Introduction:

  • Hypoxia is the lack of sufficient oxygen in the blood, tissues, and/or cells to maintain normal physiological function11
  • While most often associated with higher altitudes, there are several causes of hypoxia
    • Depending on the cause, pilots experience various types of hypoxia
  • Symptoms can be difficult to detect, especially when flying as a single-pilot but left untreated can quickly turn deadly
  • Several other factors can contribute to hypoxia, which can put a pilot at greater risk
  • While easily treated, prevention is straightforward, and with proper training, you can be more aware of the effects of hypoxia
  • Further, oxygen requirements will help curb risk and serve as prevention
  • Several case studies have shown that hypoxia is not only possible, it can be deadly

Possible Causes of Hypoxia:

  • Leaky oxygen system
  • Inoperative oxygen mask
  • Faulty oxygen regulator
  • Carbon monoxide poisoning
  • Excessive time at altitude

Types of Hypoxia:

  • There are four types of hypoxia:
    1. Hypoxic
    2. Hypemic
    3. Stagnant
    4. Histotoxic

    1. Hypoxic Hypoxia:

      • Also referred to as altitude hypoxia, hypoxic hypoxia is the lack of oxygen absorbed by the body due to atmospheric conditions
      • As pressure altitude increases, the partial pressure of oxygen decreases along with blood oxygen saturation
        • Note that it is a significant decrease in pressure that leads to the body's inability to absorb the oxygen
        • The concentration of oxygen in the atmosphere remains consistent at all altitudes (about 21%), whereas pressure is reduced 50% by 18,000'
      • The Earth loses 50% of its atmospheric pressure by 18,000' and 75% by 34,000'
      • Can occur due to faulty equipment, malfunctions, or improper use
    2. Hypemic Hypoxia:

      • It occurs when the blood is not able to carry a sufficient amount of oxygen to the body's cells
      • Caused by anemia, disease, blood loss, deformed blood cells, or carbon monoxide (CO) poisoning and with smokers
      • CO attaches itself to hemoglobin about 200 times more easily than oxygen
      • After CO poisoning, it can take up to 24 hours to recover
      • It can be a result of donating blood, resulting in a higher physiological altitude
    3. Stagnant Hypoxia:

      • Oxygen deficiency in the body due to poor circulation of the blood
      • Can occur from pulling excessive Gs or cold (constricting blood vessels) temperatures may reduce blood to extremities
      • May cause hyperventilation
    4. Histotoxic Hypoxia:

      • The inability of the body to use oxygen
      • Caused by alcohol and other drugs such as narcotics and poisons

Hypoxia Symptoms:

  • Because of wide individual variations in susceptibility to hypoxia, it is impossible to predict precisely when, where, or how hypoxia reactions will occur in each pilot
  • As a general rule, however, flights below 10,000 feet MSL without the use of supplemental oxygen can be considered safe, though night vision is particularly critical, and impairment of sight can occur at lower altitudes - especially for heavy smokers
  • The onset of hypoxia is insidious and progresses slowly, with symptoms including:
    • Euphoria
    • Headache
    • Increased response time
    • Impaired judgment
    • Drowsiness
    • Dizziness
    • Tingling in fingers and toes
    • Numbness
    • Blue fingernails and lips (cyanosis)
    • Limp muscles
    • Confusion or foggy decision making

Debilitating Effects:

  • Time of Useful Consciousness
    Time of Useful Consciousness
  • The effects of hypoxia are usually quite challenging to recognize, especially when they occur gradually [Figure 1]
  • This onset (gradual or rapid) and altitude will have a direct effect on the Time of Useful Consciousness (TUC) and Effective Performance Time (EPT)
    • Time of Useful Consciousness: refers to the pilot's ability to remain conscious when exposed to high-pressure altitudes [Figure 2]
      • Realize published times are in reference to a rapid decompression event
    • Effective Performance Time: refers to a pilot's ability to function, regardless of consciousness
  • The effects appear following increasingly shorter periods of exposure to the increasing altitude
    • Pilot performance can seriously deteriorate within 15 minutes at 15,000 feet
  • Although deterioration in night vision occurs at a cabin pressure altitude as low as 5,000', other significant effects of altitude hypoxia usually do not happen in the typical healthy pilot below 12,000'
  • From 12,000 to 15,000' of altitude, judgment, memory, alertness, coordination, and ability to make calculations are impaired, and headache, drowsiness, dizziness, and either a sense of well-being (euphoria) or belligerence occur
  • At cabin pressure altitudes above 15,000', the periphery of the visual field grays out to a point where only central vision remains (tunnel vision)
  • Time of Useful Consciousness
    Time of Useful Consciousness
  • 4 Stages of Hypoxia
    Four Stages of Hypoxia

Contributing Factors:

  • Several factors can lower the altitude at which significant effects of hypoxia occur
    • Carbon monoxide inhaled in smoking or from exhaust fumes, lowered hemoglobin (anemia), and certain medications can reduce the oxygen-carrying capacity of the blood to the degree that the amount of oxygen provided to body tissues will already be equivalent to the oxygen supplied to the tissues when exposed to a cabin pressure altitude of several thousand feet
    • Rate of pressure change
      • Oxygen is given up through rapid decompression causing loss of TUC by up to 50%
    • Small amounts of alcohol and low doses of certain drugs, such as antihistamines, tranquilizers, sedatives, and analgesics can, through their depressant action, render the brain much more susceptible to hypoxia
    • Extreme heat and cold, fever, and anxiety increase the body's demand for oxygen, and hence its susceptibility to hypoxia
    • Duration of exposure
    • Individual tolerances
    • Physical activity
      • If the pilot is physically fit and there are no fumes in the cockpit, the situation will probably never occur below 10,000'
    • Self-imposed stress

Treating Hypoxia:

  • Inform your instructor/crew
  • Descend to 10,000' or below
  • Select the emergency position with the diluter lever (gang load)
  • Slow breathing rate by counting to four or five between breaths
  • Check connections/equipment

Preventing Hypoxia:

  • Portable Fingertip Pulse Oximeter
    Amazon, Portable Fingertip Pulse Oximeter
  • Garmin D2 Delta PX, GPS Pilot Watch with Pulse Ox Sensor
    Amazon, Garmin D2 Delta PX,
    GPS Pilot Watch with Pulse Ox Sensor
  • Pilots lose the ability to take corrective and protective action in 20 to 30 minutes at 18,000 feet and 5 to 12 minutes at 20,000 feet, followed soon after that by unconsciousness
  • Pilots can best prevent hypoxia by heeding factors that reduce tolerance to increases in altitude (decreases in pressure) by enriching the inspired air with oxygen from an appropriate oxygen system and by maintaining a comfortable, safe cabin pressure altitude
    • Medical devices such as oximeters can indirectly monitor the oxygen saturation of passengers [Figure 3/4]
  • Avoid:
    • Smoking or exposure to exhaust fumes
    • Medications
    • Alcohol

Hypoxia Training:

  • Hypobaric Chamber
    Military Hypobaric Chamber
  • Since symptoms of hypoxia vary in an individual, the ability to recognize hypoxia can be greatly improved by experiencing and witnessing the effects of hypoxia during an altitude chamber "flight" [Video 1]
  • The Federal Aviation Administration (FAA) provides this opportunity through aviation physiology training conducted at the FAA Civil Aeromedical Institute and many military facilities across the U.S. [Figure 5]
  • To attend the Physiological Training Program, contact the Civil Aeromedical Institute, Mike Monroney Aeronautical Center, Oklahoma City, OK
    • Aerospace Medical Education Division, AAM-400, CAMI
    • Mike Monroney Aeronautical Center, P.O. Box 25082
    • Oklahoma City, OK 73125
    • Telephone: (405) 954-6212
  • Attendance of the Physiological Training Program requires an application form and a fee
  • Particulars about location, fees, scheduling procedures, course content, individual requirements, etc., are contained in the Physiological Training Application, Form Number AC 3150-7, obtained by contacting the accident prevention specialist or the office forms manager in the nearest FAA office
  • Hypobaric Chamber
    Military Hypobaric Chamber

Oxygen Requirements:

  • For optimum protection, pilots are encouraged to use supplemental oxygen above 10,000' during the day and above 5,000' at night
  • Federal Aviation Regulations 91.211 requires that at the minimum, flight crew be provided with and use supplemental oxygen after 30 minutes of exposure to cabin pressure altitudes between 12,500 and 14,000' and immediately on exposure to cabin pressure altitudes above 14,000'
  • Every occupant of the aircraft must be provided with supplemental oxygen at cabin pressure altitudes above 15,000'

Hypoxia Case Studies:

  • Lear Jet N47BA Flight Path After Becoming Hypoxic
    Lear Jet N47BA Flight Path After Becoming Hypoxic
  • NTSB Identification: CEN12FA571: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The student pilot's impairment from alcohol, marijuana, and hypoxia, which adversely affected his ability to maintain control of the airplane
  • NTSB Identification: WPR12FA154: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The in-flight loss of control due to the pilot's impairment as a result of hypoxia. Contributing to the accident was the pilot's operation of the airplane above 12,500' without the aid of supplemental oxygen
  • NTSB Identification: CEN09LA527: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The in-flight loss of control due to the pilot's impairment as a result of hypoxia. Contributing to the accident was the pilot's decision to operate the unpressurized airplane at an altitude requiring supplemental oxygen without having any oxygen available
  • NTSB Identification: ERA09FA429: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's improper modification of the certified, on-board oxygen system, which resulted in incapacitation due to hypoxia, and the airplane's subsequent uncontrolled descent into terrain
  • NTSB Identification: DCA00MA005: The National Transportation Safety Board determines the probable cause(s) of this accident as follows: Incapacitation of the flight crew-members as a result of their failure to receive supplemental oxygen following a loss of cabin pressurization, for undetermined reasons [Figure 6]
  • Lear Jet N47BA Flight Path After Becoming Hypoxic
    Lear Jet N47BA Flight Path After Becoming Hypoxic

Conclusion:

  • Note that human performance is affected by pressure altitude
    • Remember, there is no less oxygen at altitude than at sea level, but rather the pressure of that air and the body's ability to absorb it changes
    • Compliance therefore does not necessarily equate to safety
  • The body's respiratory drive responds primarily to carbon dioxide and only weakly to oxygen levels, which makes the onset of hypoxia insidious
  • Note the connection between supplemental oxygen requirements and hypoxia in case studies
    • These rules are in place because others have died from it
    • Recognize judgement will be impaired when hypoxic
  • Although the regulations spell out specific oxygen requirements, a simple way to remember and conservative practice is to use oxygen above 5,000' at night and 10,000' during the day
  • In fact, a high-altitude endrosement is required if acting as pilot in command of a pressurized airplane with a service ceiling or maximum operating altitude above 25,000 feet
  • Training may be made available through the FAA/Military to help pilots understand their body's response to hypoxia
  • Rapid decompression is immediately recognizable and therefore easier to respond to whereas a gradual decompression is harder to detect, increasing the risk of hypoxia with no clear warnings
  • If you or your passengers experience signs or symptoms of hypoxia, report it! So that inspections on the aircraft can occur, and engineering inspections, if required, are initiated
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