These phrases are increasingly recognized by the world’s traveler but can be misleading and are not new ideas. In fact, an association of blood clots forming in leg veins and air travel was first reported in 1954 by often quoted Boston surgeon John Homans, MD (1).

It is now nearly 60 years since Dr. Homans published the report of his patient, also a physician, who developed a deep venous thrombosis (blood clot in a leg vein) while on a transcontinental flight. Since that initial account, a better understanding has developed between the relationship of travel and the development of leg vein blood clots.

Blood clots occur in legs due to a variety of reasons and situations. However, three basic factors identified about 150 years ago seem to be essential ingredients for clots to develop in veins:

1. Alterations (stasis) in normal blood flow.
   Regardless of the cause, any slowing of blood flow in the veins as they work to return blood to the heart is called “stasis”. There are numerous causes for this in humans. Standing in one place or sitting for long periods are the simplest examples.
   
   
2. Injury to the inner lining (endothelium) of the vein wall.
   This injury does not have to be the result of a typical trauma. It can occur as a result of an increase in blood pressure within the vein itself, a chemical, infectious, or drug injury to the inner lining of the vein or an injury due to unusual turbulence of blood flowing through enlarged or varicose veins.
   
   
3. Blood becomes sticky or “hypercoagulable”.
   Conditions that create this change in blood are: dehydration, recent major surgery, typical trauma, tobacco use, birth control or other hormonal replacement medications, most malignancies and obesity. Even pregnancy and delivery has been discovered to possess a normal and welcomed, albeit temporary, hypercoagulable state.
   
   This triad of slower blood flow, injury to the lining of a vein and increased stickiness of blood are still believed to be the local conditional changes that need to exist for a blood clot to develop in a vein.
   
   There is a medical explanation that must be understood at this point before we go any further discussing a possible link between travel and venous thromboembolism.
   
   Venous thromboembolism (clot in a vein) consists of two related conditions:
   
   1. Deep Vein Thrombosis (DVT) is a clot forming in a major vein of the leg or rarely the arm.
   
   2. Pulmonary Embolism (PE) is a venous blood clot that travels from its vein origin to the lungs.
   
   DVT results in a partial or complete blockage of a deep vein by a blood clot, most commonly in the legs. This clot may mature and fuse into the wall of a vein becoming immobile or it can fragment and travel through the remaining venous system to the lungs. Both scenarios are considered serious and emergent medical issues. Pulmonary embolism can have a high and occasionally sudden fatality rate.
   
   Symptoms of deep venous thrombosis (DVT) include swelling of the effected leg, subtle redness or warmth of the skin, pain or tenderness of the calf, thigh, and occasionally the groin. If a clot has migrated to the lung (PE), then chest, mid-back, low back or flank pain is usual, often made worse by taking a deep breath. Breathlessness at rest, the sensation of not being able to take a deep breath or not being able to catch one’s breath often ensues. Even more severe signs that can evolve quickly are dizziness, fainting, or collapse. Anxiety as well as low grade fever and malaise can be early or late signs. Rarely do just one of these symptoms present. A skilled clinician can usually make the diagnosis and quickly begin treatment. I have seen all of these symptoms with PE, individually and in combination.
   
   Unfortunately, DVT and PE can occur in the absence of overt signs of a leg blood clot, somewhat like other medical problems which have been known to occur without classic signs and symptoms. An experienced and conscientious physician can occasionally be fooled into missing the diagnosis of DVT or PE. For that reason, understanding prevention and risk reduction have become serious educational subjects for doctors in Western countries.
   
   General risk factors for DVT presumably increase the risk of a traveler acquiring DVT and PE. These accepted medical risk factors include but are not limited to:
   
   Recent major surgery—abdominal, pelvic, orthopedic, neurological, and cardiothoracic
   
   Paralytic spinal cord injury
   
   Multiple trauma injury
   
   Malignancy
   
   Congestive heart failure
   
   Respiratory failure—end stage lung disease
   
   Hormonal replacement therapy—including oral contraceptives
   
   Previous episode of DVT or PE
   
   Inherited hypercoaguable condition
   
   Acquired hypercoaguable condition
   
   Pregnancy and recent delivery
   
   Obesity
   
   The rapid acceleration of the number of people in our world who travel by train, automobile, bus and aircraft has heightened the awareness of physicians to a possible link between travel and blood clots in the legs. Physician researchers have begun to understand that a traveler’s degree of immobility during the transportation process may contribute to one if not all three of the basic vein/blood conditional changes discussed previously (2,3).
   
   Commercial air travel has become a popular focus for DVT medical research. The goal of these investigations is to discover if a link between deep venous thrombosis and flying exists. Traveling in a commercial aircraft is probably the most confining of all commercial travel modalities and results in the severest restrictions on physical activity when compared to train, bus and even automobile travel. In addition, physical immobility when flying commercially has been further exacerbated by the necessity of airlines to accommodate more passengers per flight so as to improve economic efficiencies, thereby reducing space per seat.
   
   Airlines cannow also accommodate passengers with significant chronic medical illnesses, and more travelers are able to afford flights of greater than 4 hours duration creating additional circumstances that could predispose to DVT and PE.
   
   Studies of flying passengers as well as travelers using other modes of ground transportation have focused on examining extended periods of space confinement. These reports have begun to sift out conditions that seem to increase the risk of DVT and eliminate factors that do not contribute to the risk.
   
   A large Dutch study (4) of adults being treated for their first episode of DVT discovered that confined space travel doubled the risk of acquiring a DVT compared to those without a history of travel. This risk of acquiring a deep vein clot was preserved for at least a week after travel and was slightly elevated up to two months after a trip. Immobility or confinement of at least four hours was required before this doubling of DVT risk was appreciated.
   
   Travel by aircraft in this Dutch study (4) increased the risk of DVT to the same extent as travel by bus, train or car. This suggests that prolonged immobility and not necessarily the method of travel is responsible for creating the needed changes in the body’s systems that increase venous blood clot risk. In fact, a recent British study (5) probably has ruled out aircraft cabin environmental factors alone such as low humidity and cabin pressure as direct contributors to DVT risk.
   
   The British study (5) also reported that individual passenger medical factors indeed added to the risk conferred by prolonged immobility. For example, women using oral contraceptives, passengers with body mass index greater than 30kg/m2, or height equal or above 6’ 3”, increased their risk for developing a DVT beyond the risk associated with confinement alone. A combination of these additional risk factors created a “synergistic” risk. In other words, when the risks were added together, the total risk was higher than predicted.
   
   A third study (6) analyzing 8755 employees of several international organizations confirmed the above report findings. The risk of travel related DVT was higher in women using oral contraceptives, as well as when body mass index was increased and when height was greater than 6’ 1”. Risk of travel related DVT was found to be higher with increasing travel duration and with the number of times the employee flew within an 8 week period. The risk of DVT tripled (over non-travelers) in employees taking 5 or more long haul (>4 hours) flights. Each extra flight increased the risk of DVT 1.4 fold. The risk of DVT was highest in the first two weeks after a long haul flight and gradually decreased to pre-travel risk after 8 weeks.
   
   In a study (7) of healthy Dutch (mostly male) pilots, the rate of DVT was not different from that in the general Dutch population. In this unique investigation there was no association between the number of hours flown and development of DVT. It is conceivable that this “exceedingly healthy group” had few if any of the above medical risks, were trained to exercise in their seats, and commanded a number of flights less then 4 hours long.
   
   Diagnosis: As alluded to in the above discussion, diagnosis of a DVT can often be obtained swiftly, non-invasively and inexpensively. An ultrasound examination of the legs performed by a Registered Vascular Technician and interpreted by a Vascular Specialist or Radiologist can make the diagnosis of DVT within 30 minutes. Ultrasound technology is now portable and can be brought to virtually any location. CAT scans, MRI, and nuclear scans are used to diagnose pulmonary embolism (PE) but can also be utilized if an ultrasound exam is indeterminate for DVT.
   
   Treatment:
   
   Urgent hospitalization and swift treatment are indicated in most instances of DVT or PE. Blood thinner and/or clot busting medications are prescribed as well as bed rest for several days followed by ambulation, usually with compression stockings. In the last two years, invasive and unique catheters that mechanically break up and chemically dissolve clots near and above the groin have been used effectively to remove freshly formed DVT. This technology has not been applied for clots in the lung, but variations of this catheter may eventually be used for such a purpose. All patients who can tolerate blood thinners are usually kept on these medications for at least three months after a clot is diagnosed regardless of the initial treatment method. In some instances, these medications are recommended for extended periods, occasionally for life. When these medications cannot be safely given to a patient because of other medical issues, surgically implanted clot filters are often prescribed.
   
   There are instances when patients diagnosed with DVT limited to the knee or calf can be managed outside of a hospital only with medications. This treatment is safe and common but does require serial ultrasound exams of the clot location during the first 4 weeks of treatment. It must be determined that the clot is not extending up the leg despite taking the medications as prescribed.
   
   Preventative Measures:
   
   There have been a number of preventative measures for DVT and PE studied both retrospectively and prospectively. Surprisingly, the only preventative measure shown to reduce the risk of traveler’s thrombosis is graded compressive stockings, ideally with 15-30mmHg circumferential pressure at the ankle. Despite the availability of a host of clot prevention medications, ranging from aspirin to powerful blood thinners, no other measure has been shown to so profoundly reduce the risk of DVT when traveling than graded compression stockings (8). Avoiding dehydration, eliminating alcohol and reducing other diuretic beverages is also prudent. Simple leg exercises every 15 minutes while awake are encouraged.
   
   Graded compression stockings obtained with a prescription from a Vascular Specialist’s office are your best prevention strategy. These stockings should be worn during any travel where you will be in a confined space for > 4 hours. There is some evidence that they should be worn for an additional week after such trips since the risk of traveler’s venous thrombosis lingers for reasons which are not yet clear.
   
   The Art of Medicine:
   
   Your doctor may recommend to you more than stockings to prevent a blood clot in your veins when traveling. There are legitimate reasons for a physician to prescribe for you more than just compression stockings as a preventative measure. This flexibility is what I personally enjoy about the practice of medicine.
   
   Doctors routinely weigh the risks and benefits of a treatment. We couple this exercise with our knowledge and opinion (critical review) of published studies; studies that are often performed on otherwise healthy people. Finally, we consider a patient’s unique medical and social condition and reflect on our own professional experience. What culminates is a recommendation. This recommendation may go beyond or occasionally fall short of medical evidence-based treatment guidelines. Recommendations often vary from patient to patient but should always consider the presence of additional risk factors which may contribute to that patient’s chance of developing traveler’s vein thrombosis. This balance of medical knowledge, experience and in-depth understanding of a patient’s condition is how a physician provides outstanding medical care.
   
   

1. Homans, J. “Thrombosis of the Leg Veins due to Prolonged Sitting.” New England Journal of Medicine. 1954, 250: 148-149.
   
   
2. Nielsen, H.K. “Pathophysiology of Venous Thromboembolism.” Seminars in Thrombosis and Hemostasis. 1991, Suppl 3: 250-253.
   
   
3. Geroulokos, G. “The Risk of Venous Thromboembolism from Air Travel.” British Medical Journal. 2001, 322 (7280): 188.
   
   
4. Cannegieter, S.C., Doggen, C.J., Van Houwelingen, H.C., Rosendaal, F.R. “Travel-Related Venous Thrombosis: Results from a Large Population-Based Case Control Study.” (MEGA Study). PLoS Med., 2006; 3:e307.
   
   
5. Toff, W.D., Jones, C.I., Ford, I. Pearse, R.J., Watson, H.G., Watt, S.J., Ross, J.A.S., Gradwell, D.P., Batchelor, A.J., Abrams, K.R., Meijers, J.C.M., Goodall, A.H., Greaves, M. “Effect of Hypobaric Hypoxia, Simulating Conditions during Long-Haul Air Travel on Coagulation, Fibrinolysis, Platlet Function, and Endothelial Activation.” Journal of the American Medical Association. 2006; 295: 2251-61.
   
   
6. Kuipers, S., Connegieter, S.C., Middeldorp, S. et.al. “The Absolute Risk of Venous Thrombosis after Air Travel: A Cohort Study of 8,755 Employees of International Organizations.” PLos Med. 2007; 4(9): e290.
   
   
7. Kuipers, S., Schreijer, A.J.M., Cannegieter, S.C., Middeldorp , S., Buller, H.R., Rosendaal, F.R. “Incidence of Venous Thromboembolism Among Dutch Airline Pilots.” Journal of Thrombosis and Haemostasis. 2005; 35: P2256.
   
   
8. Hopewell, S., Juszczak, E., Eisinga, A., Kjeldstrom, M. “Compression Stockings for Preventing Deep Vein Thrombosis in Airline Passengers.” Cochrane Database System Review 2006, CD 00400