Key Point

The hypoxia altitude simulation test is a simple approach to determining whether children or adults with respiratory disease can travel by air without risk of an adverse event or need for oxygen supplementation.

Among the approximately one billion passengers worldwide who travel by airplane each year, respiratory complaints—along with gastrointestinal and cardiac ailments—top the list of reasons for medical emergencies. For patients with preexisting pulmonary diseases, air travel may pose particular concerns, due to the effects of the elevated cabin pressure.

According to a series of reports in the April Chest, the hypoxia altitude simulation test (HAST)—also referred to as the hypoxia inhalation test (HIT)—can identify which patients are most at risk for adverse events while flying. In one article that served as an overview, the HAST was described as a “simple” procedure that can identify those patients who may benefit from oxygen supplementation during air travel. In another article, postflight HAST results for adults with COPD were shown to correlate closely with pulse oximetry results obtained during an actual airplane ride. In children with a history of respiratory disease, a hypoxia test limit of 85% may be an appropriate cutoff to predict fitness to fly, according to a third report.

GUIDELINE RECOMMENDATIONS

During HAST, which was first described in 1984 by Gong et al, a patient is asked to breathe a mixture of gases with an oxygen saturation of 15.1% for 15 to 20 minutes, using a tight-fitting mask or mouthpiece, to simulate the cabin pressure maintained on a commercial flight (ie, partial pressure of inspired oxygen that corresponds to a maximum of 8,000 feet), explained C. Jessica Dine, MD, and Mary Elizabeth Kreider, MD, both from the Division of Pulmonary, Allergy, and Critical Care Medicine at the University of Pennsylvania, Philadelphia, in their overview. Assessment for symptoms, continuous ECG monitoring, and arterial blood gas measurement are part of the test. Research has shown that the HAST is as predictive as using a hypobaric chamber in measuring oxygenation, the investigators noted.

Guidelines issued by the British Thoracic Society—called “the most practical” compared with their Canadian, North American, and other European-country counterparts—recommend preflight screening with HAST not only for patients with COPD but also for those with restrictive lung disease, cystic fibrosis, a history of recent respiratory illnesses or infections, pulmonary tuberculosis, significant comorbidities, or past difficulties with air travel. The guidelines advise that adult patients receive in-flight supplemental oxygen if the Pao₂ falls below 50 mm Hg or if the oxygen saturation as measured by pulse oximetry (Spo₂) falls below 85% during the HAST.

REAL-LIFE COMPARISON

To assess the capability of HAST to predict in-flight hypoxemia, Paul T. Kelly, MSc, from Christchurch Hospital, New Zealand, and colleagues recruited for study participation seven women and six men with documented COPD who were planning to travel by air of their own accord. The participants underwent preflight respiratory function tests (ie, spirometry, lung volumes, and Dlco) within one month of travel.

In-flight, participants wore a portable pulse oximeter to continuously measure Spo₂ and pulse rate. They recorded symptoms of breathlessness using the Borg dyspnea scale and kept a log of in-flight activities. Continuous in-flight measurements of cabin pressure via a wrist altimeter were taken every 60 seconds and were averaged for each phase of the flight. The six-minute walk test and HIT were performed about two weeks after the flight. “The HIT was performed postflight due to the ethical implications of a possible ‘failed’ test that was not otherwise medically requested,” the investigators remarked.

None of the participants required oxygen during the flight, and no adverse events were recorded. The mean peak in-flight pulse rate was significantly higher than the mean preflight pulse rate (110 and 84 beats/min, respectively). Preflight Spo₂ (mean, 95%) decreased with the reduction in cabin pressure until aircraft achieved cruise altitude; the mean in-flight Spo₂ of 86% was significantly lower. The mean nadir Spo₂ during such activity as being seated or visiting the lavatory was 78%. Postflight Spo₂ returned to preflight values.

There was a strong relationship between the mean desaturation during postflight HIT (84%) and that during actual air travel. Mean Borg dyspnea scale values preflight and in-flight—which increased from 1 to 2, respectively—were replicated in the HIT.

“HIT is the best widely available laboratory test for predicting the occurrence of in-flight hypoxia,” the researchers contended. They suggested that further research is needed to help establish “appropriate evidence-based air-travel guidelines for passengers with respiratory disease.”

TESTING IN CHILDREN

Andrew C. Martin, MD, from the University of Western Australia, Perth, and colleagues conducted a prospective interventional study of 35 children younger than 5 years with neonatal chronic lung disease (nCLD) who had not used supplemental oxygen for at least four weeks prior to study enrollment and 34 healthy children who served as controls.

All children underwent HIT. Among the healthy children, the test was continued for 20 minutes regardless of Spo₂; however, for those with a history of nCLD, the test was discontinued if Spo₂ decreased to less than 85% for more than one minute.

Since an Spo₂ cutoff of neither 85% nor 90% significantly differentiated between children with nCLD and controls and no child older than 2 had an Spo₂ nadir of less than 90%, the investigators reanalyzed the data of only children younger than 2. In this analysis, an Spo₂ cutoff of less than 85% significantly differentiated between the two groups (only one child in the control group failed the HIT at the 85% limit). For children younger than 2, increasing total number of days of oxygen therapy (ie, increased severity of nCLD) was the only factor that was significantly associated with failing the HIT at the 85% cutoff level.

“Our data suggest that physicians can be reasonably confident that even among children with a history of nCLD, those who are older than 2 years of age would pass the hypoxia test and therefore could undertake commercial air travel safely,” the researchers noted. “Applying a lower cutoff value of 85% better discriminates between healthy children and those with a history of nCLD, who are theoretically at greater risk of hypoxia when flying at altitude,” they added, although they acknowledged that further study is needed.

POTENTIAL ROLE OF HAST

“Collectively, the three articles regarding the HAST … point out several things of importance for the practicing pulmonologist,” said Lawrence C. Mohr, MD, Professor of Medicine and Director of the Environmental Biosciences Program at the Medical University of South Carolina in Charleston, in an accompanying editorial. Among these things are the increasing use and interest in the preflight evaluation of cardiopulmonary patients and the usefulness of testing in patients of all ages. Through more research of this nature, HAST may “reach its full potential as the ‘gold standard’ for the evaluation of patients prior to commercial air travel,” Dr. Mohr asserted.

—Adriene Marshall

Suggested Reading
Dine JC, Kreider ME. Hypoxia altitude simulation test. Chest. 2008;133(4):1002-1005.
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(6):980-986.
Kelly PT, Swanney MP, Seccombe LM, et al. Air travel hypoxemia vs the hypoxia inhalation test in passengers with COPD. Chest. 2008;133(4): 920-926.
Martin AC, Verheggen M, Stick SM, et al. Definition of cutoff values for the hypoxia test used for preflight testing in young children with neonatal chronic lung disease. Chest. 2008;133(4):914-919.
Mohr LC. The hypoxia altitude simulation test: an increasingly performed test for the evaluation of patients prior to air travel. Chest. 2008;133(4): 839-842.

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