II.

Aviation Medicine

Specialists in aviation medicine study the reactions of human beings to the stresses of air travel. They are concerned with the proper screening of candidates for flight training, the maintenance of maximum efficiency among aircrews, and with clinically oriented research into the effects of flight on the body. They also cooperate actively with aeronautical engineers in the development of safe aircraft.

A.

History

Aviation medicine is rooted in the early 18th-century physiological studies of balloonists, some of whom were also doctors. In 1784, a year after the first balloon flight by the French physicist Jean Pilâtre de Rozier, a Boston doctor, John Jeffries, made the first study of upper-air composition from a balloon. The first comprehensive studies of health effects in air flight were made by the French doctor Paul Bert, who published his research on the effects of altered air pressure and composition on humans in 1878 under the title La pression barometrique. In 1894 the Viennese physiologist Herman Von Schrötter designed an oxygen mask with which the meteorologist Artur Berson set an altitude record of 9,150 m (30,000 ft). With the advent of aircraft, the first standards for military pilots were established in 1912. Significant work in this area was directed by the doctor Theodore Lyster, an American pioneer in aviation medicine. Technical advances included the first pressurized suit, designed and worn by the American aviator Wiley Post in 1934, and the first anti-gravity suit, designed by W. R. Franks in Great Britain in 1942. In an effort to help design better restraint systems for military jet aircraft, the United States flight surgeon John Stapp conducted a series of tests on a rocket-powered sledge, culminating on December 10, 1954, when Colonel Stapp underwent deceleration from a velocity of 286 m (937 ft)/sec in 1.4 sec.

B.

Physiological Considerations

Aviation medicine is concerned primarily with the effects on human beings of high speed and high altitude, and involves the study of such factors as acceleration and deceleration, atmospheric pressure, and decompression. In civil-aviation medicine, an additional concern is passenger airsickness.

B.1.

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 the Earth's gravity at sea level, or gs. 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 backwards in a seated position, properly supported human test subjects have been able to tolerate a deceleration force of 50 g without severe injury.

B.2.

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 build-up of toxic products soon limits activity. Brain and eye tissues are the most sensitive to oxygen deficiency.

The Earth's atmosphere, which contains 21 per cent of oxygen by volume, is under a normal sea-level pressure of 760 torrs (14.7 lb/sq in). The barometric pressure up to about 4,575 m (15,000 ft) is sufficient to sustain human life. Above this altitude the air must be artificially pressurized to meet the respiratory needs of human beings.

High-altitude military aircraft are provided with oxygen equipment, and military personnel are required to use it at all times when participating in flight above 3,050 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).

Commercial carriers provide oxygen systems and pressurized cabins in accordance with civil air regulations. An airliner flying at 6,710 m (22,000 ft), for example, must maintain a “cabin altitude” of 1,830 m (6,000 ft).

B.3.

Altitude Sickness

This physiological condition results from a state of acute oxygen deficiency, known medically as hypoxia, at high altitudes. Ascending from the lower atmosphere, called the troposphere, the atmosphere is thin enough at 3,900 m (13,000 ft) to produce symptoms of hypoxia, or oxygen hunger. At the lower limit of the stratosphere, about 10,675 m (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 judgement 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.

B.4.

Aeroembolism

Because of the reduction of barometric pressure at altitudes above 9,150 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. Pre-flight 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. See also Deep Vein Thrombosis.

B.5.

Airsickness

Airsickness is produced by a disturbance of the labyrinthine mechanism of the inner ear, although psychogenic factors such as apprehension can also play a part. Motion sickness can be prevented by taking drugs containing scopolamine or some antihistamines before flying.

B.6.

Time Change

As transport planes became faster, pilots and passengers were able to travel across many time zones in less than a day. The resulting disturbance in the body's “biological clock” or circadian (“about a day”) rhythm can produce disorientation and reduce concentration and efficiency. This condition is popularly known as jet lag. While troublesome to passengers, the problem is more acute for pilots, who may have to fly another assignment in a short time. Concern has been expressed about the possible effect of this situation on air safety, although no air accident has yet been clearly identified as jet-lag-induced.