis
Publicity has highlighted the perceived increased risk of passengers and aircrew developing blood clots in the legs during air travel. This condition is known as Deep Venous Thrombosis (DVT). If blood clots formed in the legs were to dislodge and migrate to another part of the body, usually the lungs, by embolization, the condition is Venous Thrombo-Embolic disease, or VTE. A term first used by Symington and Stack in 1977 linking DVT with air travel is the “Economy Class Syndrome” (ECS).
Although much has been written about DVT risks from air travel in both the scientific literature and the lay press, robust scientific studies that clearly define the risks, incidence and consequences of this possible condition are lacking. As a result of this paucity of evidence, estimates about the magnitude of the phenomenon and its’ consequences vary widely. This article will discuss the variety of opinions regarding DVT and ECS. The article will also highlight some of the research to date and give the interested reader additional references to pursue. Finally, recommendations to minimize the theoretical risks of developing DVT during flight are discussed.
Definition of DVT
Deep Venous Thrombosis is a condition in which one or multiple blood clots (thrombi) form in the deep veins of the legs. The deep veins are located within the muscles of the legs, and are generally not visible on the surface as are superficial varicose veins. Because veins do not have a thick muscular wall, they are not able to help pump blood to different parts of the body as the arteries do. Instead, blood moves through the veins by either gravity or by the contraction of surrounding muscles, which squeezes blood in the veins back to the heart using a system of one-way valves in the veins.
If blood is allowed to stagnate, clots may form in the veins. Stagnation may occur by several mechanisms. First, inactivity of the muscles will not provide the squeezing action on the veins. This is why people in casts or following surgery are at increased risk for DVT. Next, pressure against a vein may block blood flowing through the vein. This pressure may come from compression of the vein by the edge of a firm seat, kinking of the vein due to leg position or other factors.
Once blood stagnates and clots begin to form, they may grow in size, a process called propagation. Clots may then block the entire diameter of the vessel, forcing blood to bypass the clot to return to the heart. Blood tends to become congested below the level of the clot. This may cause leg swelling and pain on contraction of the calf muscles.
DVT is a significant medical condition by itself. Treatment usually requires putting the individual on blood thinners (anti-coagulants) such as Coumadin, Xarelto or low molecular weight heparin (Lovenox). These medications are temporarily disqualifying for pilots, but are waiverable by the FAA. A much more serious potential complication is the result of a clot breaking loose, traveling up the veins to the right side of the heart, and lodging in the lungs. This condition is called a Pulmonary Embolus (PE). A PE may cause sudden shortness of breath, coughing, chest discomfort, low oxygen levels in the blood or sudden death.
Risk factors for DVT are numerous. Some are universally accepted, while others are more controversial. Definite risk factors include pregnancy , blood clotting disorders, previous or current history of cancer, a personal of family history of DVT and recent major surgery. Other risk factors include medications, particularly oral contraceptives and estrogens, immobilization of the legs and dehydration. Many individuals feel that smoking, obesity, age greater than 40 years or tall height (due to longer distance to push blood against gravity to the heart) are risk factors.
All forms of travel are thought to be a risk factor for DVT because of prolonged seating. The risk of DVT associated with prolonged seating was first described in British air raid shelters during World War II. There is no specific evidence that air travel adds to the risk of DVT versus other forms of travel. Symptoms of DVT associated with travel have been reported developing from the time of travel through several weeks after travel. This range makes establishing a definite cause and effect relationship problematic. A research article published in the December 2002 issue of Aviation, Space and Environmental Medicine, Pulmonary Embolism at Autopsy in a Normal Population: Implications for Air Travel Fatalities by D. F. H. Pheby and B. W. Codling, did not find evidence of increased deaths due to pulmonary embolism caused by air travel.
Unique risks associated with all forms of travel include prolonged duration of travel, increased frequency of long distance travel, seated posture when sleeping, elective or enforced immobility and restricted leg room. Theoretical additional risk factors with air travel are the low cabin humidity and increased dehydration if non-alcoholic/non-caffeinated fluids are not consumed, reduced oxygen pressures at cabin altitudes, long duration flights (trans-oceanic), and reduced cabin pressures leading to abdominal bloating and compression of veins near the groin.
The incidence of DVT is poorly defined in the population as a whole. Reported cases of DVT in the United States vary from 260,000 to 800,000 per year. Some researchers feel that one half to two thirds of cases go unreported or unrecognized each year. The British House of Lords Select Committee on Science and Technology Fifth Report on Air Travel and Health states that the incidence of DVT in the general (non-traveling) population is one case per thousand individuals per year. The report also states the increased risk associated with travel may be zero to 0.4 cases per thousand, although 75% of people who develop risk DVT during travel had one or more risk factors for the condition independent of travel.
These numbers would indicate that one thousand cases of DVT would develop annually per million travelers independent of their travel. Some estimates of DVT in the general population are up to 80% higher. Theoretically, zero to 400 additional cases of DVT per million people would develop because of air travel. The databases of the FAA and NTSB do not list thrombotic events involving a passenger, but newspapers and other articles have listed events involving DVT in both passengers and aircrew.
A 12 May 2001 study in Lancet by John Scurr of the University of London Hospital showed that of 200 people scanned before flight without blood clots in their legs, 10% had clots detected after flight. Note that the presence of clots does not equate to symptomatic DVT, and clots may dissolve spontaneously without the individual being aware of their presence.
The Fifth Report also warns against the use of the term “Economy Class Syndrome” when describing DVT possibly associated with air travel. It points out that business class, first class and aircrew are not immune from DVT, nor are other types of travelers. More appropriate terms suggested include “Flight Related DVT” or Traveller’s [sic] Thrombosis.
A comprehensive review of the subject was published in the international journal, Aviation, Space and Environmental Medicine. The article was written by Dr. Michael Bagshaw, Chair and the Air Transport Medicine Committee of the Aerospace Medicine Association. This distinguished committee of 55 aerospace medicine physicians from numerous countries and backgrounds concludes that there is no epidemiological studies published supporting an increased incidence of DVT due to travel in the absence of pre-existing risk factors. (ASEM, Sept 2001, 72:848-51). The Committee supports further study of the subject by the UK Department of Health, the Australasian multi-center case-control study and the World Health Organization.
A related autopsy study of passengers dying from pulmonary emboli following aircraft flight, Pre-existing Pulmonary Thromboembolic Disease in Passengers with the “Economy Class Syndrome” showed that 5 of 14 deaths demonstrated evidence of pre-existing pulmonary emboli.
The September 13, 2001 issue of the New England Journal of Medicine contains an article and an editorial on the risk of DVT with air travel. The article indicates there is increasing risk of DVT with increasing duration of flight time. The editorial has an excellent review of the literature, indicates more research is needed and encourages the airlines to continue their efforts to educate passengers about strategies to prevent DVT.
In 2014 an article in Aviation Space and Environmental Medicine on behalf of the Aerospace Medical Association Air Transport Medicine Committee, Travelers’ Thrombosis, pointed out that “there is no unique factor in the air travel cabin environment that has been shown to have any effect on the coagulation cascade.”
Recommendations to Minimize Risks of DVT
The recommendations to reduce the risk of DVT possibly associated with air travel are theoretical in nature, since no studies have been conducted to evaluate their effectiveness. The recommendations listed are derived from a variety of scientific sources listed below. Many airlines have their own recommendations and information for travelers. Some recommendations are more relevant to passengers than aircrew, while others may be easier for aircrew members to incorporate.
Recommendations for those at known increased risk for DVT include:
- Drink plenty of non-alcoholic, non-caffeinated fluids.
- Recline the seat when practical, particularly when sleeping.
- Use seating with increased pitch and legroom
- Walk around the cabin regularly, hourly if possible.
- Flex the calf muscles frequently by pulling up the toes or pushing firmly on the floor.
- Consider wearing support stockings or medical compression stockings.
Non-flying pilots may run their seat back and extend their legs when seated. Although required to remain at the controls when flying, allowances for crewmembers to get up from the seat are made for “physiologic needs.” (FAR 91.105). The well-hydrated pilot will reduce the risk of blood clots both by keeping the blood more fluid and by regular trips to the lavatory allowing exercise of the calf muscles.
Those individuals at increased risk for DVT may consider additional precautions.
- Consult with personal physician about minimizing risks
- Consider taking low-dose aspirin (less than one tablet daily) with physician concurrence
- Wear medical compression stockings
- Delay travel
- Purchase first class seating, particularly on long flights
Those with a history of DVT may consult with their physician about the role of low molecular weight heparin to reduce clots if they have to travel.
FAA Policy – Waivers for Thrombosis
Pilots with a history of DVT may be waivered by the FAA to fly while taking blood-thinning medication. In general, the individual’s blood studies (INR or Prothrombin Time) must be in the therapeutic range on a fixed dose of medication three consecutive tests spaced over several weeks each. In early 2014, the FAA published criteria stating there must be 6 weeks of stable INRs before consideration for waiver, but this is inconsistently applied. The symptoms of the DVT should be resolved and there should be no restrictions on a pilot’s activity. The FAA will require reports of a pilot’s current status from the treating physician and lab work every six months as long as blood-thinning medication is used. Some newer medications such as Xarelto do not required regular lab testing.
If a pilot were to develop a pulmonary embolus as a complication of DVT, additional studies would be required prior to consideration for medical certification waiver. Lung function must return to normal, both in terms of breathing capacity and blood flow. Documentation by pulmonary function tests for those individuals who had respiratory distress initially is useful when requesting a waiver.
Pilots should consider speaking with their treating physician about the increased risk of occupationally related DVT. At this time, however, the risk can only be considered theoretical, as scientific studies have not conclusively shown a link between flying and symptomatic DVT greater than that of DVT in the general population.
Pilots needing assistance in obtaining waivers for a history of deep venous thrombosis may contact AMAS for confidential consultation.
Pilots should be aware of the potential increased risk of DVT associated with air travel. Several easy to follow guidelines should minimize any increased risk. Additional research is needed to better define the risk and the effectiveness of preventive measures. Suspicious symptoms should be evaluated promptly to avoid complications of DVT.