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Hypoxia

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 in fact several causes of hypoxia
    • Depending on the cause, various types of hypoxia are experienced as a result
  • Symptoms can be difficult to detect, especially when flying as as 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 easier and with the proper training you can be more aware of the effects of hypoxia
Hypobaric Chamber
Figure 1: Military Hypobaric Chamber

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 4 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 atmospheric conditions
      • As pressure altitude increases, the partial pressure of oxygen decreases along with blood oxygen saturation
        • Note that it is the significant decrease in pressure that leads to the body's inability to absorb the oxygen
        • The concentration of oxygen in the atmosphere remains about 21% from the ground out to space whereas pressure is reduced 50% by 18,000'
      • 50% of Earth's atmospheric pressure is lost by 18,000' and 75% by 34,000'
      • Can occur due to faulty equipment, malfunctions or improper use
    2. Hypemic Hypoxia:

      • 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
      • 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 (constructing blood vessels) temperatures may reduce blood to extremities
      • May cause hyperventilation
    4. Histotoxic Hypoxia:

      • Inability for the body to use oxygen
      • Caused by alcohol and other drugs such as narcotics and poisons
4 Stages of Hypoxia
Figure 2: Four Stages of Hypoxia
Figure 3: Altitude Chamber

Hypoxia Symptoms:

  • Because of wide individual variations in susceptibility to hypoxia, it is impossible to predict exactly 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
  • Symptoms include:

    • Euphoria
    • Headache
    • Increased response time
    • Impaired judgment
    • Drowsiness
    • Dizziness
    • Tingling in fingers and toes
    • Numbness
    • Blue fingernails and lips (cyanosis)
    • Limp muscles

Debilitating Effects:

  • The effects of hypoxia are usually quite difficult to recognize, especially when they occur gradually
  • This onset 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
    • Effective Performance Time: refers to a pilot's ability to function, regardless of consciousness
  • The effects appear following increasingly shorter periods of exposure to increasing altitude. In fact, pilot performance can seriously deteriorate within 15 minutes at 15,000 feet
  • Although a deterioration in night vision occurs at a cabin pressure altitude as low as 5,000', other significant effects of altitude hypoxia usually do not occur in the normal 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)
  • 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

Contributing Factors:

  • The altitude at which significant effects of hypoxia occur can be lowered by a number of factors
    • 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 provided to the tissues when exposed to a cabin pressure altitude of several thousand feet
    • Rate of pressure change
      • Oxygen actually 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
Time of Useful Consciousness
Figure 4: Time of Useful Consciousness

Treatment:

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

Prevention:

  • Hypoxia is best prevented 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 monitors the oxygen saturation of a passengers
  • Avoid:
    • Smoking or exposure to exhaust fumes
    • Medications
    • Alcohol

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'

Training:

  • 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, which is conducted at the FAA Civil Aeromedical Institute and at many military facilities across the U.S.
  • 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, which is obtained by contacting the accident prevention specialist or the office forms manager in the nearest FAA office

Case Studies:

  • 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 5]

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
  • 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 our case studies
    • These rules are in place because others have died from it
  • Training is available through the FAA/Military to help pilots understand their body's reaction to hypoxia
  • If you or your passengers experience signs or symptoms of hypoxia, report it! So that inspections on the aircraft can occur and engineering inspections can be initiated, as required
Lear Jet N47BA Flight Path After Becoming Hypoxic
Figure 5: Lear Jet N47BA Flight Path After Becoming Hypoxic

References:

  1. Advisory Circular 61-21A (Chapter 1) Hypoxia
  2. Advisory Circular (61-107) Operations of Aircraft at Altitudes Above 25,000 Feet and/or Mach Numbers (MMO) Greater Than .75
  3. Aeronautical Information Manual (8-1-2) Effects of Altitude
  4. Aircraft Owners and Pilots Association - Form and Function
  5. Aviation Medicine Blog
  6. CFI Notebook.net - Carbon Monoxide Poisoning
  7. CFI Notebook.net - Hyperventilation
  8. CFI Notebook.net - Hypoxia Lesson Plan
  9. CFI Notebook.net - Supplemental Oxygen
  10. Federal Aviation Administration - Aerospace Physiology Training
  11. Federal Aviation Administration - Beware of Hypoxia
  12. Federal Aviation Administration - Hyperventilation
  13. Federal Aviation Regulations (91.211) Supplemental Oxygen
  14. Flying Magazine - Technicalities: Hypoxia at Your Fingertips
  15. National Transportation Safety Board (NTSB)