Decompression tables

The practice of decompression by divers comprises the planning and monitoring of the profile indicated by the algorithms or tables of the chosen decompression model, to allow asymptomatic and harmless release of excess inert gases dissolved in the tissues as a result of breathing at ambient pressures greater than surface atmospheric pressure, the equipment available and appropriate to the circumstances of the dive, and the procedures authorized for the equipment and profile to be used. There is a large range of options in all of these aspects.

Decompression may be continuous or staged, where the ascent is interrupted by stops at regular depth intervals, but the entire ascent is part of the decompression, and ascent rate can be critical to harmless elimination of inert gas. What is commonly known as no-decompression diving, or more accurately no-stop decompression, relies on limiting ascent rate for avoidance of excessive bubble formation. Staged decompression may include deep stops depending on the theoretical model used for calculating the ascent schedule. Omission of decompression theoretically required for a dive profile exposes the diver to significantly higher risk of symptomatic decompression sickness, and in severe cases, serious injury or death. The risk is related to the severity of exposure and the level of supersaturation of tissues in the diver. Procedures for emergency management of omitted decompression and symptomatic decompression sickness have been published. These procedures are generally effective, but vary in effectiveness from case to case.

The procedures used for decompression depend on the mode of diving, the available equipment, the site and environment, and the actual dive profile. Standardized procedures have been developed which provide an acceptable level of risk in the circumstances for which they are appropriate. Different sets of procedures are used by commercial, military, scientific and recreational divers, though there is considerable overlap where similar equipment is used, and some concepts are common to all decompression procedures.

Decompression in the context of diving derives from the reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during this reduction in pressure. When a diver descends in the water column the ambient pressure rises. Breathing gas is supplied at the same pressure as the surrounding water, and some of this gas dissolves into the diver's blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the diver is in a state of equilibrium with the breathing gas in the diver's lungs, (see: "Saturation diving"), or the diver moves up in the water column and reduces the ambient pressure of the breathing gas until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again. Dissolved inert gases such as nitrogen or helium can form bubbles in the blood and tissues of the diver if the partial pressures of the dissolved gases in the diver gets too high above the ambient pressure. These bubbles and products of injury caused by the bubbles can cause damage to tissues known as decompression sickness, or "the bends". The immediate goal of controlled decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long-term goal is to also avoid complications due to sub-clinical decompression injury.

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