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Decompression Theory: Paul Bert and John Scott Haldane
Bert is remembered more as a man of science than as a politician or administrator. His classical work, La Pression barometrique (1878), laid the foundation of knowledge of the physiological effects of air-pressure, both above and below atmospheric pressure.
Bert became interested in the problems that low air pressure caused for mountain climbers and balloonists. This led him to study the problems that divers had with increased pressure as well. He reviewed the current reports of research in this area. He was struck in particular by the experiences that Dr. Alphonse Gal had while diving in Greece. Dr. Gal was the first doctor to actually dive in order to study how the body reacted underwater. Bert studied Gal's own diving experiences and his reports on divers who were injured or killed.
Bert’s research and experiments led to his conclusion that pressure does not effect us physically, but rather chemically by changing the proportions of oxygen in the blood. Too little creates oxygen deprivation and too much creates oxygen poisoning. He showed that pure oxygen under high pressure can be deadly and to this day Central Nervous System (CNS) oxygen toxicity is known as the ‘Paul Bert Effect’.
Perhaps his most important discovery was the effects of nitrogen under high pressure, which for the first time explained decompression. In investigating the causes of decompression illness Bert exposed 24 dogs to pressure of 7-9 ¾ atmospheres (equivalent to a depth of 87.5msw or 290fsw) and decompressed them rapidly in 1-4 minutes. The result was that 21 died, while only one showed no s ymptoms. In one of his cases, in which the apparatus burst while at a pressure of 9 ½ atmospheres, death was instantaneous and the body was enormously distended, with the right heart full of gas. However, he also found that dogs exposed, for moderate periods, to similar pressures suffered no ill effects provided that the pressure was relieved gradually, in 1-1 ¾ hours.
He determined that the symptoms observed were due to the formation of gas bubbles in the blood and tissues. He also identified nitrogen as the gas which was producing the bubbles. He went on to explain that it was the increase in partial pressure of Nitrogen which caused Nitrogen to become dissolved in the bodies tissues and then the subsequent reduction in pressure caused the nitrogen to come out of solution and form bubbles. As a result of this research Bert concluded that divers and caisson workers decompress slowly and at a constant rate “for they must not only allow time for the nitrogen of the blood to escape but also to allow the nitrogen of the tissues time to pass into the blood”.
He also went on to suggest stopping divers halfway to the surface during decompression after a deep dive and as such was the first to suggest what are now known as deep stops.
Bert carried out a number of experiments into methods of treating the compressed air illness once the symptoms had appeared. His experiments showed that once bent the symptoms could be relieved by returning into the compressed air environment of the caisson or tunnel and then decompressing the patient slowly. This was clearly the start of recompression therapy which has been shown to be the most effective way of treating decompression illness. He also showed that breathing pure oxygen was highly effective in relieving the symptoms of decompression illness. In one of his experiments on animals he noted: “The favorable action of oxygen was . . . evident; after several inhalations (of oxygen) the distressing symptoms disappeared.”
In a later entry, Bert attempted to explain why oxygen worked. “I thought that if the subject were caused to breathe a gas containing no nitrogen — pure oxygen for example — the diffusion would take place much more rapidly and perhaps would even be rapid enough to cause all the gas (nitrogen) to disappear from the blood.”
This is indeed why oxygen is so useful in treating decompression illness. Bert was the first to propose the concept of oxygen recompression therapy, though the actual practice wasn’t implemented until many years later.
Haldane was born in Edinburgh into a notable family. He was trained in medicine at the University of Edinburgh and graduated in 1884. After graduation he moved to Queen’s College, Dundee (which at the time was part of St Andrews University) before transferring to Oxford University. At Oxford he lectured on medicine and conducted medical research. In 1906, in collaboration with John Gillies Priestley (1880-1941), he discovered that the respiratory reflex is triggered by an excess of carbon dioxide in the blood rather than a lack of oxygen.
In his later years Haldane became an authority on the effects of pulmonary diseases on industrial workers and in 1912 was appointed Director of the Mining Research Laboratory in Doncaster. Haldane also founded the Journal of Hygene and it in this publication that the first set of diving decompression tables were published. During his lifetime ha also published Organism and Environment (1917), Respiration (1922) and The Philosophy of a Biologist (1936). Haldane died of pneumonia in 1936.
It is Haldane’s work on decompression for which he is most widely remembered, especially amongst divers. In 1905 Haldane was approached by the Royal Navy’s Deep Diving Committee to carry out research on a number of aspects of their diving operations. The most important aspect of this work was looking at ways to avoid the bends or “caissons disease” as it was then widely known.
It had long been observed that men working in pressurised bridge and tunnel construction areas, known as caissons, would sometimes complain of paint in their joints. As the depth they were working increased and so the pressure inside the caisson increased the severity of the symptoms increased. Many suffered total paralysis and there were frequent deaths. Research and practical observation suggested that gasses, breathed under pressure by the workers, were diffusing into the body’s tissues and when these gasses came out, in the form of bubbles in the body, the workers got caisson disease, or what we now call decompression sickness (DCS).
The same symptoms were seen amongst divers who were breathing air under pressure. Divers were told to minimise this risk by ascending slowly to begin with, and then rising faster as they got nearer the surface. Thanks to Haldane's work, we know now that this was incorrect and potentially dangerous.
Haldane began experimenting on goats as they were readily available subjects and are of a similar size to humans. He found that the body could tolerate a certain amount of excess gas with no apparent ill effects. Caisson workers pressurised at two atmospheres (10 msw/33 fsw) experienced no problems, no matter how long they worked. Similarly goats saturated to 50 msw (165fsw) did not develop DCS if decompressed to half ambient pressure.
Haldane wrote “the formation of bubbles depends, evidently, on the existence of a state of supersaturation of the body fluids with nitrogen. Nevertheless there was abundant evidence that, when the excess of atmospheric pressure does not exceed about one-and-a-quarter atmospheres, there is complete immunity from symptoms due to the bubbles, however long the exposure to the compressed air may have been, and however rapid the decompression. Thus, bubbles of nitrogen are not liberated within the body unless the supersaturation corresponds to more than a decompression from a total pressure of two-and-a-quarter atmospheres to a total pressure of one atmosphere (i.e. that normally existing on the surface of the earth).”
In order to explain these observations Haldane suggested that we consider the body as a group of tissues which absorbed and released gases at different rates. This meant the tissues were all exposed simultaneously to the breathing gasses at ambient pressure, but each tissue reacted to the gas pressure in a different way. He then went on to suggest a mathematical model to describe how each of the tissues absorbs and releases gases and put limits on the amount of over pressurization that the tissues could tolerate.
Haldane introduced the concept of half times to model the uptake and release of nitrogen into the blood. The half time is the time required for a particular tissue to become half saturated with a gas. He suggested 5 tissue compartments with half times of 5, 10, 20, 40 and 75 minutes.
He also demonstrated that decompression was most dangerous nearest the surface. One of the key elements of Haldane’s work, and one that is still as relevant today, is that he identified that it is the relative pressure differences that are important rather than just the absolute depth changes. As we are now well aware, a diver ascending from 60m would have to travel 35m, to a depth of 25m, before the absolute pressure on him was halved (7 bar to 3.5 bar), but would only have to ascend 15m, to a depth of 5m, to achieve the same result from 20m (3 bar to 1.5 bar). He wrote “Hence it seemed to me probable that it would be just as safe to diminish the pressure rapidly from four atmospheres to two, or from six atmospheres to three, as from two atmospheres to one. If this were the case, a system of stage decompression would be possible and would enable the diver to get rid of the excess of nitrogen through the lungs far more rapidly than if he came up at an even rate. The duration of exposure to high pressure could also be shortened very considerably without shortening the period available for work on the bottom”.
Haldane also developed practical dive tables based on his research that included slower ascent rates as the diver approached the surface. The results of this research and Haldane’s diving tables were published in 1908 in the Journal of Medicine (Boycott, A.E., Damant, G.C.C., and Haldane, J.S. "The Prevention of Compressed Air Illness," Journal of Hygiene, Volume 8, (1908), pp. 342-443.)
Following the report of the Admiralty’s Deep Diving Committee it was decided to publish the committee’s conclusions in the form of a blue book available to the public. The conclusions were universally acceped and it became the foundation of all diving operations, both in the UK and abroad. In 1912 the US Navy adopted the tables published by Boycott, Damant and Haldane and these tables were used by all US Navy divers up until 1956.
For more information on decompression theory click on the links below for information on the development of the US Navy decompression tables and the Buhlmann Tables as well as more information on half times and tissue compartments.