Dysbarism refers to medical conditions resulting from changes in ambient pressure.

Activities associated with pressure changes: Underwater diving, caisson workers, and in people who move between different altitudes.

Ambient pressure is the pressure in the water around the diver or the air, with caisson workers.

As a diver descends, the ambient pressure increases. 

At 10 meters (33 feet) in salt water, it is twice the normal pressure than that at the surface, and at 40 meters (a common recommended limit for recreational diving) it is 5 times the pressure than 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 does not drop to 50% of sea level pressure until 20,000 feet or 6,000 meters altitude.

Air spaces: sinuses, middle ears, gas in our bowels, cavities in our teeth, and lungs. 

Pressure in air spaces is usually exactly the same as the pressure outside, because our air spaces are connected to the outside world. 

When a pressure difference exists between the outside world and air spaces, painful pressure occurs  on the walls of that air space, as air pushes from the higher-pressure side to the lower-pressure side: e.g.painful ears on air trips.

A percentage of the air we breathe is always dissolved in our blood.

At a higher ambient pressure, the gas inhaled is at a higher pressure, so more of it dissolves in the blood and diffuses into body tissues.

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. 

However, if ones move quickly to a lower ambient pressure, then the gas comes out of our blood and tissues violently, in large bubbles.

Types of illness that result from increases in pressure: descent during an underwater dive, descent during a plane flight.

Types of illness that result from decreases in pressure: coming up from a caisson, or ascending a mountain.

Dysbarism comprises several types of illness:

Decompression sickness the most well-known complication of scuba diving. 

Decompression sickness 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. 

When DCS occurs, bubbles disrupt tissues in the joints, brain, spinal cord, lungs, and other organs. 

Symptoms vary: 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, paraplegia, shortness of breath, or death.

Nitrogen narcosis: Nitrogen comprises 79% of the air, but at surface pressures it has no sedating effect. 

At greater depths, however, nitrogen affects the brain like anesthetic gases. 

The onset and disappearance of effects are near instantaneous. 

Barotrauma is injury caused by pressure effects on gas spaces, that occur during ascent or descent. 

The ears are the most commonly affected barotrauma body part.

The most serious injury barotrauma injury occur in the lung: pneumothorax, pneumomediastinum, pneumopericardium, subcutaneous emphysema, and arterial gas embolism. 

At the most extreme of barotrauma: ruptured eardrums, bleeding sinuses, exploding tooth cavities, and the lung injuries.

Divers do not hold their breath during ascent, to keep the airways open and avoid pressure differences between their lungs and ambient pressure.

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. 

It can present in similar ways to arterial blockages seen in other medical situations. 

Patients may suffer strokes, heart attacks, pulmonary embolism with shortness of breath and chest pain. 

It is difficult to distinguish AGE from DCS, but is rarely necessary for physicians to be able to distinguish between the two: the treatment is the same. 

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