Altitude Exposures during Aircraft Flight

Altitude Exposures during Aircraft Flight* Flying Higher Joseph]. Cottrell, M.D., F.C.C.P.

Commercial aircraft flight represents a highly variable altitude exposure that may result in significant hypoxemia for patients with cardiac or pulmonary disease. To develop better guidelines for travel by patients with cardiopulmonary disease, we measured inflight cabin altitude on 204 regularly scheduled commercial aircraft flights. Measurements were carried out on 16 different types of aircraft, operated by 28 airlines. The median altitude exposure for all flights was 6,214 feet (1894 m), Cabin altitudes ranged from

sea level to 8,915 feet (2717 m), Inspired partial pressure of oxygen falls from 159 mm Hg at sea level to 127 mm Hg at 6,200 feet and further declines to 113 mm Hg at 9,000 ft. There was no significant difference between domestic and international flights. New generation aircraft fly at higher altitudes than older aircraft and are associated with greater altitude exposures to passengers (p = 0.002). The risk of hypoxemia may increase as newer model aircraft replace older ones.

Most commercial airliners are pressurized in that they maintain a cabin pressure higher than the pressure of the surrounding atmosphere through which they fly. Pressurized aircraft do not, however, maintain a sea level pressure during most flight operations. The pressures associated with aircraft flight are most commonly expressed as their altitude equivalents, in feet. The two important altitudes for a pressurized aircraft are the flight altitude (FA) and the cabin altitude (CA). Ideally, CA would be maintained at sea level for all flight operations. Unfortunately, this is not practical from either a structural design or an operational efficiency perspective. Although current Federal Aviation Regulations specify as a design criteria that an aircraft must be capable of maintaining an 8,000 foot (2,438 m) cabin environment at the aircraft's highest operating altitude, 1 Federal regulations for actual airline operations are more complicated and lenient. 2 These rules allow significantly higher cabin altitudes, although CAs greater than 10,000 feet (3,048 m) are effectively prohibited by a requirement that the flight crew wear oxygen at that level. Table 1 displays the fall in oxygen tensions at altitudes between sea level and 10,000 ft. The development of the 8,000 foot (2,438 m) design criteria, largely based on medical knowledge, has been reviewed previously." It was, and still is, quite adequate for healthy individuals. It is well known, however, that patients with significant pulmonary disease may develop significant hypoxemia at altitudes well

below this level. 4,5 For this reason, it has been suggested that altitudes of 3,000 to 6,000 feet (914 to 1,829 m) may be optimal for transporting the general population.Y' Some patients with severe cardiac or pulmonary disease should be further restricted. 8 Unfortunately, higher pressurization to provide lower CAs increases financial costs. It demands greater structural integrity of the aircraft, increases weight, and causes more fuel to be consumed. In this era of airline deregulation, and with the advent of a new generation of commercial aircraft designed to fly at higher altitudes, we undertook the measurement of cabin altitudes during a wide variety of commercial aircraft flights.

*From the Department of Medicine, University of Illinois College of

Medicine, Chicago. This study was supported in part by the facilities of the Research Resources Center, University of Illinois. Manuscript received April 10; revision accepted May 5. Reprint requests: Dr. Cottrell, 1740 West Taylor Street, Room 2146, Chicago 60612

METHODS

Measurements of cabin altitudes were made by members of the Section of Respiratory and Critical Care Medicine, our housestaff or immediate family members. Measurements were made on regularly scheduled commercial flights being used for both business and vacation travel. For each flight, the following data were recorded: date, airline, type aircraft, flight altitude (either as announced over the aircraft public address system or obtained from the flight crew after arrival), departure airport, and arrival airport. Inflight cabin altitude was continuously observed and peak values were manually recorded from a handheld altimeter. (Safesport A1400) These altimeters have a stated accuracy of 200 ft, and this was confirmed for all altimeters in our altitude chamber. Altimeters were set to zero before each flight. Actual cabin altitude was determined by adding

Table I-Fall in Oxygen Tensions with Altitude Altitude, ft Sea level 2,000 4,000 6,000 8,000 10,000

Inspired Po 2, mm Hg

Tracheal Po 2 , mm Hg

159 148 137 127 118 109

149 138 127 117 108 99

CHEST / 92 / 1 / JANUARY, 1988

81

Table 2-Distribution of Flights by Aircraft Type No. of Flights

Aircraft B-727 DC-9 (all series) B-737 B-767 DC-lO B-747 DC-8 BA-146 BAC-lll FH-227/F-27 A-3oo Concord FH-Metroliner L-lOll A-3l0 B-757

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CABIN ALTITUDE (THOUSANDS OF FEET)

FIGURE 2. Distribution of cabin altitudes.

2 1 1

airport elevation" to the peak measured altitude recorded during the flight. An aircraft flight includes three distinct phases: ascent, cruise, and descent. During the ascent phase, which lasts for approximately 20 minutes, CA gradually rises to a plateau. This is followed by the cruise phase of the flight during which CA tends to be stable at its highest levels. These values are reported herein. The duration of exposure at this altitude is primarily dependent upon the distance being traveled. Finally, during aircraft descent, cabin pressure increases to the atmospheric pressure of the arrival airport. This final phase averages approximately 20 minutes in length. STATISTICAL ANALYSIS

All data are expressed as means ± 1 SD. Both unpaired Student's t-tests, as well as Mann-Whitney analysis for nonparametric data were performed. Nonparametric results are reported. Two-tailed p values of less than 0.05 were considered significant. RESULTS

Cabin altitudes were measured on 204 commercial aircraft flights over a 20-month period of time. These flights involved 16 different types of aircraft (Table 2)

OTHER (71 )

operated by 28 air carriers (Fig 1). Although flight altitudes ranged from 10,000 ft (3,048 m) to 60,000 ft (18,288 m), peak cabin altitudes ranged from sea level to 8,915 ft (2,717 m), The tracheal P0 2 at this altitude is 103 mm Hg. Figure 2 displays the frequency distribution of cabin altitudes. The mean altitude exposure for all flights was 5,673 ft (1,724 m) with a standard deviation of 2,019 ft (615 m), The median was 6,214 ft (1,894 m) with a calculated tracheal P0 2 of 116 mm Hg. Table 3 contains the data for the "domestic" and "intercontinental" groups. There was no statistically significant difference in altitude exposure between the 194 "domestic" and the ten "intercontinental" flights. When flights on two "new" design aircraft (A-310 and 767) were compared with the two most widely used "older" design aircraft (DC-9 and 727) there was a statistically higher (p = 0.002) altitude exposure in the newer model aircraft. Median altitude exposure in the "new" design aircraft was 7,412 ft (2,259 m) with a calculated tracheal P0 2 of 111 mm Hg. DISCUSSION

Although air travel represents an extremely safe form of transportation, it has certain inherent risks, including hypoxemia. Many healthcare providers are unaware of the marked fall in alveolar oxygen tension that occurs with ascent to moderate altitudes. Alveolar oxygen tension falls to 65 mm Hg at 8,000 ft (2,438 m) with a resultant fall in arterial oxygen tension (Po 2) to Table 3-Summary of Comparisons

MIDWAY (22)

FIGURE 1. Distribution of flights. The numbers in parentheses represent the number of flights. Five airlines accounted for 65 percent of the flights. The "other" category includes flights on 23 air carriers.

82

All flights

Domestic Intercontinental Old aircraft New aircraft

No.

Cabin Altitude, ft (Mean±l SD)

Median

204 194 10 120 14

5673 ± 2019 5673±2057 6074± 1004 5820± 1730 7004± 1373

6214 6214 5812 6056 7412

Significance NS p=0.002

Altitude Exposures during Aircraft Flight (Joseph J. Cottrell)

approximately 60 mm Hg in healthy individuals. Due to the unique characteristics of the oxyhemoglobin dissociation curve, this fall in arterial oxygen tension has little physiologic significance for normal individuals. Their arterial oxygen content is decreased by less than 10 percent. In patients with altered resting blood gas values, however, ascent to these same moderate altitudes can cause significant hypoxemia. 5,10 Although there has been much recent interest in the aircraft cabin environment," there is very little information on operational cabin pressurization. The only previous study" involved only two aircraft types, one of which is no longer in service. Our data, obtained from a wide variety of aircraft, confirm that cabin altitudes do reach levels that can cause significant hypoxemia in a small subgroup of travelers. In addition, the data indicate that this risk of hypoxemia will increase in the future as newer model aircraft replace older models. Unfortunately, prediction of hypoxemia for the individual air traveler remains quite difficult. Two nomograms have been developed to predict inflight arterial P0 2 based upon preflight arterial blood gas analysis and knowledge of the cabin altitude.":" The first was developed from aeromedical evacuation experience during the Viet Nam conflict. It is based upon a young male population with an average age of22 years. In addition, only 22 of 201 subjects had a baseline arterial oxygen tension of less than 70 mm Hg. The second nomogram is based upon altitude simulation with hypoxic gas in 22 patients with chronic obstructive airways disease (COPD). Neither of these nomograms has been evaluated in a prospective fashion, and since they differ in their predictions, optimal choice is unclear. Application of either of these nomograms may be difficult, for they are both dependent on knowledge of arterial blood gas analysis and cabin altitude. It is unknown how close to the time of departure an arterial blood gas must be drawn. In a group ofCOPD patients subjected to low altitude flight in a nonpressurized aircraft (CA = 2250 m), it was observed that the P02 measured two hours before the flight correlated well with the measured inflight Po 2 • Unfortunately, arterial P0 2 levels obtained within four months prior to the flight correlated poorly with the inflight values." Our measured data demonstrate that preflight prediction of cabin altitude is very difficult. Several approaches have been advocated in the literature. The suggestion that hypoxic risk can be estimated by assuming an altitude exposure of 5,000 ft (1,524 m) during "routine" aircraft flight and 7,000 ft (2,134 m) during "intercontinental" flight" is clearly incorrect. Although our data indicate that a "worst case" strategy could allow one to assume an altitude exposure of9, 000 ft, this would result in many unnecessary prescriptions for oxygen. One committee has suggested that knowl-

Table 4-Aircraft Cabin Pressurization* Differential Pressure Aircraft Type

psij

Cabin Altitude, ft

B-727 B-757 B-767 B-747 B-737 DC-8 DC-9 DC-10 A-300 A-320 L-1011 BAC-111 Concord

8.6 8.6 8.6 8.9 7.45 8.77 7.76 8.6 8.25 8.3 8.4 7.5 10.7

5,400 5,400 5,400 4,700 8,000 5,000 7,300 5,400 6,100 6,000 5,800 7,900 1,000

*Cabin altitude is the design specification at a flight altitude of 35,000 ft. Cabin altitude varies with flight altitude. t1 psi is 51.7 mm Hg. Differential pressures were supplied by the manufacturer or the Federal Aviation Administration.

edge of the aircraft being used, and the intended flight altitude, may be used to predict CA. 14 The maximum pressure difference that an aircraft can maintain between the outside atmosphere and the cabin is the differential pressure. The greater the differential pressure, the lower the CA at any given flight altitude. The differential pressure is expressed in pounds per square inch (psi) and varies from aircraft type to aircraft type. Table 4 lists the differential pressurizations of some widely used aircraft. For purposes of comparison, it also presents the CA for each type of aircraft at a FA of 35,000 ft (10,668 m), There are several limitations to the clinical application of this approach. Some aircraft may not be operating at their maximum differential pressurization and will have a CA higher than predieted." Although aircraft of different types may have identical pressurization, they may be designed to cruise at different FAs, resulting in different CAs. The differences in CA between the older and newer aircraft are not the result of decreases in differential pressurization but are the result of the higher FAs used. A B-767 routinely cruises at a FA of 41,000 ft (12,497 m), several thousand feet higher than a B-727. Finally, although we have reported peak cabin altitudes, which are generally associated with the cruise phase of the flight, a cabin altitude profile may be complex and subject to change during the flight due to traffic or weather. Figure 3 is a time profile of cabin altitude recorded on a New York to Chicago flight that encountered turbulence enroute. Recognizing the above limitations, we currently make several recommendations to our patients with significant cardiac or pulmonary disease who are planning to travel by air. These include avoiding ethanol and tobacco both before and during to avoid the exacerbating effects of these agents on hypoxemia. Based on the data in this study, we suggest avoiding CHEST / 93 / 1 / JANUARY, 1988

83

available, simulation of altitude by inhalation of hypoxic gas mixtures has also been advocated." In summary, our data confirm that commercial aircraft flight is associated with significant altitude exposures that can result in hypoxemia for patients with cardiopulmonary disease. This exposure can be expected to increase over the next few years as higherflying, newer aircraft enter service.

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ACKNOWLEDGMENT: I am indebted to George Alex and Joseph Cottrell Sr. for their help in data collection.

1

10

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30

40

50

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70

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90 100110120

TIME IN MINUTES FIGURE 3. Cabin altitude profile. Actual cabin altitude profile recorded on a New York to Chicago flight. The increase in cabin altitude resulted from a flight altitude change due to turbulence.

newer design aircraft when feasible. We also use three different approaches to air travel depending upon the urgency of the need to travel and the patient's arterial blood gas analysis. The first and most accurate approach, which we advocate for air ambulance operation, involves noninvasive pulse oximetry and titration of supplemental oxygen based upon measured inflight arterial saturation. This is not practical for routine travel. For ambulatory patients who have normal arterial carbon dioxide levels, we agree with previous empiric recommendations that inflight arterial P0 2 should be maintained at a minimum value of 50 to 55 mm Hg. 13 ,16,17 Based upon a median exposure of 6,214 ft (1,894 m), a preflight arterial oxygen tension of approximately 70 mm Hg is required to maintain an adequate inflight saturation. If the patient has a baseline arterial Po.Iower than 70 mm Hg, we recommend supplemental oxygen during the flight. This is available by advance arrangement on most airlines, at a cost to the patient of $35 to $50 per flight. Federal regulations prevent the patient from providing his or her own supply. Our third approach applies to those individuals with elevated arterial levels of carbon dioxide. For individuals in this group, for whom excessive oxygen can cause worsening of hypoventilation, we currently recommend a flight simulation in our altitude chamber, with measurement of arterial blood gas values both with and without supplemental oxygen. If such a facility is not

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REFERENCES 1 Code of Federal Regulations, Title 14, Part 25.841. Washington, DC:US Government Printing Office, 1986 2 Code of Federal Regulations, Title 14, Part 121.327-.331. Washington, DC: US Government Printing Office, 1986 3 McFarland RA. Human factors in relation to the development of pressurized cabins. Aerospace Med 1971; 42:1303-18 4 Shillito FH, Tomashefski JF, Ashe WE The exposure of ambulatory patients to moderate altitudes. Aerospace Med 1963; 34:850-57 5 Henry IN, Krenis LF, Cutting RT Hypoxemia during aeromedical evacuation. Surg Gynecol Obstet 1973; 136:49-53 6 Ernsting J. Prevention of hypoxemia-acceptable compromises. Aviat Space Environ Med 1978; 49:495-502 7 McFarland RA. Human factors in air transport design. New York: McGraw-Hill Book Company, 1946 8 Committee on Physiologic Therapy, American College of Chest Physicians. Air travel in cardiorespiratory disease. Dis Chest 1960; 37:579-88 9 Navarre M. Flight guide. Long Beach, CA: Airguide Publications, 1985 10 Schwartz JS, Bencowitz HZ, Moser KM. Air travel hypoxemia with chronic obstructive lung disease. Ann Intern Med 1984; 100:473-77 11 Committee on Airliner Cabin Air Quality, National Research Council. The airliner cabin environment: air quality and safety. Washington, DC:National Academy Press, 1986 12 Aldrete JA, Aldrete LE. Oxygen concentrations in commercial aircraft flights. South Med J 1983; 76:12-14 13 Gong H Jr, Tashkin D~ Lee EY, Simmons MS. Hypoxia-altitude simulation test. Am Rev Respir Dis 1984; 130:980-86 14 Cardiovascular Committee, Cystic Fibrosis Foundation. Airline travel for children with chronic pulmonary disease. Pediatrics 1976; 57:408-10 15 Spoor DH. The passenger and the patient in flight. In: DeHart RL, ed. Fundamentals of aerospace medicine. Philadelphia: Lea & Febiger, 1985; 595-610 16 Gong H Jr. Air travel and patients with chronic obstructive pulmonary disease. Ann Intern Med 1984; 100:595-97 17 AMA Commission on Emergency Medical Services. Medical aspects of transportation aboard commercial aircraft. JAMA1982; 247:1007-11

Altitude Exposures during Aircraft Flight (Joseph J. Cottrell)