Flying into Thin Air: Neurological Effects of Hypoxia

The most important effects of hypoxia is on Central Nervous System (CNS) and vision. This insult to CNS which affects the performance in flight varies as per the altitude and the resulting stages of hypoxia. The effects are discussed as per the stages of hypoxia.

Indifferent Stage.  One of the earliest symptoms of Hypoxia is its effect on the eyes. Vision especially night vision, is known to deteriorate even at altitudes less than 5,000 ft. Night vision may be lost by 5-10% at 5,000 ft and may be compromised by 25% at 12,000 ft. Other symptoms of hypoxia may also manifest if the human body is compromised, say due to anaemia or cigarette smoking or on exposure to carbon monoxide due to fumes in the cockpit. But the aircrew may not be aware that s-/he is hypoxic.

Compensatory stage.   During this stage the body and mind can be severely affected but in subtle ways. At 12,000 to 15,000 ft, the effects of hypoxia on the nervous system can become increasingly incapacitating. Even if as little as 10 to 15 min are spent at this altitude, impairment of psychomotor skills become evident. In addition there is a false sense of well being and euphoria, which may not allow the aviator to recognise or acknowledge that there is compromised performance due to hypoxia. There may be other symptoms as well, including drowsiness, pjudgment and frequent subtle errors in flying.

However compensatory actions by the circulatory and respiratory system can provide some defence against hypoxia during this stage. This happens by increase in heart rate; increased and more forceful pumping of blood by the heart to improve circulation; and increased rate and depth of respiration.Although compensatory responses occur spontaneously, once aware about the threat of hypoxia, the pilot must take conscious corrective actions promptly.

Disturbance Stage.   In this stage, the physiological compensations do not suffice to provide adequate oxygen to the tissues. Subjective symptoms may include fatigue, lassitude (weariness of mind and body), somnolence (sleepiness), dizziness, headache, breathlessness, and euphoria. Occasionally there are no subjective sensations up to the time of loss of consciousness. Objective symptoms are:-

  • Special Senses. The visual acuity is diminished and peripheral and central vision is impaired. Muscles responsible for movements of the eyeballs are weak, and the range of accommodation for near vision is decreased. Touch and pain are diminished or lost. Hearing, incidentally, is one of the last senses to be lost.
  • Mental Process.  Intellectual impairment is an early sign and makes it impossible for the individual to comprehend his own disability. Thinking is slow, and calculations unreliable. Memory is faulty, particularly for events in the immediate past. Judgment is poor. Reaction time is delayed.
  • Personality Traits.  There may be release of basic personality traits and emotions akin to what happens with intoxication with alcoholic. There may be euphoria, elation, overconfidence, or an individual may become morose.
  • Psychomotor functions.  There is poor muscular coordination. Delicate or fine muscular movements may be impossible. This results in stammering, illegible hand writing, and poor coordination in aerobatics and formation flying.
  • Hyperventilation syndrome.

Critical Stage.  This is the stage at which consciousness is lost. This may be the result of circulatory or central nervous system failure. The former is more common with prolonged hypoxia, the latter with acute hypoxia. With either type, there may be convulsions (fits) and eventual failure of the respiratory center in the brain, which if not corrected can lead to death.

An important concept to remember here is Time of Useful Consciousness (TUC).

TUC is defined as the period of time which elapses between the moments an aviator is exposed to reduced oxygen tension till there is impairment of performance of useful flying duties, due to hypoxic insult. This is the crucial window period, from the moment of interruption of oxygen supply at critical altitudes, during which a pilot can comprehend the emergency and take corrective action before being incapacitated due to hypoxia. Two major factors that dramatically reduce the TUC are Rapid decompression (RDC) and physical exertion. Certain other factors that influence TUC are individual endurance, fitness, age, extent of training environmental temperature and degree of oxygenation prior to the exposure.

The most useful concept is to regard the time of useful consciousness as the period during which an affected pilot can act to correct (her)his predicament. TUC is also called as Effective Performance time (EPT). Table below shows TUC (EPT) at different altitude.

Read the Third and Final part: Flying into Thin Air: Preventing Hypoxia


1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.

2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.

3. Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001.

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