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11.12.1 Hypoxia


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The Effects Of Altitude

The atmosphere is a mixture of gases of constant proportions up to an altitude of 60,000 ft. The approximate figures are:

  • Oxygen 21%
  • Nitrogen 78%
  • Other gases 1%

As altitude increases, pressure and density decrease and the amount of Oxygen available to the red blood cells decreases.

Two gases cause further complicating factors:

  • Water Vapour: Ever present in the atmosphere, water vapour content varies depending upon the climatic conditions. In the lungs, the alveolar air is always saturated with water vapour. This accounts for 6% of the volume of air in the lungs at sea level.
  • Carbon Dioxide: The amount of carbon dioxide in the atmosphere is approximately 0.03%. In the lungs, because of the respiration process, the amount of CO2 is higher; equivalent to 5.5% of the available volume at ground level.

These gases have to be taken into account when considering the amount of Oxygen available to the respiration process. At sea level, because of the amount of water vapour and CO2, the volume of Oxygen in the lungs available for the respiration process is reduced to 14.5%.

Tracheal Air

When inhaled air is drawn into the respiratory passages, it becomes saturated with water vapour and is warmed to body temperature. This water vapour has a constant pressure of 47 mmHg at normal body temperature. This is regardless of the barometric pressure. The inspired gases available for the respiration process are reduced by the amount of water vapour present.

Alveolar Air

The tracheal air enters the lungs and Oxygen and CO2 are exchanged in the respiration process. The expired air has less Oxygen and more carbon dioxide content. The partial pressure of O2 (ppO2) in the alveoli varies with the CO2 partial pressure. A constant, ventilation rate creates a CO2 partial pressure of approximately 40 mmHg. Using these values the ppO2 at any altitude can be calculated. In the transition from tracheal air to alveolar air, the ppO2 is reduced and ppCO2 is increased. We assume that the ppN2 remains constant.

A pressure gradient is required to ensure that Oxygen diffuses from the alveoli into the red blood cells. If this pressure gradient falls, Oxygen movement into the blood is impaired. Some degree of protection is given to the body up to 10,000 ft because of the affinity of haemoglobin for Oxygen. The body has a "surplus" of Oxygen for use to this height.

Above 10,000 ft the partial pressure of Oxygen in the alveoli falls off rapidly and the over protection is lost. The body begins to suffer from a lack of Oxygen; a process known as Hypoxia.


Hypoxia occurs when the Oxygen available in the blood supply is insufficient to meet body tissues needs. The greatest risk of Hypoxia to a pilot is normally a result of a rise in altitude associated with a fall in pressure. Early signs of Hypoxia are related to the higher mental functions and are similar to those of alcohol. The rate of onset depends on the altitude:

  • 15,000 ft: The signs and symptoms are relatively slow in onset and difficult to detect.
  • 40,000 ft: The signs and symptoms are so quick that an individual may not recognise what is happening.

In 1979 a Beech Super King Air was flying westwards at FL310 along the south coast of England on a conversion exercise. As it approached Exeter the crew asked ATC for permission to practise an emergency descent. This was granted and they were instructed to execute a right hand turn and contact Exeter ATC as they initiated descent. The crew acknowledged this instruction, adding that they 'would be out of contact for a few seconds as they would be donning masks and things'. Shortly afterwards the aircraft entered a turn to the left, which became a left orbit. The aircraft continued to orbit left for the next 6 hours, slowly drifting southwards with the wind until it crashed in north east France. No further contact had been made with the crew.

During the investigation into the accident it was discovered that the training captain had, whilst conducting such flights with previous students, actually depressurised the aircraft and Oxygen masks had been really necessary. Examination of the wreckage revealed that the pilots had donned their masks but that the mask hoses had not been fully connected to the Oxygen supply system. Further testing in an identical aircraft depressurised at FL 300, with descent initiated as soon as the test commenced, revealed that a doctor taking his mask off at such an altitude was rendered incapable after 15 seconds and unconscious after 30 seconds.

In this accident when the crew were breathing air these test times would have been reduced by a significant amount, causing rapid onset of Hypoxia with death following in a few minutes.

Signs and Symptoms of Hypoxia

Mild Hypoxia may produce a state similar to drunkenness. More serious cases will result in coma. All episodes of Hypoxia are damaging to tissues. If exposure is prolonged then damage may be permanent; the most vulnerable area being the brain.

At normal body temperatures the brain is unable to tolerate total lack of Oxygen for more than 3 minutes without irreversible damage. The symptoms of Hypoxia are many and individuals will differ in their reactions to the onset. The symptoms are listed below:

  • Personality Change: Changes in behaviour occur. The mild mannered may become aggressive in nature. A "Laissez Faire" attitude is also apparent at this stage.
  • Impaired Judgement: Lack of self-criticism. The sufferer is usually the last person to see any deterioration in performance.
  • Muscular Impairment: The pilot begins to lose muscular co-ordination. Accurate flying becomes difficult. Minor errors quickly turn into major events.
  • Memory Impairment: Short term memory is lost. Simple arithmetic problems become difficult and accuracy in calculation is difficult. Long term memory actions can still be accessed.
  • Sensory Loss: Colour vision is affected very early in the onset of Hypoxia. Touch becomes dull,hearing becomes limited and spatial orientation problems may occur.
  • Cyanosis: The extremities of the body become blue in colour. Haemoglobin in the de-oxygenated state gives the capillaries this bluish tinge.
  • Hyperventilation: As a pilot begins to suffer from the onset of Hypoxia the need for Oxygen results in a tendency to overbreathe.

Other sensations include tingling or warm sensations, sweating, headache and nausea. All the above symptoms will be experienced by a person suffering from Hypoxia; however, each person will exhibit his own symptom pattern which occurs on each exposure to Hypoxia.

Impairment of Consciousness

As Hypoxia progresses so does an individual's level of consciousness. Initial confusion is followed by semiconsciousness and unconsciousness. Without Oxygen, DEATH will follow.

Forms of Hypoxia

There are three forms of Hypoxia, Anaemic, Stagnant and Histotoxic Hypoxia.

Hypoxic Hypoxia

Caused by an insufficient partial pressure of Oxygen in the inspired air. This reduction of Oxygen becomes apparent above an altitude of 10,000 ft. Most likely in aviation when an aircraft has a decompression.

Anaemic Hypoxia

Anaemic Hypoxia, also known as Hypaemic Hypoxia, is caused by a reduction in the Oxygen carrying capacity of the blood. This reduction can be caused by a lowering in the amount of circulating haemoglobin, Anaemia. Haemoglobin forming a bond with carbon monoxide produces the same result.

Stagnant Hypoxia

Defined as an Oxygen deficiency in the body due to poor blood circulation. Caused by a failure of the circulatory system. When flying, this type of Hypoxia, can be caused by problems such as pressure breathing or excessive "G" forces.

Histotoxic Hypoxia

The inability of the body to utilize Oxygen. Caused by a failure of the body tissues to use the available Oxygen efficiently because of impairment to cellular respiration. Poisons such as drugs and alcohol are the usual cause.

Oxygen Requirements

As altitude increases the Oxygen pressure decreases:

  • By 8,000 ft the atmospheric pressure is only ¾ of the sea level pressure.
  • At 18,000 ft the atmospheric pressure is ½ that at sea level.
  • By 33,500 the atmospheric pressure is ¼ of the sea level pressure.

As altitude increases, the percentage of Oxygen that needs to be added to the gas a pilot breathes needs to increase to ensure that the alveolar partial pressure is maintained.

Alveolar Partial Pressure:

  • Sea Level 103 mm Hg
  • 10,000 ft 61 mm Hg

Above 10,000 ft extra Oxygen needs to be added. The percentage of Oxygen added increases until 33,700 ft where 100% Oxygen is required to give the equivalent alveolar partial pressure to that at sea level (103 mmHg). Above this height the partial pressure can be allowed to fall to the 10,000 ft equivalent of 61mmHg - this occurs at 40,000 ft. Above 40,000 ft positive pressure breathing, the forcing of Oxygen under pressure into the lungs, is required.

Stages of Hypoxia

There are four stages of Hypoxia which vary according to the altitude and the severity of symptoms.

Indifferent Stage

Night vision shows the effects of Hypoxia. A loss of 40% of night vision can be experienced at altitudes as low as 4000 ft.

Compensatory Stage

The circulatory and respiratory system provide a defence against Hypoxia. Pulse rate, systolic blood pressure, circulation rate, and cardiac output increase to offset the lack of Oxygen. Respiration will increase in rate and depth. At 12 to 15 000 feet the effects of Hypoxia on the nervous system are increasingly apparent. After 10 to 15 minutes, the impairment in efficiency becomes obvious. Crewmembers start to become drowsy and frequent errors of judgement are made. Simple tasks become difficult, especially those requiring alertness or moderate muscular co-ordination. At these altitudes Hypoxia is slow in onset and is difficult to detect especially in the hard working environment of the modern cockpit.

Disturbance Stage

The body can no longer compensate for the Oxygen deficiency. Occasionally, pilots become unconscious from Hypoxia without undergoing the subjective symptoms; Fatigue, sleepiness, dizziness, headache, breathlessness, and euphoria are the symptoms most often reported. However, the symptoms above are all valid.

Critical Stage

Within three to five minutes, judgement and co-ordination usually deteriorate. Subsequently, mental confusion, dizziness, incapacitation, and unconsciousness occur.

Susceptibility to Hypoxia

Susceptibility to Hypoxia can be increased by the following:

  • Altitude: At higher altitudes Hypoxia onset can be measured in seconds not minutes.
  • Time: The longer the pilot is without Oxygen the greater the effect.
  • Exercise: Exercise increases the need for the body to produce more energy. Hence, the need for more Oxygen.
  • Cold: When cold, the body uses energy to get warm. To warm the body heat is generated from the oxidation of food.
  • Illness: Illness increases the demands on the body's need for energy.
  • Fatigue: Tiredness and fatigue lower the body's resistance to the onset of Hypoxia.
  • Drugs/Alcohol: Hypoxia impairs the body's higher mental functions. Drugs and alcohol have a similar effect. The combination of the two has an obvious cumulative effect.
  • Smoking: CO has a greater affinity for haemoglobin than Oxygen. By reducing the amount of haemoglobin available for an Oxygen bond the body is already part way to being Hypoxic.
Time of Useful Consciousness

The definition of the Time of Useful Cosciousness is accepted as:

The time available to a pilot to recognise the development of Hypoxia and do something about it.

Limitations of Time

Time of useful consciousness:

  • 18,000 ft 30 Minutes
  • 25,000 ft 2 - 3 Minutes
  • 30,000 ft 45 - 75 Seconds
  • 35,000 ft 30 seconds
  • 40,000 ft 10 - 20 Seconds
  • 45,000 ft 12 Seconds

An aircraft not equipped with Oxygen should not fly at altitudes above 10,000 feet for extended periods of time. An unpressurised aircraft should not exceed 14,000 ft without supplemental Oxygen being used.

Treatment of Hypoxia and Hyperventilation

The symptoms of Hypoxia and Hyperventilation are so similar that to differentiate between them can be difficult. Use the following guidelines:

  • Above 10,000 ft: Assume Hypoxia, Oxygen must be given to the sufferer. A descent below 10,000 ft is essential.
  • Below 10,000 ft: Hypoxia should not be a problem except in those people who are old or have respiratory problems. The rate and depth of breathing should be slowed down. If hyperventilation is identified as the problem then re-breathing the expired air can help the recovery. Restricting the breathing by use of a sick-bag or Oxygen mask are common methods used.