Exploring the evolution and purpose of decompression theory can be a fascinating journey. The development of the first hyperbaric chambers in the 1600s and the discovery of the effects of oxygen and nitrogen on our bodies have all contributed to our ability to dive more safely. Furthermore, this knowledge has enabled us to treat diving injuries effectively.
Brief History of Decompression Theory
Reverend Nathaniel Henshaw 1628-1673
In 1662, Reverend Nathaniel Henshaw of Britain created what is believed to be the first hyperbaric chamber. He named it "domicilium". Henshaw believed that increased air pressure could alleviate some acute injuries, while lower pressures might be useful in treating chronic illnesses. The Domicilium was a large fifteen-by-fifteen-foot chamber with the capacity to pressurize to a range of two to four atmospheres, greater than most modern chambers, which can only compress to a maximum of three atmospheres.
Henshaw initially proposed that the chamber could help with digestion, insensible breathing, facilitate breathing, and assist in the expulsion of sputum, making it a valuable tool in preventing lung infections. However, many of these claims were unsubstantiated.
Henshaw's idea was later applied in different countries in Europe to improve health with "Compressed Air Baths". People breathed air, not oxygen, but by increasing the air pressure, the partial pressure of oxygen goes up. This increase proved to be beneficial but not yet understood by the physicians who applied the chamber sessions.
Although the results of the therapy were unknown, it is interesting to note that this was done before Boyle confirmed the relationship between the volume and pressure of gases in the 1670s, the actual discovery of oxygen in 1774 by Joseph Priestly, or the development of Dalton's and Henry's laws of gases in the early 1800s.
Sir Robert Boyle 1627-1691,
Sir Roberto Boyle is often stated to be the first person to document decompression sickness. In 1660, he conducted an experiment on a bird that he subjected to reduced ambient pressure by use of a primitive vacuum 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. In 1670, he experimented with putting a viper (among many other animal experiments) in a vacuum jar and rapidly reduced the pressure. A bubble appeared in the snake's eye. When the pressure was increased, the bubble vanished, and the snake was "content". No theory was proposed, as he had no idea what had happened, and it would be a good two centuries until there were any further discoveries on decompression sickness. Still, it was from this experiment that the idea began to arise.
Much of our understanding of decompression sickness, however, still stems from the observations and scientific endeavours of the 1800s.
John Dalton 1766-1844
Dalton's Law (also called Dalton's Law of partial pressures) states that "The total pressure exerted by a mixture of gases is equal to the sum of the pressures of each of the different gases making up the mixture – each gas acting as if it alone were present and occupied the total volume."
He formulated the theory of partial pressure in 1801, and its applications to diving are wide and extremely important, especially in technical diving.
William Henry 1774-1836
Henry's Law says that "the amount of gas that dissolves into a liquid at a given temperature is directly proportional to the partial pressure of that gas".
It is alleged that here he formed a strong association with John Dalton. In 1802, he published Henry's Law.
While Archimedes' Principle, Charles's Law and Boyle's Law focus on the physical effect, Henry's Law and Dalton's Law explain the physiological effect.
Charles-Jean (Jacques) Triger 1801-1867
During the Industrial Revolution, the steam engine was invented and later adapted to run air compressors. In 1840, Charles-Jean Triger used compressed air to extract coal in France through a pressurized chamber called a caisson, meaning "box" in French. The caisson, with an open bottom, was lowered into the ground, and compressed air was pumped into it, pushing water down and out of the bottom. This allowed workers to dig deeper into the ground and access valuable coal deposits below the water table. The workers entered the working chamber through an airlock at the top, and the whole system was managed manually through inlet and outlet valves. This method allowed for the first known injuries caused by compressed air and marked the beginning of a decades-long search for the cause of the mysterious and deadly "caisson disease."
Tiger was a unique employer at the time, as he paid close attention to the safety and well-being of the workers. Because of this, he hired two physicians, B. Pol and T.J.J. Watelle, to oversee the medical needs of future projects using the caisson.
Many workers suffered from health issues while working on Triger's projects conducted at high pressure, including one of the physicians who suffered from caisson disease.
Pol and Watelle were the first to suggest recompression as a treatment for caisson disease in 1854, but they incorrectly believed that it was caused by decreased oxygen in the circulation.
There were few advancements in understanding decompression sickness for the next two decades.
Doctors treated decompression sickness symptoms with various methods, including cold water baths, natural oils, cordials, and even submersion in liquid mercury. However, little progress was made in understanding the disease until more physicians began investigating its causes and potential cures due to the increasing number of cases.
Dr Alphonse Jaminet and Dr Andrew Smith
In 1868, Captain James Buchanan Eads began constructing the St. Louis Bridge. The construction proceeded without any injuries until the workers reached a depth of 18 meters/60 feet, which was only half of the required depth. The workers began to complain about joint pain, partial paralysis of lower limbs, headaches, and itchiness. Many of them emerged from the caisson in a bent-over posture due to joint pain, shortness of breath, and abdominal pain. This posture resembled the fashionable female posture of the 1820s, known as the Grecian Bend, and therefore, the workers who contracted caisson disease were referred to as having "the bends".
In 1870, Dr. Alphonse Jaminet set up a medical facility on a barge to treat injured workers at a construction site. This was one of the first on-site occupational clinics in the US and required workers to remain under Dr Jaminet's care for at least an hour after work, drinking three-quarters of a pint of strong beef tea.
Jaminet pioneered early treatments for decompression sickness despite limited knowledge. He established standard rates for compression and decompression, though he wrongly linked "the bends" to fast compression. Despite fast decompression rates by modern standards, his work set the stage for preventing decompression injuries.
The Brooklyn Bridge commenced in 1870 with caissons three times larger than that of the St Louis Bridge. Dr Andrew Smith was an ENT specialist responsible for the workers. He noticed that many workers were suffering from a disease with symptoms such as joint pain, headache, shortness of breath, vomiting, and paralysis. It was Dr Smith who first named the illness "caisson disease."
Dr. Smith found that compressed air exposure caused workers' symptoms to worsen. Increased heart rates, sweating, and urination were observed. Dr Smith correctly linked the higher heart rates to elevated carbon dioxide levels, likely from gas lamps' incomplete combustion. He also noted the hot and humid working conditions in the caissons caused excessive sweating and urination.
He could not prevent the disease because its cause was still unknown, so he did his best to treat it with morphine or atropine. He did, however, make the important observation that connected "obese" workers with an increased risk of caisson disease.
Paul Bert 1833-1886
was a French zoologist, physiologist, and politician. He earned the nickname "Father of Aviation Medicine," but despite his intelligence, he held many unfounded and irrational racist beliefs.
As a physiologist, he was interested in the problems that low air pressure caused for mountain climbers and balloonists and built many chambers to experiment with barometric pressure.
His most important work, La Pression Barometrique (1878), laid the foundation for understanding how air pressure affects us, both above and below atmospheric pressure.
Bert concluded through his research and experiments that pressure does not affect us physically but rather chemically by altering the proportions of oxygen in the blood. When oxygen levels are too low, it leads to oxygen deprivation, while too much oxygen can cause oxygen poisoning. Bert's experiments showed that exposure to pure oxygen under high pressure can be fatal. As a result, Central Nervous System (CNS) oxygen toxicity is now known as the "Paul Bert Effect" to this day.
Bert's research led to one of the most significant discoveries in the field of decompression illness. He discovered that nitrogen under high pressure could cause decompression. Like many scientists of his time, Bert was not hesitant to experiment on animals.
In 1870, he conducted numerous experiments on the rapid decompression of mice, dogs, and birds. He subjected these animals to extended periods in chambers pressurized to 10 times that of normal atmospheric levels, then rapidly decompressed them in a matter of minutes or seconds. In an experiment with 24 dogs, he decompressed them rapidly after exposing them to pressure equivalent to a depth of 87.5 mt/290 ft. Unfortunately, 21 dogs died, but Bert found that dogs exposed to similar pressures for a moderate period of time suffered no ill effects if the pressure was gradually relieved over 1-1 ¾ hours.
Bert conducted research and determined that the symptoms observed were a result of gas bubbles forming in the blood and tissues. Nitrogen was identified as the gas responsible for the bubbles. He explained that an increase in the partial pressure of nitrogen caused it to dissolve in the body's tissues, and when the pressure was subsequently reduced, the nitrogen came out of solution and formed bubbles.
Based on his findings, Bert concluded that divers and caisson workers must decompress slowly and at a constant rate. This is because they not only need to allow time for the nitrogen in their blood to escape but also to let the nitrogen in their tissues pass into the blood.
In relation to diving, he was the first to suggest deep stops, proposing to stop divers midway during decompression after a deep dive.
Bert conducted various experiments to find ways of treating compressed air illness once the symptoms appeared. His experiments showed that returning the patient to the compressed air environment of the caisson or tunnel and then decompressing them slowly could relieve the symptoms of being bent. This marked the beginning of recompression therapy, which has been found to be the most effective way of treating decompression illness. Additionally, he discovered that breathing pure oxygen was highly effective in relieving the symptoms of decompression illness. In one of his animal experiments, he observed that "The favourable action of oxygen was...evident; after several inhalations (of oxygen), the distressing symptoms disappeared."
In a later entry, Bert tried to explain why oxygen was so effective. He 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, in fact, why oxygen is so useful in treating decompression illness. Bert was the first to propose the concept of oxygen recompression therapy, although it wasn't put into practice until many years later.
These practices of slow decompression, oxygen administration, and recompression are mainstays of the current treatment of decompression illness over 150 years later.
John Scott Haldane 1860-1936
Hailed as the father of modern dive decompression theory. Now, a century later, virtually all dive computers and dive tables in use today are derived from the Dive Decompression Theory theoretical model that he invented.
Born in Scotland, he was a renowned British physician, physiologist, and philosopher; he was known for his intrepid self-experimentation. He conducted several experiments on himself, including locking himself in sealed chambers and breathing potentially lethal cocktails of gases while recording their effects on his mind and body. Haldane's discoveries about the human body and the nature of gases were significant. He also conducted experiments on his wife and son, J. B. S. Haldane, even when his son was still quite young. Haldane’s son JBS Haldane, was a well-known and accomplished geneticist in his own right (Aldous Huxley based a character on him in his famous work, Brave New World).
John Scott Haldane was the first scientist to apply a scientific approach to predicting and preventing decompression sickness and created the first accurate decompression tables for those working in compressed air environments. In 1905, he demonstrated that the respiratory drive is influenced by the pH of the blood, and a year later, in 1906, he discovered that the respiratory reflex is triggered by an excess of carbon dioxide in the blood rather than a lack of oxygen. In 1919, he pioneered the use of pure therapeutic oxygen in treating victims of carbon monoxide poisoning, which remains a cornerstone of modern emergency treatment for the condition.
Haldane is well-known for his contribution to the field of decompression, which holds significant importance for divers. In 1906, the Royal Navy's Deep Diving Committee approached Haldane to conduct research on various aspects of their diving operations. The primary objective of this research was to find ways to prevent the 'bends' or "caisson disease," which was commonly known at that time. Haldane drew on the work of William Henry (Henry's Law), Paul Bert, and practical observation, which suggested that gases breathed under pressure by the workers seeped into the tissues of the body. When these gases came out of the body in the form of bubbles, the workers developed caisson disease, which is now referred to as decompression sickness (DCS).
Haldane proposed a model that explains how nitrogen (or any other inert gas) accumulates in our bodies. He used Henry's law, which states that the amount of gas that dissolves in a liquid is proportional to the partial pressure of that gas, and he needed to put time to that.
Divers who breathed air under pressure experienced the same symptoms as caisson workers. In the past, divers were advised to ascend slowly and then rise faster as they approached the surface to minimize the risk. Thanks to Haldane's research on dive decompression theory, we now know that this approach was incorrect and potentially dangerous.
Haldane conducted numerous experiments in his chamber in the early 1900s, using both nonhuman and human subjects. He observed that many of the symptoms were caused by the nervous system's effects. He even viewed nitrogen bubbles in the white matter on microscopy. As white matter is mostly made up of fat, Haldane appropriately hypothesized that the severity of caisson disease is linked to the amount of fat in the patient, much as Dr Andrew Smith had decades before. Based on this and other research, restrictions were placed on who could work in caissons. Specifically, "really fat men should never be allowed to work in compressed air, and plump men should be excluded from high-pressure caissons."
In 1907, Haldane constructed a decompression chamber at the Lister Institute for the purpose of making underwater diving safer. He conducted experimental work and developed the first decompression tables using his concept of stage decompression. Haldane conducted extensive experiments with animals and divers in deep-water lochs in Scotland. The experiments examined the effects of depth and pressure exposure, duration, and patterns of decompression. Initially, rabbits, guinea pigs, rats, and mice were used, but it was difficult to detect symptoms in smaller animals. His animal of choice for experiments involving a decompression chamber was goats. Although pigs have tissue characteristics that are more similar to humans, goats were more readily available and had a body-mass-to-cardiac-output ratio similar to that of a lean man. Haldane was not viewed as a serial goat bender in those days but rather as a scientist who worked diligently to determine the underlying conditions of the experiments. His work laid the foundation for modern decompression medicine.
The hundreds of experiments in his chamber led him to conclude that the quantity of nitrogen absorbed within a particular body tissue depended on the amount of fat present in that tissue as well as the blood flow through it. He found that the body could tolerate a certain amount of excess gas with no apparent ill effects. Caisson workers pressurized at two atmospheres (10 msw/30 ftsw) experienced no problems, no matter how long they worked. Similarly, goats saturated to 50 msw/165 ftsw did not develop DCS if decompressed to half ambient pressure. He also introduced the concept of supersaturation, which suggested that the body's tissues could retain nitrogen for a limited period after decompression before forming bubbles. This finding helped to explain Bert's discovery of a delay in the onset of symptoms following rapid decompression.
One of Haldane's most effective approaches was to understand that he didn't have to model every single aspect of a goat and know all its details. Instead, he just needed to have representative numbers that were close enough so that their effects overlapped. These representative numbers are referred to as compartments.
Haldane's dive decompression theory included the development of the tissue half-times theory to model the absorption and release of nitrogen in the bloodstream. He proposed that the body should be viewed as a collection of tissues (compartments) that take in and release gases at different rates. Each tissue was exposed to breathing gases at ambient pressure simultaneously but responded differently to the gas pressure. Haldane then introduced a mathematical model to explain how each tissue absorbed and released gases while also setting limits on the amount of over-pressurization that the tissues could handle.
Half-time refers to the time it takes for a specific tissue to become half-saturated with a gas or for the slowest tissue to desaturate to half of the partial pressure of its supersaturated state. Haldane suggested five tissue compartments with half-times of 5, 10, 20, 40, and 75 minutes. His research led to practical dive tables that recommended slower ascent rates as the diver approached the surface.
Haldane made an important discovery about the behaviour of dissolved nitrogen in the human body. He found that at a certain point when the partial pressure of nitrogen reached double the absolute pressure, it would start to spontaneously undissolve and form bubbles. While the exact threshold is now understood to be more complex than this, Haldane's insight was still a significant step forward at the time. Using his findings, Haldane developed a table to help divers avoid reaching the critical 'double' pressure, which helped prevent the British Navy from routinely injuring divers and reduced the Admiralty's expenses.
He showed that decompression was the most risky near the surface. One of the main elements of Haldane's work, which is still relevant today, is that he identified that it is the differences in relative pressure that matter more than just the absolute changes in depth. As a result, he concluded that staged decompressions would be possible and allow the diver to eliminate excess nitrogen through the lungs much more quickly than if he ascended at a uniform rate. The duration of exposure to high pressure could also be significantly reduced without shortening the time available for work at the bottom.
After the Admiralty's Deep Diving Committee submitted their report, it was decided to publish their conclusions on dive decompression theory in the form of a blue book. This book was made available to the public, and the committee's conclusions were widely accepted, becoming the foundation for diving operations both in the UK and abroad. In 1912, the US Navy also adopted these tables, which were then used by all US Navy divers until 1956.
Doppler Ultrasound Flowmeter
The link between diving and medicine is due to the ability of Doppler ultrasound technology to detect bubbles moving in blood vessels, which helps in detecting the presence of bubbles in those vessels after diving.
Decompression from most dives causes a degree of bubble formation in the veins. It is generally believed that the number of these bubbles is an indicator of the probability of decompression sickness (DCS) developing. Higher numbers of bubbles are more likely to result in DCS.
However, it's important to note that silent bubbles and a few of them do not cause symptoms of DCS.
When it comes to the significance of bubbles themselves, there are a few problems. Bubbles are commonly detected in the veins following dives that do not produce DCS. While the risk of developing DCS does appear to be greater following dives that produce high bubble grades, a significant proportion of such dives still do not result in obvious problems. Therefore, Doppler bubble detection is not a valid diagnostic test for DCS, and high bubble grades on Doppler in the absence of DCS symptoms would not be an indication for recompression treatment.
In part two, we will discuss Instructor development decompression theory with exams.
Anttila, M., PhD. (2016, February 12). Gradient Factors - Dive rite. Dive Rite. https://www.diverite.com/articles/gradient-factors/
Decompression theory for goats. (n.d.). https://www.nigelhewitt.co.uk/diving/maths/deco.html
Dive-Tech, M. P. (n.d.). Dive-Tech: Decompression theory - Paul Bert and John Scott Haldane. http://www.dive-tech.co.uk/bert%20and%20haldane.htm
Ninokawa, S., & Nordham, K. D. (2021). Discovery of caisson disease: a dive into the history of decompression sickness. Baylor University Medical Center Proceedings, 35(1), 129–132. https://doi.org/10.1080/08998280.2021.1967021
Nuttle, W. (2022, March 8). Working under pressure Triger | Eiffel’s Paris — An engineer’s Guide. Medium. https://medium.com/eiffels-paris-an-engineers-guide/working-under-pressure-b34571c9f83e
Pioneers of Diving: John Scott Haldane • Scuba Diver Life. (2020, August 3). Scuba Diver Life. https://scubadiverlife.com/pioneers-diving-john-scott-haldane/
Wikipedia contributors. (2023, November 9). Paul Bert. Wikipedia. https://en.wikipedia.org/wiki/Paul_Bert
Wikipedia contributors. (2024a, January 27). History of decompression research and development. Wikipedia. https://en.wikipedia.org/wiki/History_of_decompression_research_and_development
Wikipedia contributors. (2024b, January 27). John Scott Haldane. Wikipedia. https://en.wikipedia.org/wiki/John_Scott_Haldane
History of the hyperbaric chamber | Biobarica. (n.d.). https://www.biobarica.com/en/news/history-of-the-hyperbaric-chamber-jWo2k097pH3Xe3hPXksM/
Hyperbaric Physics. (2024, January 1). PubMed. https://pubmed.ncbi.nlm.nih.gov/28846268/
History of Hyperbaric Oxygen therapy | Part I. (2024, February 8). https://www.healingwithhyperbarics.com/history-of-hyperbaric-oxygen-therapy-part-one