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 tissues. 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 get too high when compared to the ambient pressure. These bubbles, and products of injury caused by the bubbles, can cause damage to tissues generally 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.
The symptoms of decompression sickness are known to be caused by damage resulting from the formation and growth of bubbles of inert gas within the tissues and by blockage of arterial blood supply to tissues by gas bubbles and other emboli consequential to bubble formation and tissue damage. The precise mechanisms of bubble formation and the damage they cause has been the subject of medical research for a considerable time and several hypotheses have been advanced and tested. Tables and algorithms for predicting the outcome of decompression schedules for specified hyperbaric exposures have been proposed, tested, and used, and usually found to be of some use but not entirely reliable. Decompression remains a procedure with some risk, but this has been reduced and is generally considered to be acceptable for dives within the well-tested range of commercial, military and recreational diving.
The first recorded experimental work related to decompression was conducted by Robert Boyle, who subjected experimental animals to reduced ambient pressure by use of a primitive vacuum pump. In the earliest experiments the subjects died from asphyxiation, but in later experiments, signs of what was later to become known as decompression sickness were observed. Later, when technological advances allowed the use of pressurisation of mines and caissons to exclude water ingress, miners were observed to present symptoms of what would become known as caisson disease, the bends, and decompression sickness. Once it was recognized that the symptoms were caused by gas bubbles, and that recompression could relieve the symptoms, further work showed that it was possible to avoid symptoms by slow decompression, and subsequently various theoretical models have been derived to predict low-risk decompression profiles and treatment of decompression sickness.
Timeline
1660 – Sir Robert Boyle conducted an experiment on a bird in an air pump. This predated actual intentional investigations into decompression, but the experiment was effectively a rapid decompression and caused the death of the bird by asphyxiation. 1670 – Sir Robert Boyle performed an experiment with a viper in a vacuum. A bubble was observed in its eye and it displayed signs of extreme discomfort. This was the first recorded description of decompression sickness. 1841 – Jacques Triger documented the first cases of decompression sickness in humans when two miners involved in pressurised caisson work developed symptoms. 1847 – The effectiveness of recompression for the treatment of decompression sickness (DCS) in caisson workers was described by B. Pol and T.J. Watelle. 1857 – Felix Hoppe-Seyler repeated Boyle's experiments and suggested that sudden death in compressed air workers was caused by bubble formation, and recommended recompression therapy. 1861 - Bucquoy proposed the hypothesis that "les gaz du sang... repassent à l'état libre sous l'influence de la décompression ...et occasionnent des accidents comparables à ceux d'une injection d'air dans les veines" ("the blood gases ... return to the free state under the influence of decompression ... and cause accidents comparable to those of an injection of air in the veins"). 1868 – described decompression sickness as an occupational illness of sponge divers. 1873 – Dr. Andrew Smith first used the terms "caisson disease" and "compressed air illness", describing 110 cases of decompression sickness as the physician in charge during construction of the Brooklyn Bridge. The nickname "the bends" was used after workers emerging from pressurized construction on the Brooklyn Bridge adopted a posture similar to fashionable ladies of the period "the Grecian Bend". 1878 – Paul Bert determined that decompression sickness is caused by nitrogen gas bubbles released from tissues and blood during or after decompression, and showed the advantages of breathing oxygen after developing decompression sickness. 1889–90 - Ernest William Moir builds the first medical airlock when he noticed that about 25% of the workforce digging the Hudson River Tunnel were dying from decompression sickness and realised that the solution was recompression. 1897 – N. Zuntz proposed a perfusion-based tissue model. 1906 – V. Schrotter suggested a uniform decompression of 20 minutes per atmosphere of pressure. J.S. Haldane was commissioned by the British Admiralty to study decompression sickness. 1908 – John Scott Haldane prepared the first recognised decompression table for the British Admiralty. This table was based on experiments performed on goats using an end point of symptomatic DCS. 1912 – Chief Gunner George D. Stillson of the United States Navy created a program to test and refine Haldane's tables. This program ultimately led to the first publication of the United States Navy Diving Manual and the establishment of a Navy Diving School in Newport, Rhode Island. Diver training programs were later cut at the end of World War I. 1912 – Leonard Erskine Hill proposed continuous uniform decompression 1915 - US Navy published C&R tables. 1916 - UN Navy established its Deep Sea Diving School in Newport, Rhode Island. 1924 - US Navy published first US Navy Diving Manual. 1927 – Naval School, Diving and Salvage was re-established at the Washington Navy Yard. At that time the United States moved their Navy Experimental Diving Unit (NEDU) to the same naval yard. In the following years, the Experimental Diving Unit developed the US Navy Air Decompression Tables which became the accepted world standard for diving with compressed air. 1930's – J.A. Hawkins, C.W. Schilling and R.A. Hansen conducted extensive experimental dives to determine allowable supersaturation ratios for different tissue compartments for Haldanean model. 1935 – Albert R. Behnke et al. experimented with oxygen for recompression therapy. 1937 – US Navy 1937 tables developed by O.D. Yarbrough were published. 1938 – Edgar End and Max Nohl made the first intentional saturation dive. 1941 – Altitude decompression sickness was treated with hyperbaric oxygen for the first time. 1942 – Aggazzotti and Ligabue detect evidence that gas particles in the free state exist in living animal tissue. 1942 – Albert R. Behnke proposed exposing humans to increased ambient pressures long enough for all the tissues to become saturated with inert gases. 1942 – Momsen referred to "partial pressure vacancy" (PPV) where he used partial pressures of oxygen and helium as high as 2–3 ATA to create a maximal PPV. 1956 – US Navy Decompression Tables developed by M. Des Granges (1956) were published. 1957 – George F. Bond began the Genesis project at the Naval Submarine Medical Research Laboratory proving that humans could withstand prolonged exposure to different breathing gases and increased environmental pressures. 1960 – F.C. Golding et al. split the classification of DCS into Type 1 and 2. 1965 – LeMessurier and Hills published their paper, A thermodynamic approach arising from a study on Torres Strait diving techniques, which suggested that decompression by conventional models results in bubble formation which is then eliminated by re-dissolving at the decompression stops which is slower than off-gassing while still in solution. This indicates the importance of minimizing bubble phase for efficient gas elimination. 1965 – French Navy GERS (Groupe d'Etudes et Recherches Sous-marines) 1965 table was published. 1965 – M.W. Goodman and Robert D. Workman introduced recompression (treatment) tables utilizing oxygen to accelerate elimination of inert gas 1965 – US Navy published Nitrox and Heliox tables developed by Robert D. Workman. 1965 – The first commercial saturation dives were performed in 1965 by Westinghouse at . 1965 – Conshelf III experiment was carried out by Jacques Cousteau and a group of divers for three weeks at the depth of 100 m. 1967 – The term "oxygen window" was first used by Albert R. Behnke. 1971 – Albano proposed a method for decompression based on the assumption that tissue microbubble growth could be controlled by ensuring that a maximum supersaturation in tissues is not exceeded. 1971 – In the US, the Williams-Steiger Occupational Safety and Health Act of 1970 triggered investigation of the safety of US Navy tables in reaction to an attempt to legislate their use for commercial diving. 1972 – Royal Naval Physiological Laboratory (RNPL) published tables based on Hempleman's tissue slab diffusion model. 1973 – Isobaric counterdiffusion was first described by D.J. Graves, J. Idicula, Christian Lambertson, and J.A. Quinn in subjects who breathed one inert gas mixture while being surrounded by another. 1973 – French civilian Tables du Ministère du Travail 1974 (MT74) were published. 1974-76 – Further work was done by the United States Navy Experimental Diving Unit on excursion dives from saturation 1976 – M.P. Spencer showed that the sensitivity of decompression testing is increased by the use of ultrasonic methods which can detect mobile venous bubbles before symptoms of DCS emerge. 1976 – Edward Beckman published findings of a comparison of US Navy air tables with RNPL, Buhlmann and other tables and indicating that the US Navy tables for diving below 100 fsw which were reputed to produce unacceptable rates of decompression sickness for civilian applications, were significantly less conservative than the other models in the comparison. 1979 – The first US Navy saturation procedures were published in the US Navy Diving Manual revision 2. 1981 – Huggins model and tables using Spencer's formula for no-decompression limits were published. 1981 – D.E. Yount described the Varied Permeability Model. 1981 – Peter B. Bennetts Atlantis III saturation experiment to 2250 fsw (686 msw). 1982 – Paul K Weathersby, Louis D Homer and Edward T Flynn introduced survival analysis into the study of decompression sickness. 1983 – E.D. Thalmann published the E-L model for constant PO2 nitrox and heliox closed circuit rebreathers. 1983/4 – Albert A. Bühlmann published Decompression–Decompression sickness. Bühlmann recognized the problems associated with altitude diving and proposed a method which calculated maximum nitrogen loading in the tissues at a particular ambient pressure. Set of parameters : ZH-L 12.