TORONTO--Air travel presents special problems for patients with chronic obstructive pulmonary disease (COPD), who are less able than healthy people to endure the atmospheric conditions typical of commercial airliners. Because of federal regulations, these conditions simulate an altitude of no more than 8,000 feet; during flight, passengers experience a below-normal inspired oxygen tension and an above-normal gas volume.

In healthy persons, the increased gas volume can lead to gas expansion in noncommunicating bullae and other trapped spaces; consequences can include increased sinus pressure, abdominal bloating, flatulence, and (in rare cases) pneumothorax. The lower inspired oxygen tension may trigger hyperventilation and respiratory alkalosis; alveolar hypoxia may lead to pulmonary hypertension. These risks are even greater for COPD patients.

"The hypoxemia may be more severe," explained James Stoller, MD. In addition, the likelihood of pulmonary hypertension is greater in patients who are ventilation-limited and cannot hyperventilate, a common occurrence in severe COPD. Acute right heart failure is a possibility in these patients.

Dr. Stoller highlighted the hazards of air travel as it relates to COPD at the annual meeting of the American Thoracic Society. He also described ways to predict which COPD patients will need in-flight supplemental oxygen and offered tips on how to make air travel easier for such patients.

PREDICTING WHO NEEDS OXYGEN

"About one in 20,000 to 30,000 air travelers experiences a medical emergency, either in the airport, rushing to the plane, or in the plane itself," said Dr. Stoller, Vice Chairman of the Division of Medicine and Head of the Section of Respiratory Therapy at the Cleveland Clinic Foundation in Ohio.[1] The frequency of death in these circumstances is much lower--about 0.3 per million.[2]

Cardiac events are responsible for up to 56% of these kinds of emergencies, whereas syncope accounts for about one quarter. "Asthma and COPD comprise an important minority, though--about 10%," Dr. Stoller pointed out.

To reduce the risk of air travel-related problems in COPD patients, current recommendations call for in-flight supplemental oxygen if the patient's arterial oxygen tension (PaO2) during travel is predicted to be 50 mm Hg or less. In research centers, the predicted PaO2 can be obtained through hypoxic altitude simulations, which often involve measuring PaO2 while having the patient breathe a reduced-oxygen mixture of 15.1% oxygen (the concentration at 8,000 feet) or through the use of an altitude chamber.

"For most clinical circumstances, we depend on regression equations," said Dr. Stoller. These equations, he noted, can be programmed into a calculator or computer and thus are relatively simple to use. However, using the equations requires knowing both the patient's PaO2 at sea level and spirometric data measured without the use of a bronchodilator. Comparable equations using pulse oximetry data are not yet available, Dr. Stoller conceded.

The accuracy of these regression equations is good. In a classic study of 22 clinically stable, normocapnic patients with chronic airway obstruction, a sea-level PaO2 below about 68 to 72 mm Hg predicted a PaO2 below 50 mm Hg at 8,000 feet with a sensitivity of 87%.[3]

Later studies produced similar findings, although they used slightly different regression equations. "You can pick your favorite," said Dr. Stoller. "They all perform reasonably well."

HOW MUCH OXYGEN?

COPD patients with a predicted PaO2 of 50 mm Hg or less generally require an extra 2 L/min of supplemental oxygen during air travel. For example, patients who need 2 L/min of supplemental oxygen on the ground should receive 4 L/min in the air.

This recommendation, said Dr. Stoller, is derived from a number of studies. In one, Cramer et al[4] exposed 30 subjects--10 healthy, 10 with obstructive lung disease, and 10 with restrictive disease--to 15% oxygen in a body box. Under these conditions, oxygen saturation (as measured by pulse oximetry) ranged from 70% to 97%, with a median of 90%. "Bleeding in 2 L [of oxygen] a minute restored the pulse oximetry saturation to above 90% in all but one of these patients," said Dr. Stoller. The remaining patient required 3 L/min.

The extra 2 L/min recommendation assumes that patients will remain seated during most of the flight, pointed out Dr. Stoller. The supplemental oxygen requirement is likely to be greater in those who frequently get up and walk around.

An even more important caveat, he noted, is that studies of predicted PaO2 and in-flight oxygen needs only examined stable COPD patients and thus may not apply to unstable patients--yet these patients commonly travel by air. In addition, the studies did not include--and, therefore, may not apply to--COPD patients with hypercapnia or comorbidities, such as stroke or coronary artery disease, or to patients with other forms of restrictive lung disease.

MAKING AIR TRAVEL EASIER

To facilitate air travel for COPD patients, physicians should provide multiple copies of letters describing their patients' disease and in-flight oxygen requirements. They also should prescribe an emergency supply of any medications patients may need and provide contact information for physicians at their patients' travel destinations.

It may be necessary for physicians to contact airlines, notifying them well in advance of patients' needs for in-flight supplemental oxygen. "We should advise our patients to fly nonstop, if possible, because it's less inconvenient and often less expensive," said Dr. Stoller, explaining that some airlines charge by the number of flight segments for supplemental oxygen.

"If patients who must use supplemental oxygen at ground level need to travel, they should favor an oxygen supplier with networks that can dovetail with oxygen suppliers in other cities," he added. They should "arrange for oxygen during layovers, as these are not provided by the airline."

To assess the cost and availability of supplemental oxygen during air travel, researchers led by Dr. Stoller called all 33 (11 domestic, 22 international) of the commercial air carriers in the 1997 Cleveland Metropolitan Yellow Pages.[5] Of these, 25 (76%) offered in-flight oxygen, typically requiring 48 to 72 hours' advance notice.

"The equipment available ran the gamut from nasal cannula alone to Ventimasks with a variety of liter flow rates," said Dr. Stoller. The air carriers did not necessarily offer in-flight oxygen at every flow rate that might be prescribed.

Six carriers supplied the oxygen free of charge, while 18 supplied it only for a fee, which ranged from $64 to $1,500. However, the cost for supplemental oxygen during a standard six-hour round trip was typically $100 to $250. Only one carrier allowed travelers to bring their own oxygen--a single E-cylinder--at no extra charge.

--Timothy Begany

References
1. Cummins RO, Schubach JA. Frequency and types of medical emergencies among commercial air travelers. JAMA. 1989;261:1295-1299.
2. Cummins RO, Chapman PJ, Chamberlain DA, et al. In-flight deaths during commercial air travel. How big is the problem? JAMA. 1988;259:1983-1988.
3. Gong H Jr, Tashkin DP, Lee EY, Simmons MS. Hypoxia-altitude simulation test. Evaluation of patients with chronic airway obstruction. Am Rev Respir Dis. 1984;130:980-986.
4. Cramer D, Ward S, Geddes D. Assessment of oxygen supplementation during air travel. Thorax. 1996;51:202-203.
5. Stoller JK, Hoisington E, Auger GA. A comparative analysis of arranging in-flight oxygen aboard commercial air carriers. Chest. 1999;115:991-995.

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