In 1667 Robert Boyle observed that following exposure to a compressed atmosphere, and subsequent decompression, an air bubble formed in the eye of a viper. The animal also appeared distressed by the experience.
In the 19th century, work began on the Brooklyn Bridge in New York. In order to build the foundations for the large stone towers of the bridge it was necessary to dig down below the surface of the riverbed. To provide a dry environment for the men working on the riverbed, underwater enclosures called caissons were used.
The air inside these underwater enclosures was pressurised to counteract the weight of the surrounding water. Following their shifts, some men would return to the surface suffering pain that made it difficult for them to stand straight. Their appearance was similar to the 'Grecian bend' adopted by fashionable women of the time - hence the ‘bends’.
The connection between the workers' return to the surface and their symptoms led to the introduction of surface based recompression chambers to treat the afflicted. However, the reason for the condition was not fully recognised until 1878, when Paul Bert published his theory that the cause was the formation of nitrogen bubbles within the body. He also correctly stated that it was possible to avoid their harmful affects by ascending to the surface gradually - and that hyperbaric chambers worked, in part, because they decreased the size of bubbles.
The modern term for the ‘bends’ is Decompression sickness (DCS), which along with another diving related disorder called Cerebral Arterial Gas Embolism (CAGE) is also known as a Decompression illness (DCI).
Take a bottle of carbonated water and gently shake it. Open the bottle slowly, allowing the carbon dioxide gas dissolved in the liquid to come out of solution at a controlled rate. Few bubbles should form as a result. Now, take another bottle and repeat the above, but this time open it quickly. Many more bubbles will form. This is analogous to what happens when the body’s tissues are decompressed too quickly, although in the case of decompression sickness it is the inert gas in the breathing gas mixture that forms bubbles.
In order to appreciate the way in which inert gasses, like nitrogen and helium, can lead to decompression sickness, it is necessary to first understand a few facts.
Air consists of approximately 21% oxygen (O2) and 79% nitrogen (N2). Atmospheric pressure is caused by the weight of the air above, which in turn is the effect gravity has on the various gasses in it. This pressure decreases with altitude. The more molecules there are in a given volume of a gas, the higher the pressure.
Under the surface of a body of water, the weight of the water molecules causes hydrostatic pressure, which increases proportionally with the amount of water above (depth).
There are two ways to enable divers to breathe underwater. The first is by enclosing the diver in a protective air-filled capsule, such as submarines. The other method is to provide the diver with breathing air at the same pressure as the surrounding water.
Dalton’s law states that every gas in a mixture of gases exerts its own pressure independently of the others. The pressure of a single gas in a mixture is known as the 'partial pressure'.
