Dysbarism. Dysbarism or dysbaric disorders refers to medical conditions resulting from changes in ambient pressure. Various activities are associated with pressure changes. Underwater diving is the most frequently cited example, but pressure changes also affect people who work in other pressurized environments (for example, caisson workers), and people who move between different altitudes. A dysbaric disorder may be acute or chronic.

Ambient pressure 
Ambient pressure on a diver is the pressure in the water around the diver (or the air, with caisson workers etc.). It is the sum of the atmospheric pressure at the surface and hydrostatic pressure due to the depth. As a diver descends, the ambient pressure increases. At  in seawater, it is twice the normal atmospheric pressure at the surface. At 40 meters (a common recommended limit for recreational diving) it is 5 times the pressure at sea level.

Pressure decreases with altitude above sea level, but less dramatically. At 3000 feet altitude (almost 1000 meters), the ambient pressure is almost 90% of sea level pressure. Ambient pressure does not drop to 50% of sea level pressure until 20,000 feet or 6,000 meters altitude.

Effects of pressure on the body

Direct effects on tissues 
This is not of practical importance, because the body is mostly composed of barely compressible materials such as water. People often wonder whether scuba divers feel their body being crushed by the pressure, but divers would have to reach depths of thousands of feet before their flesh began to suffer significant compression.

Gas filled spaces 
Gas is very compressible. Humans have many air spaces: sinuses, middle ears, gas in the gastro-intestinal tract, cavities in the teeth, and largest of all, the lungs. On land in dail life, the pressure in these air spaces is usually exactly the same as the pressure outside, because the air spaces are connected to the outside world. If there is a pressure difference between the outside world and one of these air spaces, the air space will change volume as much as the surrounding tissues allow, after which the wall will be stretched painfully and may rupture.

Dissolved gas 
A percentage of the gas we breathe (air) is always dissolved in our blood, like the gas dissolved in a carbonated drink bottle with the lid on. If a person moves to a higher ambient pressure, then the gas inhaled is at a higher pressure, so more of it dissolves in the blood and diffuses into body tissues (Henry's and Fick's gas laws). If they slowly move back to a lower pressure, then the extra gas comes out slowly until they are back to their normal amount of dissolved gas. But if they move quickly to a lower ambient pressure, then the gas comes out of our blood and tissues violently, in large bubbles, in the same way that quickly removing the cap from a bottle of soft drink produces far more bubbles than slowly opening the bottle.

Types of dysbarism 
Different types of illness result from increases in pressure (for example, descent during an underwater dive, descent during a plane flight), versus decreases in pressure (for example, coming up from a caisson, or ascending a mountain). Dysbarism comprises several types of illness:

Decompression sickness (DCS) 

Decompression sickness, also called caisson workers' disease and the bends, is the most well-known complication of scuba diving. It occurs as divers ascend, and often from ascending too fast or without doing decompression stops. Bubbles are large enough and numerous enough to cause physical injury. It is quite possible that all divers have microbubbles in their blood to some extent, but that most of the time these bubbles are so few and so small that they cause no harm. When DCS occurs, bubbles disrupt tissues in the joints, brain, spinal cord, lungs, and other organs. Symptoms vary enormously. DCS may be as subtle as unusual tiredness after a dive, or an aching elbow, or a mottled skin rash. Or, it may present dramatically, with unconsciousness, seizures, paralysis, shortness of breath, or death. Paraplegia is possible.

Nitrogen narcosis 

Nitrogen narcosis is also called “L’ivresse des grandes profondeurs” or "rapture of the deep". Nitrogen constitutes 78% of dry air, but at surface pressures it has no sedating effect. At greater depths, however, nitrogen affects the brain in the same way as nitrous oxide (also known as laughing gas) and other anaesthetic gases. The effect is similar to the effects of alcohol, and to some extent there is cross-tolerance. Unlike alcohol, the onset and disappearance are near instantaneous. A diver may be quite clear-headed at 20 meters, and yet giddy and silly at 30 meters. Ascending to 20 meters will almost instantly clear the head.

High-pressure nervous syndrome (HPNS) 

High-pressure nervous syndrome (HPNS) – also known as high-pressure neurological syndrome, and helium tremors, is a neurological and physiological diving disorder which can result when a diver descends below about  using a breathing gas containing a high proportion of helium. The effects experienced, and the severity of those effects, depend on the rate of descent, the depth and the percentage of helium. Symptoms of HPNS include tremors, myoclonic jerking, somnolence, EEG changes, visual disturbance, nausea, dizziness, and decreased mental performance.

Barotrauma 

Barotrauma is injury caused by pressure effects on gas spaces. This may occur during ascent or descent. The ears are the most commonly affected body part. The most serious injury is lung barotrauma, which can result in pneumothorax, pneumomediastinum, pneumopericardium, subcutaneous emphysema, and arterial gas embolism. All divers, commercial air travelers, people traveling overland between different altitudes, and people who work in pressurized environments have had to deal with some degree of barotrauma effect upon their ears, sinuses, and other air spaces. At the most extreme, barotrauma can cause ruptured eardrums, bleeding sinuses, exploding tooth cavities, and the lung injuries described above. This is the reason why divers follow a procedure of not holding their breath during ascent. By breathing continuously, they keep the airways open and avoid pressure differences between their lungs and ambient pressure.

Arterial gas embolism (AGE) 

Arterial gas embolism (AGE) is a complication of lung barotrauma of ascent. It occurs when breathing gas is introduced to the circulation on the arterial side via lung over-pressure trauma. AGE can present in similar ways to arterial blockages seen in other medical situations. Affected people may suffer strokes, with paralysis or numbness down one side; they may suffer heart attacks; they may suffer pulmonary embolism with shortness of breath and chest pain. It is often impossible to distinguish AGE from DCS, but luckily it is rarely necessary for physicians to be able to distinguish between the two, as treatment is the same. Sometimes AGE and DCS are lumped into a single entity, Decompression Illness (DCI). This problem does not normally occur on breathhold dives as the gas in the lungs will return to the volume it had at the start of the dive, and not expand further.

Classification 
In addition to the foregoing, dysbarism is sometimes classified according to the source of the excess gas, with "trapped gas" dysbarism referring to the expansion of pockets that were already in a gaseous state in the body, and "evolved gas" dysbarism referring to gasses (primarily nitrogen or helium) dissolved in the body coming out of solution to form gas bubbles.

Compression arthralgia

Compression arthralgia is pain in the joints caused by exposure to high ambient pressure at a relatively high rate of compression, experienced by underwater divers. Also referred to in the U.S. Navy Diving Manual as compression pains.

Compression arthralgia has been recorded as deep aching pain in the knees, shoulders, fingers, back, hips, neck and ribs. Pain may be sudden and intense in onset and may be accompanied by a feeling of roughness in the joints.

Onset commonly occurs around 60 msw (meters of sea water), and symptoms are variable depending on depth, compression rate and personal susceptibility. Intensity increases with depth and may be aggravated by exercise. Compression arthralgia is generally a problem of deep diving, particularly deep saturation diving, where at sufficient depth even slow compression may produce symptoms. Peter B. Bennett et al. (1974) found that the use of trimix could reduce the symptoms.

See also

References

External links 

Underwater diving safety
Underwater diving disorders