[Mark Powell]

For those of you interested in the history of decompression theory here are some notes I have assembled/written. Although I have tried to check all facts and dates I would welcome any feedback on specifics.

Caissons Disease
----------------

Decompression sickness was first discovered in the early 19th century although at that time there was no clear understanding of the cause of the disease. Workers involved in mining, tunnel construction or building work in pressurised underwater containers as well as the early hard hat divers began to notice a variety of symptoms when leaving the tunnel or construction chamber or when ascending after a hard hat dive.

In 1844 Royal Engineer Colonel William Pasley (later Maj Gen Sir William Pasley) was allocated the task of clearing the wreck of HMS Royal George which had sunk in the Solent and was proving a hazard to navigaton for ships entering Portsmouth, an important Naval harbour. Pasley decided to use the opportunity to test and evaluate hard hat diving equipment. Workers on the project regularly undertook long strenuous dives, frequently working up to seven hours a day at depths of 20m (65ft). Despite the success of the project, all of the divers involved reported a number of symptoms including aches and pains in various parts of the body. The cause of the symptoms was unknown and initially these symptoms were referred to as the Mysterious Malady.

During this same time diving bells were increasing in size. The industrial revolution produced high-capacity air pumps capable of sufficient pressure to keep water from entering these bells. Diving bells then evolved into dry underwater chambers allowing several men to work in a dry environment. This innovation greatly simplifies the task of constructing bridge footings and tunnels. These dry, pressurised work areas were known as plenum pneumatic or caissons, French for ''big box.'' This design was a major leap forward in engineering because it allowed workers easy access from the surface, and the pressure inside kept the work area dry. An air-lock was used to pass materials in and or to change out work shifts. The use of the caisson grew rapidly as larger and more ambitious engineering works were undertaken. The men who worked in these big boxes became known as caisson workers or just caissons.

As the use of the caisson grew a large number of workers began complaining of symptoms similar to those encountered by the hard hat divers. Caisson workers reported a number of symptoms including dizzy spells, shortness of breath and sharp acute pain in the joints or abdomen. After a period the symptoms would reduce but the worker was sometimes left with symptoms which refused to disappear completely. It was also noted that those who suffered from the mysterious malady felt better when they returned to the caisson. As projects got bigger and working pressures increased the attacks of the mysterious malady increased, not only in number of victims, but also in severity, so much so that fatalities started to occur frequently.

The term ‘maladie du caisson’ or casisson’s disease was first used in 1841 by a French mining engineer named Triger who noticed that coal miners working under increased atmospheric pressure suffered from muscular cramping and pains after returning to the surface.

In 1866, James Eads, was called on to bridge the Mississippi at St. Louis. Instead of the usual truss bridge, he proposed to build three huge cantilever arches, 500 feet. Eads was an unconventional choice for building the bridge; he had never built a bridge before although, despite no formal education, he had a wide range of design and engineering experience. In his 20s he designed a diving bell and worked at salvaging cargo from sunken river boats. During the Civil War, Eads built ironclad gunboats.

The project would stretch current engineering practice as it involved placing the piers right in the middle of the Mississippi river. Eads knew that the bottom of the Mississippi was made up of swirling mess of moving mud and silt. Furthermore he knew that the bedrock lay 30m (100 feet) below the mud and silt. Clearly the use of caissons was the only answer, but they would need to be used on a scale that had not previously been imagined.

The caissons were 80x60-foot steel boxes, nine feet high and open at the bottom. Masonry piers were built on top of them, causing them to sink down to the riverbed. Air was then pumped into the caisson under pressure in order to keep the water and mud out.

Workers descended a stairwell through the masonry to the caisson and entered it through an airlock. Inside, they dug out mud while the whole structure gradually settled downward toward bedrock. Workers above kept building up the masonry pier on top of the gradually sinking caisson.

The approach seemed to be working well and it allowed workers to proceed with the project. However, when the pressure in the caisson reached twice atmospheric pressure, workers felt pains in their joints as they returned to the surface. When they reached a depth where the pressure was three atmospheres the symptoms became worse and a number of workers were hospitalized. Some suffered total paralysis and some symptoms were so serious that the some of the workers died of their injuries. Out of a total workforce of 600 workmen, 119 were affected to some extent and 14 died.

It was noticed that visitors to the caissons who had a brief visit and then returned to the surface never experienced any problems. Obviously, the less time you spent below, the less you suffered. It was also noticed that if you ascended slowly from the caisson the symptoms were reduced. Eads gradually reduced the working shifts to only 45 minutes and in 1870 ordered that workers always carried out a slow return to the surface. These measures seemed to work. Doctors, however, were baffled and couldn’t explain the etiology or physiological cause behind the disease.

Even in Webster's 1913 Dictionary the definition of Caissons Disease shows a clear explanation of the symptoms but no clear understanding of the causes.

Definition: \Cais"son dis*ease"\ (Med.)
A disease frequently induced by remaining for some time in an
atmosphere of high pressure, as in caissons, diving bells,
etc. It is characterized by neuralgic pains and paralytic
symptoms. It is variously explained, most probably as due to
congestion of internal organs with subsequent stasis of the
blood.

Caisson’s disease continued to affect workers throughout the 1800s. During the construction of the Brooklyn Bridge between 1870 and 1883 3 people died and 15% of those who suffered symptoms were left with some level of permanent paralysis. Unfortunately, due to personality conflicts between James Eads and Washington Roebling who took over from his father John as Chief Engineer on the Brooklyn Bridge, Eads never passed on the knowledge that had been gained during construction of the St Louis Bridge. Washington Roebling was himself struck down with caissons disease in 1872 and remained painfully paralysed for the rest of his life.

It was during the construction of the Brooklyn Bridge that the term “bends” was introduced for the first time. As the workers often had trouble standing straight their posture was similar to the “Grecian bend” adopted by fashionable women of the time and so they began to call the problem “the bends”, a name that has stuck to this day.