Effects of Aircraft Cabin Altitude on Passenger Comfort and Discomfort
Condition: Acute Mountain Sickness
MeSH terms: Altitude Sickness
Study type: Interventional

Summary: 
      Commercial aircraft passengers are exposed to atmospheric pressures ranging from the pressure
      found at ground level to that encountered in the external environment at 8,000 feet. There is
      some evidence in the medical literature that symptoms of acute mountain sickness can result
      from ascent to altitudes of 6,300 to 10,000 feet by unacclimated persons during the first few
      days following ascent, probably due to the hypoxia that results from breathing air at the
      reduced ambient pressures at altitude. The logical hypothesis that follows is that exposure
      to 8,000 feet could cause hypoxia sufficient to adversely affect the comfort and well being
      of some commercial aircraft passengers on prolonged flights. There is insufficient data in
      the literature to validate this hypothesis.

      Exercise at sea level and at altitude reduces arterial oxygen levels. The logical hypothesis
      that follows is that the combination of moderate exercise and exposure to altitude could
      cause hypoxia sufficiently severe to adversely affect the comfort and well being of some
      people and that the combined effect of exercise and altitude on comfort and well being is
      greater than the effect of exercise or altitude alone. Again, there is insufficient evidence
      in the literature to substantiate this possibility.

      The purpose of this investigation is to test these hypotheses.
    

Detailed description: 
      Altitude affects human health and well being through its effect on tissue oxygenation by
      altering the partial pressure of oxygen in the gas that enters the lungs. The partial
      pressure of any component of a mixed gas is equal to the total pressure of the gas multiplied
      by the fraction of the gas that is made up by the component--approximately 78% for nitrogen
      and 21% for oxygen in air at all altitudes. As altitude increases, total air pressure
      decreases and consequently, the partial pressures of all component gases, including oxygen,
      decrease. As air is inhaled, it becomes saturated with water vapor from body tissues, further
      reducing its partial pressure of oxygen. Tissue oxygenation, measured in terms of partial
      pressure of oxygen in arterial blood (paO2), is directly related to the partial pressure of
      oxygen in the gas entering the lungs.

      The United States Federal Aviation Agency requires that commercial aircraft be designed so
      that the barometric pressure in the cabin at maximum cruise altitude exceeds that found in
      the external atmosphere at 8,000 feet (565 mm Hg).1 The scientific basis for this limit is
      unclear. Although there is a large body of knowledge concerning the effects of altitude on
      humans, most of it involves healthy young people at altitudes higher than 8,000 feet. An
      investigation of the effects of altitude on commercial airline passengers performed by
      McFarland in 1937 found "…older persons up to 72 years of age respond to moderate altitudes,
      i.e., up to 16,000 feet, without unusual difficulties." However, the same author went on to
      conclude, "The human factors analysis presented here would suggest that the comfort and
      well-being of airline passengers would be significantly benefited by as near sea level
      conditions as possible. In any event, cabin altitudes of 3,000 - 5,000 feet should not be
      exceeded." 2, 3 In 1986, the Committee on Airliner Cabin Air Quality of the National Research
      Council (NRC) concluded, "Similarly, the decrease in oxygen partial pressure (pO2) that
      occurs at 8,000 feet is safe for normal people, but possibly hazardous for patients with
      COPD." 4 In a report published in 2001, the NRC concluded that research into the effect of
      cabin pressure on susceptible persons should be given high priority.5

      Acute Mountain Sickness (AMS), a syndrome characterized by headache, anorexia, nausea,
      vomiting, lassitude and sleep disturbance, the onset of which closely follows ascent to
      altitude6 has been reported in 12 to 42% of visitors to altitudes ranging between 6,300 and
      10,000 feet in the mountains of Colorado.7-12 The time course of these symptoms was not
      reported in sufficient detail to allow characterization of their onset during the first
      several hours after arrival at altitude.

      Hypoxia-related medical problems during flight are rare..13-18 However, an unpublished
      statistical model 19 predicts that at 8,000 feet a substantial proportion of persons whose
      ages are similar to those of commercial aircraft passengers will experience paO2s below the
      levels at which use of supplemental oxygen is recommended by various medical
      authorities.20-24 If the recommendations for use of supplemental oxygen are valid,25, 26 it
      is possible that exposure to these levels of hypoxia for up to 20 hours (the maximum
      anticipated duration of commercial aircraft flight segments) could adversely affect the
      comfort and well being of passengers.

      Physical exercise has been demonstrated to reduce blood oxygenation in persons with normal
      and abnormal pulmonary health.27-29 Some members of the aircraft crew perform moderate levels
      of work as part of their job30-34, and exercise is recommended by some to alleviate the
      discomfort experienced as a result of the prolonged inactivity encountered in flight. It is
      unknown if moderate exercise has a beneficial or deleterious effect on the comfort and well
      being of persons at altitude.

      The terms "comfort" and "well being" are commonly used but imprecisely defined. "Comfort" has
      been defined as the "... circumstance under which one is able to concentrate fully on
      selected tasks. No attention is given to the maintaining of well being." 35 "Well being" is
      defined as "The state of being healthy, happy, or prosperous." 36 This investigation will
      measure the single concept of "comfort and well being" by using the following factors of the
      Environmental Symptoms Questionnaire IV:

      Factor 1, Cerebral Acute Mountain Sickness; Factor 2, Respiratory Acute Mountain Sickness;
      and Factor 5, Distress.37

      Low scores indicate a greater degree of comfort and well being than do high scores.

      Mild hypoxia has been reported to cause asymptomatic changes in oxygen saturation, heart
      rate, blood pressure, visual function, short term memory, and fine motor coordination.38-41
      Unpublished studies have found asymptomatic disorders of control of the pupillary muscles,
      possibly due to an imbalance between sympathetic and parasympathetic nervous control.42

      7. STUDY PLAN AND SCHEDULE

      7.1 Study environment

      Because of the size of the altitude chamber, multiple "runs" will be required at each
      altitude to evaluate the required number of test subjects. A "run" will consist of
      appropriate test subject orientation, screening to exclude persons with temporarily
      disqualifying health conditions (see Section 7.6.5), entry into the chamber, depressurization
      to the selected altitude, maintenance of that pressure for a period of time not to exceed 20
      hours during which outcome data will be collected and recorded, followed by repressurization
      to sea level, final data collection and recording, and dismissal. A follow up telephone
      interview will be conducted 5 to 7 days after the chamber run is completed (Attachment 6).
      The same test subjects will not be used at multiple altitudes.

      During the runs, test subjects will be seated in commercial aircraft seats and allowed to
      sleep, eat, or walk about as they desire within the physical constraints of the altitude
      chamber. Aircraft-type entertainment (magazines, books, videotapes) will be provided. Food
      service and restroom facilities will be available in the chamber. Five test subjects will be
      randomly selected to exercise on a treadmill ergometer at 3 mph for 10 minutes once an hour
      for 9 hours to simulate create a the metabolic workload of 2500 to 3300 kcal.

      Test subjects and in-chamber test personnel will be blinded to the altitude profile of the
      run. For test altitudes equal to ground level, brief depressurization and repressurization
      will be carried out to simulate the noise and pressure changes of a run at higher altitudes.

      The following environmental parameters will be maintained within the specified limits for
      each run:

      Pressure: Ground level Ambient +/- 2 mm Hg 2000' 707 mm Hg +/- 2 mm Hg 4000' 656 mm Hg +/- 2
      mm Hg 6000' 609 mm Hg +/- 2 mm Hg 7000' 586 mm Hg +/- 2 mm Hg 8000' 564 mm Hg +/- 2 mm Hg

      The low frequency with which medical emergencies are reported among airline passengers and
      visitors to mountain resorts suggests that exposure to altitudes are safe. Therefore,
      asymptomatic effects of altitude on physiologic parameters are not, a priori, considered
      sufficiently severe to warrant consideration of design change, and are not included in the
      altitude selection logic described in paragraph 7.3. Physiologic variables will be measured,
      however, to demonstrate whether or not the hypoxia encountered during the test runs is
      sufficiently severe to have any effect on the test subjects.

      7.2.1 The following outcomes are of primary interest:

      Prevalence and severity of the following symptoms: Measured by:

      Fainting Observation Shortness of breath Questionnaire Chest Pain Questionnaire Nausea
      Questionnaire Headache Questionnaire Malaise Questionnaire Short term memory dysfunction
      Questionnaire Fatigue Questionnaire Fine Motor Coordination Questionnaire

      Physiologic/performance measures Measured by:

      Heart Rate Pulse oximeter Arterial oxygen saturation Pulse oximeter Visual acuity Snellen eye
      chart Color perception FM 100 Hue Short term memory Kentucky Comprehensive Listening Test
      Fine Motor Coordination Purdue Pegboard

      7.2.2 Outcomes of secondary interest are:

      Prevalence and severity of the following symptoms: Measured by:

      Lightheadedness Questionnaire Dizziness Questionnaire Faintness Questionnaire Flushed
      Questionnaire Chills Questionnaire Sweaty Questionnaire Shivering Questionnaire Tingling
      Questionnaire Numbness Questionnaire Ear blockage Questionnaire Blurred vision Questionnaire
      Tunnel vision Questionnaire Sleepiness Questionnaire Nervousness Questionnaire Irritability
      Questionnaire Restlessness Questionnaire Depression Questionnaire Happiness Questionnaire
      Combativeness/aggressiveness Questionnaire

      An enhanced Environmental Symptoms Questionnaire (ESQ IV) will be used to ascertain symptom
      outcomes37. The enhancement consists of additional questions that elicit information
      concerning the test subjects' perceptions of the chamber environment, recent food and drink
      intake, and level of activity. It will be distributed prior to depressurization and
      periodically throughout the run to ascertain the prevalence of symptoms, and an in-chamber
      attendant will perform the physiologic and performance testing at selected times throughout
      the run.

      7.5 Location of study:

      The Center for Aerospace and Hyperbaric Medicine Oklahoma State University Center for Health
      Sciences 801 East 91st Tulsa Oklahoma 74132

      7.6 Test subjects:

      Test subjects will be recruited from Tulsa and the surrounding areas. Test subjects will be
      given $18.00 per hour by Oklahoma State University as compensation for their participation in
      this study.

      The following test subject selection criteria will be used to select a total of 216 test
      subjects, 108 of whom will be randomly assigned in age strata to 0 and 8,000'. Should the
      null hypothesis be rejected, 108 test subjects will be recruited for similar assignment to
      each test altitude. This will be repeated until the altitude selection algorithm has been
      completed.

      7.6.1. Age Distribution:52

      Age Sedentary Proportion Sedentary (63 subjects) Exercising Proportion Exercising (45
      subjects) Total # < 30 11% 7 28% 13 20 30 - 39.9 17% 10 42% 19 29 40 - 49.9 25% 16 24% 11 27
      50 - 59.9 25% 16 6% 2 18 60+ 22% 14 - - 14 Total 100% 63 100% 45 108

      7.6.2. Gender Distribution for test subjects will be approximately 50% male and 50% female.

      7.6.3. Have flown on commercial flights during past 60 months, but not for 3 or more hours in
      past 1 month.

      7.6.4. Have not been above 4,000' in past 1 month.

      7.6.5. Medical History

      Do not have a history of:

      Lung disease requiring medication or oxygen therapy at ground level Any of the following
      conditions at AMA Class 3 or worse Chronic Obstructive Lung Disease Chronic
      bronchitis/emphysema Asthma Restrictive Lung Diseases Any of the following cardiac conditions
      at AMA Class 3 or worse Coronary Artery Disease Arrhythmia Congestive Heart Failure
      Cerebrovascular disease Cerebrovascular accident (stroke) Lower Extremity Deep Vein
      Thrombosis Diabetes mellitus, poorly controlled (change in medication doseage/ frequency in
      past month) Seizure within past 6 months. Claustrophobia that would impair chamber
      participation Musculoskeletal disorder that impairs self care and self locomotion. Current
      anemia of any cause Sickle Cell Anemia Sickle Cell Trait Other conditions, which, in the
      opinion of the Principal Investigator, might result in excessive risk upon exposure to a
      hypobaric environment.

      Shall not, at the time of the test:

      have a contagious disease (including upper respiratory illness) be pregnant be within 4 weeks
      of a major surgical procedure. have dental abscesses have been scuba diving within 24 hours
      of test have donated blood within 24 hours of test be under the influence of drugs or alcohol

      Health status to be ascertained by questionnaire and interview before each chamber run.
      Applicants who successfully complete the health questionnaire will undergo a phyical
      examination by a qualified physician focusing on cardiovascular and pulmonary status before
      final determination of eligibility will be made. Normal cardiovascular and pulmonary
      examinations will be required for the applicant to be eligible for inclusion in the study.
      Test subjects over 40 years of age who will be exercising at altitude will be required to
      demonstrate their ability to exercise safely to 4.0 METS in a graded exercise tolerance test
      to be performed before the day of the altitude chamber run.

      A "PreRun" questionnaire will be administered the morning of the run before test subjects
      enter the altitude chamber to determine if the test subjects have developed any disqualifying
      conditions since undergoing the physician review. All PreRun questionnaires will be reviewed
      by a Physician Investigator.

      7.6.6. Do not have special dietary needs.

      7.6.7. Can read and speak English.

      All questionnaires, routine and emergency instructions will be given in English. There are no
      racial or ethnic exclusionary criteria.

      7.6.8. Body Size: Body mass index less than 35 and Height less than 74".

      The size of the commercial aircraft seats and the altitude chamber interior necessitate a
      size limitation for the safety and comfort of participants.

      7.6.9 Informed Consent: No applicant who has not voluntarily given informed consent to
      participate will be allowed to participate in this study. Applicants will be given a draft
      copy of the Informed Consent form to review when they are informed of the dates of
      orientation sessions. The Informed Consent will be explained in detail during the
      orientation, and all test subjects' questions concerning it and their participation in the
      study will be explained. Only Physician and Scientific Researchers will obtain informed
      consent from the candidates. All signed Informed Consent forms will be retained in accordance
      with the Boeing Safety, Health, and Environmental Master Records Retention Schedule.

      8. STUDY EQUIPMENT AND PROTOCOLS

      8.2 Protocols:

      The detailed timeline of each run is given in Attachment 4. No test equipment will be
      attached to the test subjects for the duration of the run. However, the detector of the pulse
      oximeter will be intermittently clipped to each test subject's finger to measure blood
      oxygenation. For each measurement, the detector will be attached for approximately 1 minute.
      Throughout the test protocol from hour 2.5 till hour 14, the test subjects will be provided
      with video entertainment and magazines, as they would have on a flight. Videos will be
      stopped during survey times. The test subjects will be asked to bring with them those items
      they normally take on a flight (books, work, etc.) and to wear clothing that they would wear
      for a 20 hour flight.

      At least one member of the team of investigators will be present in the altitude chamber at
      all times during the test runs to conduct the testing and to monitor the condition of the
      test subjects.

      Test subjects will be removed from the altitude chamber for the following reasons:

        1. Test subject request

        2. Any of the following medical conditions:

           Loss of consciousness Chest pain that persists more than 5 minutes Shortness of breath
           not relieved by reassurance Repeated vomiting

        3. Any other condition, which, in the judgment of the in-chamber observer or physician,
           warrants removal for the safety of the test subjects.

      Once a test subject leaves the chamber, he/she will not be allowed to return.

      Altitude chamber runs will be scheduled to begin (Hour 0) at 9:00 am. Test subjects will
      complete testing at approximately 6:30 am the following morning

      The duration of the runs may be decreased if it is found that symptom prevalence and severity
      plateau before 20 hours. This will be determined after all the sea level and 8,000' runs have
      been completed.

      8.3 Research Methodology, Sampling and Data Analysis

      8.3.1 Research Methodology

      To determine the effects of moderate altitude exposure on comfort and well being, 108 test
      subjects will be exposed to 8,000 foot-equivalent barometric pressure in an altitude chamber
      for 20 hours and their responses to symptom survey questions will be compared to responses
      from 100 test subjects exposed to ambient (ground) pressure for 20 hours in the same chamber.

      The study will employ a Split-Split Plot Factorial design. The chamber runs are the
      whole-plot units and the altitude (0', 8,000' and any intermediate altitudes) is the
      whole-plot factor, a test subject is the split-plot unit with exercise as the split-plot
      factor [Ex (exercise) vs Sed (sedentary)], and the repeated measurements on each test subject
      represent the split-split unit with exposure time as the split-split plot factor. So under
      this design the appropriate experimental unit for testing altitude differences is a chamber
      run. The repeated measures in this study include the responses to the symptom items on the
      questionnaires, which will be administered x times over the 20-hour time period, and the
      measurements of PaO2, heart rate, etc., which will also be taken repeatedly over the time of
      the study.

      8.3.2 Sampling

      Two hundred sixteen test subjects will serve in the study-approximately half will be female
      and approximately half will be male. The age distribution breakdown is given in the table
      below (note that for a given age range, approximately half of the test subjects will be male
      and approximately half will be female). First, 216 test subjects will be selected that fit
      the distribution given below. Then test subjects of a given gender within a given age range
      will be randomly assigned to one of four conditions (0' Condition Ex; 0' Condition Sed;
      8,000' Condition Ex; and 8,000' Condition Sed). See Attachment 8 for sample size
      considerations.

      0' Condition 8,000' Condition Age # of Subj Ex M/F Sed M/F Ex M/F Sed M/F Ex M/F Sed M/F < 30
      40 13/13 7/7 6/7 4/3 7/6 3/4 30 - 39.9 58 19/19 10/10 10/9 5/5 9/10 5/5 40 - 49.9 54 11/11
      16/16 5/6 8/8 6/5 8/8 50 - 59.9 36 2/2 16/16 1/1 8/8 1/1 8/8 60+ 28 - 14/14 - 7/7 - 7/7 Total
      216 45/45 63/63 22/23 32/31 22/23 31/32

      8.3.3 Data Collection and Confidentiality

      Survey and telephone interview forms used for collection of test subject responses are shown
      in Attachment 5. Data recording forms are shown in Attachment 6. The test subjects will be
      identified on the survey forms by Test Subject Number alone. Data will be entered into an
      electronic spread sheet using the Test Subject Number as identifier. To insure data
      integrity, a spreadsheet linking Test Subject Number to name of test subject will be
      available to the investigators. Spread sheets will reside on file servers protected by
      password security access. When not in use, survey forms, telephone interview forms, and
      laptops will be stored in locked cabinets. Paper records will be retained by The Boeing
      Company and destroyed after 10 years. Electronic data will be stored in password protected
      computers, or in similarly protected electronic format, indefinitely.

      In preparation for data analysis, test subject responses and demographic information will be
      merged using Test Subject Number, but names will not be included. Only grouped results will
      be published. Nothing will be published in a form in which the test subject from which the
      information arose can be identified.

      8.3.4 Data Analysis

      The symptom data will be analyzed for significant differences between the treatments by first
      calculating AMS-R, AMS-C and AMS-D from the EQS questions. We will carry out the appropriate
      Analysis of Variance (ANOVA) for the split-split plot factorial design to look at differences
      due to altitude and main effects due to activity (Ex vs Sed) and interaction effects between
      altitude and exercise. This ANOVA will also analyze the within test subject scores to
      determine if symptom reporting changes over time. Second, the symptoms data will be recoded
      to "no symptom=0" (0 response on the symptom scale) and "symptom=1" (a 1-6 response on the
      symptom scale). These data will be analyzed for main effects and interaction effects between
      altitude and exercise.

      The other dependent measures (SaO2, heart rate, etc.) will be analyzed using the appropriate
      ANOVA for the split-split plot factorial design. These data will also be analyzed for main
      effects (altitude and activity) and for changes over time, and for the altitude/activity
      interaction.
    

Intervention name(s): Hypobaric hypoxia
Intervention type(s): Procedure
Allocation: Randomized
Intervention model: Parallel Assignment
Masking: Single
Keywords: hypoxia; hypobaric; altitude