John Scott Haldane (1860-1936) Scottish physiologist John Scott Haldane is considered to be the father of decompression theory. Haldane was the first scientist to apply a scientific approach to predicting decompression and despite being almost 100 years old his methods still form the basis of the majority of current decompression practice. 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 Hygiene 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 before the absolute pressure on him was halved (7 bar to 3.5 bar), but only 15m 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., & 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 accepted 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. Hopefully that provides some interesting background to how we arrived at todays understanding of decompression theory. Please note it is dangerous to embark upon any form of decompression diving without the correct training and skill levels. The above article was written by Mark Powel, one of only a few full time Technical Instructors in the UK. If you would like to discuss any aspects of Technical training with Mark, please contact him direct on 07770 864327.