Goats and Gases: “The Prevention of Compressed Air Illness” by Haldane et al— A Commentary

There is scarcely a publication on decompression theory that does not refer to “The Prevention of Compressed Air Illness,” the seminal paper of Haldane et al ¹ that appeared in the Journal of Hygiene. Although the monograph of the Frenchman Paul Bert ² from 1878 is regarded as fundamental in respect to the discovery of the principal mechanisms involved in decompression, it is Haldane who is always associated with the first set of decompression tables. In general, 4 major topics are usually attributed to him: 1) the concept of “stage decompression,” 2) the “2:1”-supersaturation ratio, 3) the calculation of the on- and off-gassing of body tissues with inert gas, and 4) the calculation of the first decompression tables.

John Scott Haldane (1860–1936) gained his first merits in industrial hygiene in the 1890s in connection with mining. This applied research led him to reason about issues of pure science, namely the physiology of breathing. After researching the effects of vitiated air and carbon monoxide poisoning, he turned his attention to the general question of the regulation of respiration, leading to what many consider his most important paper in 1905. ³

In the same year he was asked to examine the problems experienced in deep-sea diving. These problems were chiefly those involving the breathing apparatuses and the so-called caisson disease, often referred to as the “bends” or “decompression illness.” Haldane was made the scientific member of a committee formed by the Admiralty to investigate these matters. His reputation as Britain's leading expert on respiration surely made him a favorite for this post, but the position of his brother, Richard Burdon Haldane (1865–1928), as War Secretary from 1905 to 1912 may well have had some influence on that decision.

The paper in the Journal of Hygiene shares obvious similarities with the report to the British Admiralty. ⁴ However, the Admiralty report focuses more on the outcomes of the research in terms of technical solutions, instructions, and the tables, whereas the Journal of Hygiene paper from 1908 deals with the nature of the experiments conducted in the decompression trials in greater detail. Approximately one third of the paper reviews the physiological foundations of decompression in diving, whereas the remainder provides details about the experiments carried out on goats and humans.

As mentioned above, Haldane is usually given credit for these experiments and the tables that resulted from them. Looking at the Admiralty report and the paper from 1908 more closely, and taking into account other contemporary diving literature,^5,6 this impression gets diluted to a significant extent. As a matter of fact, only the second point, the “2:1-ratio,” is originally from Haldane. He never denied this in the 1908 paper or in subsequent papers.

As the initial section of the paper from 1908 correctly references, nearly all the theoretical work had already been performed by others, such as Paul Bert, Nathan Zuntz, Hermann von Schrötter, and Leonard Hill, to name a few. Haldane adjusted some of their statements on perfusion rates by applying the research results from his earlier projects. A valuable source of information on the symptoms and possible causes were the case reports given to Haldane by E. W. Moir and Dr F. S. Keays, both associated with S. Pearson & Son Ltd, London. ¹

The experiments on the goats were, for the most part, conducted by the coauthors of the paper, Boycott and Damant, a circumstance that is conceded in both the report and the paper. Their main objective was to certify the decompression strategy of Haldane. Goats were chosen not only because they allegedly resemble the fat-blood distribution of humans so closely, but also because they were the animals that could be handled in a most efficient manner for this testing. Robert Davis, chief engineer of Siebe & Gorman Ltd in London, manufacturer of breathing and diving apparatuses, who was also involved in the research, remarked that Haldane would have actually preferred pigs or baboons. ⁷ But these animals were too intelligent and could not be tricked into entering the decompression chamber so easily.

Paul Bert had already devised the principle of stage decompression in 1878. However, he favored uniform decompression (ie, a continuous ascent at a constant speed) because he simply considered it more practicable. ² The instructions for such decompression were quite fuzzy, though, and the number of incidences of decompression illness remained high. In the experiments with the goats, Haldane noted that upon rapid decompression the organism tolerates the liberation of a certain amount of excess nitrogen without producing symptoms of the bends. By extensive tests carried out on goats, various levels of severity of decompression illness could be reproduced. From the observations gathered, Haldane concluded that halving the ambient pressure would not bring about symptoms of the “compressed air illness.” ¹ If, therefore, a diver had spent time at 20 m of seawater (msw) depth, he could be brought up to 10 msw without risking the bends. Likewise, a diver who spent time at 40 msw could ascend rapidly to 20 msw, and so on. After reaching such a stop, sufficient time was spent to allow the excess nitrogen to be eliminated from the body, after which the diver ascended to the next stop. Hence, the term stage decompression was applied to this process. It was Haldane who helped establish this concept as the standard decompression procedure, thus not only introducing a safer decompression strategy, but also reducing the overall decompression time considerably. The uniform decompression method, according to Haldane, wastes too much time:

It seems evident that with uniform decompression a great part of the time spent in the process is time lost, for during the greater part of the time …the difference in pressure between the nitrogen in the tissues and that in the air is much less than it might safely be made; and as a consequence the nitrogen is given off from the lungs much more slowly than would be safely possible. ⁴

To consider the different types of perfusion rates for the different tissues, 5 theoretical tissues (ie, compartments) were modeled to represent tissues with “fast” and “slow” perfusion rates. ¹ This was done by defining half-lives for nitrogen uptake and elimination. This concept follows the so-called exponential law, a principle that has many applications in physics, chemistry, and biology. ⁸ The values for the half-lives given in the paper from 1908 range from 5 to 75 minutes; today, modern decompression computers usually use 6 to 12 compartments between 5 and 240 minutes. Later investigators heavily revised the supersaturation ratio, but the basic principle of Haldane's decompression strategy remains intact up to the present day. ⁹

It is worthy of note that John Scott Haldane was not very proficient in mathematics, typically using only simple arithmetic in his research. Although he surely had a basic understanding of the mathematics involved in “his” decompression procedure, he very likely would not have bothered to calculate the tables. Fortunately, his then-15-year-old son, J. B. S. Haldane (1892–1964), was quite talented in mathematics. Thus, J. B. S. Haldane (later achieving great scientific fame in his own right) not only was allowed to do 2 test dives, but was also able to perform the computation of the tables, as his mother noted in her autobiography. ¹⁰

None of J. S. Haldane's shortcomings can diminish the significance of his contribution to decompression theory. The tables presented by Haldane, Damant, and Boycott became the standard of their time for diving operations throughout the world, drastically reducing the number of decompression fatalities. They still provide the basis for modern decompression algorithms used in diving computers and diving tables found in commercial, recreational, and military diving. The development of these original tables provide a clear example of how a significant scientific discovery is rarely made through the mind of just a single genius but rather by the collaborative efforts of several individuals. The fact that these tables are today almost entirely associated with Haldane is merely the tribute that is usually paid to the architect behind such a discovery.