As air travel is expected to globally increase by 2039, family physicians should be prepared to help patients prevent venous thromboembolisms by addressing risk factors and reducing said risk.
2021 Namey/Burnett Preventive Medicine Writing Award Submission
By Michael Cargill, OMS-III, Liberty University College of Osteopathic Medicine
Venous Thromboembolisms in Air Travel
Prior to the COVID-19 pandemic, more than 300 million people took long-distance flights each year.1 While there was a drastic decline in 2020 air travel, the International Air Transport Association forecasts a strong recovery in global air travel with a 2.1x increase in the number of air passenger journeys in 2039, as compared to 2019.2 With a predicted increase in future air transportation, travel-related impacts on health, such as venous thromboembolisms (VTEs), should be considered. VTEs encompass thrombotic events, such as deep vein thrombosis (DVT) (clotting in the deep veins), which can dislodge and relocate to the pulmonary circulation where blockage causes a pulmonary embolism (PE).
The first association between VTE and long-distance flights was documented in 1954 by John Homans.3 While overall incidence of travel-related VTE is low, it has been reported that long-distance air-travel may increase the risk two- to four-fold.1,4 Additionally, pre-existing risk factors are an important aspect of travel-related VTE. Studies have demonstrated that 75–99.5% of travel-related VTE cases had more than one other preexisting risk factor.1 Overall, risk correlates with length of travel and existence of other risk factors.1,4 As air travel is expected to globally increase in the future, it is important that primary care physicians be prepared to identify patients with preexisting risk factors for developing VTE and to provide preventative countermeasures for long-distance flights.
VTE Pathophysiology and Risk Factors
In 1856, Rudolf Virchow described three conditions in which VTEs may arise: venous stasis, vascular injury and hypercoagulability.5 These conditions are seen in scenarios in which VTEs occur.
Venous stasis is a major component in the formation of a VTE. As blood flow decreases, a hypoxic microenvironment contributes to decreasing antithrombotic factors (thrombomodulin, endothelial protein C receptor) and an increase in procoagulants (P- selectin).5 This risk factor is evident in DVT that occurs in long-distance flights. Prolonged compression of the popliteal vein leads to venous stasis in the distal lower extremity. Furthermore, as most airline seats cannot be adjusted and have limited leg room, individuals <1.6m (5 ft, 3 in) and >1.9m (6 ft, 3 in) are at greater risk of VTE.1
Risk factors that result in vascular injury may also increase coagulability. Recent trauma, surgeries, hospitalizations or prior VTEs increase the risk for travel-related VTE.1,4,5 Another condition of Virchow’s Triad—hypercoagulability—also increases the risk of a VTE occurrence.
Inherited thrombophilias may contribute to an increased risk of thrombus formation. A common example of an inherited thrombophilia is factor V Leiden, a genetic disease that conveys resistance to factor V against inhibition by protein C and is found in 5% of Caucasians in the United States.5 Other factors that increase the risk of thrombus formation include oral contraceptives, cancer, age, recent surgery and obesity.1,4,5
Reducing Risk of VTE
Prevention is the primary strategy in combating VTE induced by long-distance air travel. As risk factors are additive, it is important for the physician to identify the patient’s risk level and apply appropriate preventative measures.4 Additionally, VTEs are not only a concern during the long-distance flight, but also are a special concern for two weeks after the long-distance flight and can even be identified up to two months later.1,4
Completing a comprehensive medical history is important for identifying a multitude of VTE risk factors. An in-depth knowledge of the patient’s medications is vital because oral contraceptives, hormone replacement and other medications may increase the risk of thrombus formation.4 A thorough social history may uncover potential risk factors.
Understanding a patient’s travel history and identifying frequent long-distance flyers may guide the physician in identifying other potential risk factors. In addition to social history, a comprehensive family history may offer a starting point for identifying patients with inherited thrombophilia disorders and may guide genetic testing and counseling.
Patient education can help patients be proactive in preventing VTE. Patients can be advised to walk for five minutes every hour for longer flights.4 Simple measures, such as walking to the bathroom every 2–3 hours, may also be a suitable approach.6 Certain exercises that decrease venous stasis can be practiced by patients in their seats during flights. The Centers for Disease Control and Prevention (CDC) recommend raising and lowering one’s toes (while keeping the heel on the floor), raising and lowering one’s heels (while keeping the toes on the floor), and tightening and relaxing one’s leg muscles.7 (Figure 1)
Additionally, the World Health Organization (WHO) suggests that hand luggage not be placed in a position that limits lower extremity movement.6 For general long-distance travelers, the CDC recommends ambulating frequently, utilizing calf exercises and sitting in an aisle seat, if possible.1 (Figure 1)
In the clinic, an increased risk for VTE may provide a platform to advocate lifestyle changes in obese patients who frequently undertake long-distance flights. Patient education approaches are a key aspect to preventing travel-related VTE.
In addition to general measures, further preventative action may be taken for patients at a high risk of VTE. (Figure 1) Current CDC guidelines do not recommend global prophylactic anticoagulants or aspirin to prevent long-distance travel-related VTE.1 If pharmacologic prophylaxis is warranted, anticoagulant pharmacologic prophylaxis is recommended on an individual basis where benefits outweigh risks. High-risk patients on long-distance flights can also be prescribed a graduated compression stocking (GCS) that provides 15-30 mmHg of pressure at the ankles.1,4 GCSs are not recommended for long-distance travelers who are not at an increased risk of VTE.
Figure 1: CDC recommendations for long-distance travelers.1 Illustrations by author.
As air travel is expected to increase in the decades following the COVID-19 pandemic, it is important physicians consider patients who may be at a high risk of a travel-related VTE. Venous thromboembolisms arise in a hypercoagulable state, often due to inactivity associated with long-distance air travel. Patients with preexisting risk factors for VTE are at an even higher risk for a thromboembolic event after the flight, notably within the first two weeks. Prevention is key to combating travel-related thromboembolic events.
Current CDC recommendations for general measures against travel-related VTE include calf exercises, frequent ambulation and aisle seating if possible. For patients at an increased risk of VTE, properly fitted, below-knee GCSs are recommended. Anticoagulant prophylaxis is only recommended on an individual basis where the benefits outweigh the risks. By identifying risk factors and applying appropriate preventative measures, physicians can help prevent travel-related VTE.
- Reyes NL, Beckman MG, Abe K. Deep Vein Thrombosis & Pulmonary Embolism – Chapter 8 – 2020 Yellow Book. Centers for Disease Control and Prevention. https://www.cdc.gov/travel/yellowbook/2020/travel-by-air-land-sea/deep-vein- thrombosis-and-pulmonary-embolism. Published 24 June 2019. Accessed 25 November 2020.
- 20 Year Passenger Forecast Infographic. IATA. https://www.iata.org/en/publications/store/20-year-passenger-forecast/. Published May 2020. Accessed 25 November 2020.
- Homans J. Thrombosis of the Deep Leg Veins Due to Prolonged Sitting. New England Journal of Medicine. 1954;250(4):148-149. doi:10.1056/nejm195401282500404
- Bartholomew JR, Schaffer JL, McCormick GF. Air travel and venous thromboembolism: minimizing the risk. Cleve Clin J Med. 2011;78(2):111-120. doi:10.3949/ccjm.78a.10138
- Stone J, Hangge P, Albadawi H, et al. Deep vein thrombosis: pathogenesis, diagnosis, and medical management. Cardiovasc Diagn Ther. 2017;7(Suppl 3):S276-S284. doi:10.21037/cdt.2017.09.01
- Air travel advice. World Health Organization. https://www.who.int/news-room/q-a- detail/air-travel-advice. Published 27 April 2020. Accessed 25 November 2020.
- What is Venous Thromboembolism? Centers for Disease Control and Prevention. https://www.cdc.gov/ncbddd/dvt/facts.html. Published 7 February 2020. Accessed 25 November 2020.