Aerospace Medicine

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High Speed

In itself, high speed does not produce harmful symptoms. What can be dangerous are high acceleration or deceleration forces; these are expressed as multiples of gravity, or g’s. In pulling out of a dive, for example, a pilot may be subjected to an inertial force as high as 9 g. If a force of 4 to 6 g is sustained for more than a few seconds, the resulting symptoms range from visual impairment to total blackout. Protection is provided by a specially designed outfit, called an anti-g suit, which supplies pressure to the abdomen and legs, thus counteracting the tendency for blood to accumulate in those areas. Proper support of the head is essential during extreme deceleration in order to avoid swelling of the sinuses and severe headaches. While facing backward in a seated position, properly supported human test subjects have been able to tolerate a deceleration force of 50 g without severe injury.

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Oxygen Supply

A critical consideration in aircraft travel is the continuing physiological requirement for oxygen. The only oxygen stored by the body is that in the bloodstream. Although muscles can function temporarily without oxygen, the buildup of toxic products soon limits activity. Brain and eye tissues are the most sensitive to oxygen deficiency.

The atmosphere, which contains 21 percent of oxygen by volume, is under a normal sea-level pressure of 1,013 millibars (14.7 lb/sq in). The barometric pressure (see Barometer) up to about 4575 m (about 15,000 ft) is sufficient to sustain human life. Above this altitude the air must be artificially put under pressure to meet the respiratory needs of human beings.

High-altitude military airplanes are provided with oxygen equipment, and military personnel are required to use it at all times when participating in flight above 3050 m (10,000 ft). Military craft that can fly above 10,675 m (35,000 ft) usually also have cockpits under pressure. Positive-pressure breathing equipment is also used in all other aircraft capable of flight above 10,675 m. Full or partial pressure suits with additional oxygen equipment are required in military aircraft capable of flight above 16,775 m (55,000 ft).

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Commercial carriers provide oxygen systems and pressurized cabins in accordance with civil air regulations. An airliner flying at 6710 m (22,000 ft), for example, must maintain a “cabin altitude” of 1830 m (6000 ft).

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Altitude Sickness

This physiological condition results from a state of acute oxygen deficiency, known medically as hypoxidosis, at high altitudes. Ascending from the lower atmosphere, called the troposphere, the atmosphere is thin enough at 3900 m (13,000 ft) to produce symptoms of hypoxia, or oxygen hunger. At the lower limit of the stratosphere, about 10,675 m (about 35,000 ft), normal inhalation of pure oxygen no longer maintains an adequate saturation of oxygen in the blood.

Hypoxia produces a variety of reactions in the body. Mild intoxication and stimulation of the nervous system are followed by progressive loss of attention and judgment until unconsciousness occurs. Respiration and pulse rate increase, and the systemic oxygen content is reduced. Prolonged lack of oxygen may cause damage to the brain.

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Aeroembolism

Because of the reduction of barometric pressure at altitudes above 9150 m (30,000 ft), the body tissues can no longer retain atmospheric nitrogen in solution. As a result, liberated gas bubbles, as well as ruptured fat cells, may enter the circulatory system and form obstructions, or emboli, in the blood vessels. This condition, known medically as aeroembolism and popularly as the bends, leads to confusion, paralysis, or neurocirculatory collapse. The most characteristic symptoms of the bends are pain in the large joints resulting from pressure of the gas on tendons and nerves, together with spasm of the blood vessels. Preflight inhalation of pure oxygen to eliminate nitrogen from the system has proved valuable as a preventive measure. Rapid decompression, resulting from accidental failure at high altitudes of the pressure within the cabin, causes major damage to the heart and other organs by the ram effect of gases formed in the body cavities.

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Airsickness

This condition is produced by a disturbance of the labyrinthine mechanism of the inner ear (see Ear: Equilibrium), although psychogenic factors such as apprehension can also play a part. Motion sickness can be prevented by taking drugs containing scopolamine or some antihistamines (see Antihistamine) before flight.

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