Dysbarism
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Dysbarism refers to medical conditions resulting from changes in ambient pressure. Various activities are associated with pressure changes. SCUBA diving is the most frequently cited example, but pressure changes also affect people who work in pressurized environments (e.g. caisson workers) and people who move between different altitudes.
Ambient pressure
To understand dysbarism, the concept of ambient pressure must first be understood. As divers descend, the ambient pressure increases. At 10 meters (33 feet) in [salt water], the ambient pressure has doubled relative to normal pressure on land at [sea level]. At 40 meters (the recommended safety limit for recreational diving) ambient pressure is five times the pressure at sea level.
Pressure decreases as we rise 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 doesn’t drop to 50% of seal level pressure until a person reaches 20,000 feet or 6,000 meters.
Effects of pressure the body
1. 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. The answer is no. Divers would have to reach depths of thousands of feet before their flesh began to suffer significant compression.
2. Air spaces
Air on the other hand is very compressible. And humans have many air spaces: sinuses, middle ears, gas in our bowels, cavities in our teeth, and largest of all, our lungs. As we walk about on land in our daily lives, the pressure in our air spaces is usually exactly the same as the pressure outside, because our air spaces are connected to the outside world. If there was a pressure difference between the outside world and one of our air spaces, then we experience painful pressure on the walls of that air space, as air “tries” to get from the higher-pressure side to the lower-pressure side. This is why we sometimes get painful ears on air trips.
3. Dissolved gas
A percentage of the gas we breathe (air) is always dissolved in our blood. It’s like the gas dissolved in a soda bottle with the lid on. If we move to a higher ambient pressure, then the gas we breathe is at a higher pressure, and more of it dissolves in our blood and body tissues. If we move back to a lower pressure, and we move slowly, then the extra gas dissolves out slowly until we are back to our normal amount of dissolved gas. But if we move quickly to a lower ambient pressure, then the gas comes out of our blood and tissues violently, in large bubbles. It’s very similar to the difference between slowly opening a bottle of soda (dropping the pressure in the bottle slowly down to sea level), versus ripping the cap off quickly.
Types of Dysbarism
Different types of illness result from increases in pressure (e.g. descent during a SCUBA dive, descent during a plane flight), versus decreases in pressure (e.g. coming up from a caisson, or ascending a mountain). Dysbarism comprises several types of illness:
1. Decompression sickness (DCS).
Also referred to as Caisson's disease and sometimes called the diver's bends, this is the most well-known complication of SCUBA diving. It occurs as divers ascend, and often occurs because of an ascent that is too rapid. 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 not uncommon.
2. Arterial Gas Embolism (AGE).
Whereas DCS can be thought of as a venous-side bubbling disease, AGE occurs on the arterial side. 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 is also known as “L’ivresse des grandes profondeurs” or rapture of the deep. Nitrogen comprises 79% of the air we breathe, but at surface pressures it has no sedating effect. At greater depths, however, nitrogen affects the brain in precisely the same way as nitrous oxide (a.k.a. laughing gas). The effect is similar to the effects of alcohol, and to some extent there is cross-tolerance. Unlike alcohol, the onset and disappearance are instantaneous. A diver may be quite clear-headed at 20 meters, and yet giddy and silly at 30 meters. Simply ascending to 20 meters will almost instantly clear their head. Divers suffering nitrogen narcosis may put themselves at risk by doing stupid things such as offering their regulator (mouth piece) to fish. Because it reverses completely with ascent, divers never suffer nitrogen narcosis after a dive.
4. High pressure nervous syndrome (HPNS).
This is rarely of importance to recreational divers. Breathing any gas at great depths (hundreds of feet) can cause seizures. Interestingly it was discovered because divers were using gas mixtures without nitrogen to be able to go to great depths without experiencing nitrogen narcosis. It turns out that nitrogen actually prevents HPNS. The answer? Add very small amounts of nitrogen to gas mixes when diving at great depth, small enough to avoid nitrogen narcosis, but sufficient to prevent HPNS.
5. Barotrauma.
This refers to pressure effects on air spaces. This may occur during ascent or descent. The ears are the most commonly affected body part. The most serious injury is lung barotraumas, which can result in pneumothorax, pneumomediastinum, pneumopericardium, subcutaneous emphysema, and Arterial Gas Embolism. All divers, hikers, commercial air travelers, and all people who work in pressurized environments have had to deal with some degree of barotrauma effect upon their ears, sinuses, or other air spaces. At the most extreme, barotrauma can cause ruptured ear drums, bleeding sinuses, exploding tooth cavities, and the lung injuries described above. This is the reason why divers follow a golden rule of never holding their breath: by breathing continuously, they avoid any pressure differences between their lungs and ambient pressure.