WASHINGTON, DC—Flights of longer than 3,100 miles increase the risk of pulmonary embolism (PE)—and that risk rises sharply as flights lengthen further.

Although air travel is considered a risk factor for PE, the relationship between flight duration and severe PE incidence has never been comprehensively investigated. Therefore, researchers retrospectively studied this relationship among the more than 135 million air travelers who arrived at Charles de Gaulle Airport in France during a seven-year period.[1]

“Passengers who travel more than 3,100 miles, and particularly those who travel more than 6,200 miles, have a markedly increased risk of severe pulmonary embolism,” Stephen W. Borron, MD, one of the study’s principal researchers, told Pumlonary Reviews. The incidence of severe PE was 1.5 per million passengers on flights of more than 3,100 miles (which are usually about six hours in duration) versus only 0.01 per million passengers on shorter flights, he reported. Incidence rose to 4.8 per million passengers on flights exceeding 6,200 miles (Figure 1).

LARGE SAMPLE SIZE

For the study, which was conducted between November 1993 and December 2000, the researchers reviewed the records of all passengers who required transport to a hospital for suspected PE after arriving at Charles de Gaulle Airport. Suspicion of PE was based on the presence of one or more of the following symptoms within an hour of airport arrival: chest pain, malaise, syncope, and shortness of breath.

The diagnosis was confirmed with ventilation-perfusion lung scanning, pulmonary angiography, or high-resolution helical computed tomography. If PE was ruled out, the patient was excluded from the study.

For all patients, the researchers obtained information on flight origin, distance, and duration; class of travel; in-flight ambulation; and risk factors for thromboembolic events. The flight information was available from the Aéroports de Paris; details about ambulation and risk factors were obtained through patient interviews.

In addition, the researchers obtained a list of all patients who arrived at the airport during the study; the list included the distance and duration of each patient’s flight. This allowed the researchers to quantify the incidence of severe PE in an extraordinarily large group of air travelers.

REAL, BUT UNCOMMON, DANGER

“Although air travel raises the risk of pulmonary embolism, the absolute numbers are still very small,” stressed Dr. Borron. During the entire seven-year study, there were only 170 suspected and 56 confirmed cases of severe PE, he pointed out.

Only one of the patients with severe PE had flown less than 3,100 miles, and that patient had flown more than 2,600 miles. But even among the patients who had flown more than 3,100 miles, the risk of PE was still fairly low.

However, only 7% of the patients had factors that would have placed them at high risk of PE before their flights. “These patients did not have major trauma, cancer, or heart failure,” emphasized Dr. Borron. “They generally started out in pretty good health.” He acknowledged, though, that most of them did have factors that placed them at moderate risk for PE, such as varicose veins, oral contraceptive or hormone replacement use, age older than 40 years, obesity, or tobacco use.

The patients with PE averaged age 57 years; 75% of them were women. Most had traveled in economy class; only a few patients reported that they had left their seats during the flight.

In all cases, symptoms of PE began during the flight or while the patients were leaving the plane; no one developed symptoms once in the airport. Malaise and dyspnea were the initial symptoms in most of the patients.

The researchers admitted that they may have underestimated the thromboembolic risk associated with air travel for several reasons. First, their study was not designed to detect patients with mild PE or venous thromboembolism. Nor could it identify patients who did not present for treatment until after they had left the airport. And it did not include information about patients who died during their flights.

Other studies have found that severe PE accounts for about one fifth of PE cases, the researchers noted. But even if their estimates of the PE risk associated with air travel are quintupled, the absolute risk remains small.

Figure 1 IMPACT OF FLIGHT DISTANCE ON PE RISK

PE, pulmonary embolism. Adapted from Lapostolle et al. N Engl J Med. 2001. [1]

PREVENTIVE STRATEGIES

Nonetheless, long-distance air travel should always be considered a risk factor for thromboembolic events, Dr. Borron and his colleagues concluded. The risk of venous thrombosis—and thus PE—may rise during air travel, they suggested, because all three factors of Virchow’s triad are present. These factors include venous stasis (from sitting), vessel-wall injury (due to compression by the seat), and hypercoagulability (from being immobile).

The researchers recommended that all long-distance air travelers use simple in-flight preventive measures, although they acknowledged that the efficacy of such measures is unproven. These include adequate fluid intake; use of elastic support stockings; frequent position changes while seated; minor physical activity, such as walking around the cabin or at least moving the legs; and avoiding smoking, alcohol intake, constrictive clothing, and leg crossing.

—Timothy Begany

Reference
1. Lapostolle F, Surget V, Borron SW, et al. Severe pulmonary embolism associated with air travel. N Engl J Med. 2001;345:779-783.

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