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Health Risks of Hyperbaric Environments

This is an excerpt from Physiology of Sport and Exercise 9th Edition With HKPropel Access by W. Larry Kenney,Jack H. Wilmore & David L. Costill.

The development of scuba gear enabled deeper and longer dives. However, this capability also presented divers with additional health risks. As a diver descends, the air in the breathing apparatus must be pressurized to equal the pressure exerted by the water, which increases the partial pressures of all gases in the mixture. This increases the pressure gradient that drives oxygen and nitrogen into the body tissues, and the increased alveolar partial pressure of carbon dioxide decreases the pressure gradient that allows it to be cleared by the lungs. Thus, breathing oxygen, carbon dioxide, and nitrogen under pressure can cause the body tissues to accumulate toxic levels of these gases.

Oxygen Toxicity

Exposure to a PO₂ ranging from 318 to 1,500 mmHg has been shown to have severe effects, particularly on the lungs and the central nervous system. When PO₂ is high, enough oxygen gets dissolved in the blood plasma to supply the metabolic needs of the diver, resulting in less oxygen unloading from hemoglobin at the tissues. Because of this, the hemoglobin in the venous blood remains highly saturated with oxygen. Therefore, pure oxygen is never used in recreational diving.

Carbon dioxide does not bind as well to hemoglobin that is fully saturated with oxygen, so reduced oxygen unloading impairs CO₂ elimination via hemoglobin. In addition, when the diver breathes oxygen at a PO₂ greater than 318 mmHg (twice the normal atmospheric PO₂), cerebral blood vessels can become constricted, severely restricting the circulation to the central nervous system. This can result in such symptoms as visual distortion, rapid and shallow breathing, and convulsions. In some cases, this high PO₂ can irritate the respiratory tract, eventually leading to pneumonia. This condition, which results from excessive oxygen, is referred to as oxygen toxicity or oxygen poisoning.

Decompression Sickness

Recall that nitrogen makes up approximately 78% to 79% of air. The resulting high partial pressure of nitrogen, increased during diving, forces more nitrogen into the blood and tissues. If the diver attempts to ascend too rapidly, this additional nitrogen cannot be delivered to and released from the lungs quickly enough, but rather becomes trapped as bubbles in the blood and the tissues. This causes severe discomfort and pain and is referred to as decompression sickness, or the bends. Typically, this disorder is characterized by aching in the elbows, shoulders, and knees, where nitrogen bubbles tend to accumulate. If bubbles become emboli in the blood, they can interfere with normal circulation, and this can be fatal.

Treatment involves placing the diver in a recompression chamber (see figure 15.16). The air pressure is increased (recompressed) in the chamber to simulate that experienced while diving, and then it is gradually returned to the ambient pressure. Recompression forces the nitrogen back into solution, and then a gradual decrease in pressure allows the nitrogen to escape through the respiratory system.

To prevent decompression sickness, charts have been created that provide timetables for ascending from various depths. If, for example, a diver was to submerge to a depth of about 15 m (49 ft) for an hour, staged decompression would not be needed. But if the diver spent an hour at a depth of about 30 m (98 ft), slow, staged decompression would be necessary. Strict adherence to the timetable for a specific diving depth allows a safe ascent without decompression sickness.

Nitrogen Narcosis

Although nitrogen is not metabolically active, meaning it doesn’t participate in biological processes, at high pressures—such as during deep dives—it can act much like an anesthetic gas. The resulting condition is referred to as nitrogen narcosis, or rapture of the deep. The diver develops central nervous system symptoms similar to alcohol intoxication. The effect worsens as depth, and consequently pressure, increases, and it is secondarily worsened by time at depth. In fact, divers reference “the martini rule,” which suggests that for every 10 m (33 ft) increase in depth below 20 m (66 ft), this effect equals that of one martini ingested on an empty stomach.

Divers at depths of 30 m (98 ft) or more have impaired judgment but might not recognize the problem. Poor judgment during diving can be life threatening, so most divers who descend below 30 m usually breathe a specialized gas mixture containing mostly helium.

Spontaneous Pneumothorax

Breathing pressurized gas at depth can create a problem if the gas is not expelled during ascent. A full breath taken in and held at 2 m (6.6 ft) will expand enough during ascent to overdistend the lungs. This can rupture alveoli, allowing gas to enter the pleural space and in turn collapse the lung. This is known as a spontaneous pneumothorax. At the same time, small air bubbles can enter the pulmonary blood and form air emboli, which can become trapped in the vessels of other tissues, blocking circulation to those tissues. Severe blockage of the vessels supplying the lungs, heart, and central nervous system can cause death. Fortunately, this condition can be prevented simply by keeping the mouth open and exhaling during ascent, allowing the compressed air in the respiratory passageways to escape.

Ear Barotrauma

Most passengers have experienced the buildup of air pressure in the sinuses and middle ear when planes take off and land. Failure to equalize the air pressure in these relatively fixed spaces during ascent and descent underwater can rupture the small blood vessels and membranes that line these cavities. Pressure in the middle ear is normally equalized through the eustachian tube (which connects the middle ear to the throat). Inability to equalize the pressure in the middle ear creates unequal force against the eardrum, causing considerable pain. Under severe conditions, as during ascent or descent in deep water, this increased pressure can rupture the eardrum.

During diving, the middle ear and the sinuses usually can be equalized by blowing with moderate pressure against the closed nostrils. Because upper respiratory infections and sinusitis can cause swelling of the membranes in the sinuses and eustachian tube, scuba and breath-hold diving are not recommended for people suffering from these conditions.

Some of the health risks associated with hyperbaric conditions are depicted in figure 15.17. The preceding sections do not cover all of the risks, but we have provided an overview of some common and more serious ones. Diving can be dangerous for the inexperienced, and even the most experienced divers can get into trouble if they don’t follow proper procedures or if they ignore the health risks associated with this sport.

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