Atmospheric pressure is governed by the physical behavior of gases. The individual gaseous molecules of the earth’s atmosphere have mass and therefore weight due to the effect of gravity.Atmospheric pressure represents the weight of the mass of air extending upward from the earth’s surface. At sea level this amounts to 14 pounds per square inch, 760 mm. of mercury or I atmosphere. Atmospheric air (dry contains percent oxygen, 0.03 per cent carbon dioxide, and approximately 79 percent nitrogen at all altitudes.
Water, being much denser than air. exerts a much greater pressure because of its weight The weight of a 33-foot column of sea water equals that of a like-sized column of air extending through the entire atmosphere. Since water is incompressible, pressure increases in linear fashion with further descent, each 33 feet adding an additional atmosphere of pressure.
The medical problems created by changes in atmospheric pressure may be grouped into direct and indirect effects. The former result from mechanical forces created when pressure differentials develop across the walls of air-containing spaces within the body or upon its surface. Indirect effects result from alterations in partial pressures of the individual gases of the atmosphere.
DIRECT EFFECTS OF CHANGES IN ATMOSPHERIC PRESSURE
Gases respond to pressure changes in accordance with Boyle’s law. Their volume varies inversely and their density, or molecular concentration, directly as the absolute pressure. The fluids and solids of the human body, being incompressible, transmit pressure freely and thus assume the pressure exerted upon the body’s surface. Any alteration in the latter is reflected almost instantly by an identical pressure change in body fluids and tissues. Pressure within air containing spaces of the body can be maintained equal to external pressure only by appropriate adjustments either of the number of molecules of gas within the space or of the volume of the space.
No medical difficulties arise as long as air can pass freely between such spaces and the environment. If additional air cannot enter such a space to increase its pressure during descent, fluids and tissues of the surrounding walls, whose pressure is increasing with increasing ambient pressure, tend to move in to reduce its volume and eradicate the pressure differential. The lining of the walls becomes hyperaemia and swollen. Serum or blood may move into the relative vacuum. Conversely, during ascent, pressure within the tissues surrounding the space declines with declining environmental pressure. Unless air can be vented from the space, an increasing pressure gradient creates a distending force against the walls.
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The natural air-containing spaces of the body are the paranasal sinuses, the middle ear, the airways and lungs, and the gastrointestinal tract. Small pockets of air may also exist beneath fillings or in diseased teeth. In addition, pockets of air against various surfaces of the body may be produced by goggles, tight-fitting hoods, ear plugs, or wrinkles in diving suits. The middle ear is very susceptible to barotrauma during descent, The sinuses are much less frequently affected, but may also incur barotrauma during ascent. Respiratory infection greatly increases the likelihood of injury. Descent in water may damage tissues beneath air-containing structures closely applied to the surface of the body. All these injuries, including barotitis with ruptured tympanic membranes, tend to heal spontaneously.
The most serious mechanical injuries resulting from pressure changes involve the lungs. If the breath is held during diving, the increasing pressure compresses the thoracic cage to the position of maximal expiration. On further descent air within the lungs can no longer be compressed to counter the increasing external pressure. The relatively negative extrapulmonary pressure results eventually in pulmonary congestion, edema, and hemorrhage, a condition known as “thoracic squeeze.” Breathing the denser air at depth through equipment with excessive air flow resistance or breathing through a snorkel tube which is too long may also create a relatively negative pressure within the lungs, leading to similar consequences.
After breathing the denser air at depths, air must be released from lungs during ascent to prevent expanding air from building up sufficient pressure to over distend or rupture lung tissue; if this occurs, pneumothorax, mediastinal emphysema, or air embolism may ensue. Air embolism has been reported during ascents of as little as 9 feet and is said to be exceeded in frequency only by drowning as a cause of accidental death among
divers. Lung tissue distal to a poorly communicating diseased bronchial passage may be similarly damaged even if the diver vents his excess air during ascent. The air traveler with such bronchopulmonary disease, or one who happens to be holding his breath, may be similarly injured during a sudden loss of cabin pressure at high altitude.
Knowledge of the character of the dive, including depth, duration, air supply, and patterns of descent and ascent in relation to symptoms, often simplifies the problem of diagnosing lung injury in divers. Frothy, bloody sputum may be produced in either thoracic squeeze or lung rupture. If the victim has been skin-diving or using a snorkel tube, thoracic squeeze is likely.
In a scuba diver, however, the first consideration must be lung rupture with potentially fatal aeroembolism. (The term “scuba” is derived from “self-contained underwater breathing apparatus.”) The unconscious diver must be presumed to have air embolism or decompression sickness. Either aeroembolism or serious decompression sickness requires treatment by recompression at the earliest possible moment.
Transport to a recompression chamber should be by the fastest method available. The gain from prompt treatment far outweighs the potential danger of further decompression during air transportation at low altitudes. Meanwhile artificial respiration by the mouth-to-mouth method should be carried out if necessary, and oxygen should be administered. Placing the victim on his left side may be beneficial if aeroembolism is suspected. Thoracic squeeze usually requires no more than supportive measures.
INDIRECT EFFECTS OF CHANGES IN ATMOSPHERIC PRESSURE
Only brief mention can be made here of the effects of increases in the partial pressures of nitrogen, oxygen, and carbon dioxide. At 4 atmospheres absolute pressure (100 feet in water), divers or occupants of hyperbaric chambers may show the narcotize effect of nitrogen. Judgment, thought processes, and motor ability may become impaired and deteriorate with further increases in atmospheric pressure. Fortunately, the symptoms clear rapidly and completely on return to lower atmospheric pressure.
Hyperoartia normally produces subjective distress and stimulation of respiration, but may cause unconsciousness without warning. Scuba divers who restrict their breathing in an attempt to conserve their air supply expose themselves to this potential hazard. Loss of consciousness in skin divers resulting from “shallow-water blackout” is attributed to a large rise in partial pressure of carbon dioxide associated with an inadequate subjective or respiratory response to hypersomnia.
Convulsive seizures and coma may occur with little warning during exposure to high partial pressures of oxygen. The minimal partial pressure of oxygen capable of producing convulsions appears to be less than 2 atmospheres (Po2 = 15: mm. Hg). Divers breathing air are at little risk of oxygen convulsions. If they use pure oxygen’ however, depths below 25 feet become hazardous.
Hyperbaric oxygenation is achieved by increaseing atmospheric pressure within sealed chamber to levels varying from 2 to 4 atmospheres absolute (Po2 = 320 to 640 mm. Hg). Increased oxygenation of tissues occurs chiefly as a result of increase solution of oxygen in plasma. Hyperbaric oxygenation has been reported to be of value during surgical correction of certain congenital cardiac vascular disorders and in the treatment of carbon monoxide poisoning and certain anaerobic infections. Precautions must be taken to protect the patient and medical personnel from all of the direct and indirect effects of altered atmospheric pressure.