hyperbaric chamber

hyperbaric chamber

Hyperbaric chambers in industry

Although Henshaw, an English clergyman, constructed a pressurized chamber — or ‘domicilium’, as he termed it — in 1662, the technology which allowed the construction of large vessels capable of holding greater pressure developed in the early to mid nineteenth century, and was associated with the construction of bridges and tunnels. Water ingress was a major problem in such workings, and in 1830 Admiral Lord Thomas Cochrane patented the technique of using compressed air in tunnels and caissons to exclude water. In 1839 Triger used the technique during the sinking of a shaft through quicksand to a seam of coal at Châlons in France. The technique was successful, was applied elsewhere, and its use was soon followed by reports of decompression illness in the workers. In 1854 Pol and Watelle recorded the relief obtained by workers so afflicted who went back into compressed air, and the workers, finding this cure out themselves, voluntarily went back to the pressurized caisson to obtain relief when they got the ‘bends’. These observations were strongly supported by Paul Bert's demonstration of the success of recompression as a treatment in animals, but it was only after large projects, such as the New York tunnels under the Hudson and East rivers in 1889 and 1893, which caused more than 5000 instances of the disease, that the benefit of systematic recompression therapy for decompression illness was fully established. In order to avoid the inconvenience and danger of carrying patients back into the workings, special hyperbaric chambers were used at tunnelling sites. These were called medical locks, and the early ones were nothing more than boilers mounted horizontally with an airtight door at one end. The chamber formed could be divided by a bulkhead incorporating a door, and this formed a lock whereby the inner chamber could be entered without lowering its pressure. The chamber was equipped with electric light, bunks, and the necessary compressed air connections and controls. Following their development for compressed air workers, hyperbaric chambers were also widely used to treat divers and, later on, aviators with decompression illness.

Present-day hyperbaric chambers are categorized in terms of the number of locks that they contain, whether they can accommodate more than one person, and to what pressure they can be compressed. A chamber for one person is called a monoplace chamber, and these have only one lock or compartment. These chambers may be metal or even fabric, but are commonly constructed of acrylic tubes with metal end plates. They are now almost exclusively used for medical applications. Multiplace chambers accommodate more than one person and come in any number of sizes and pressure ratings depending on their purpose. They are equipped with pressure-proof windows and have a small interlock let into the hull which is used to allow the passage of small items, such as food and drink, without decompressing the main lock. They are used still in industry for the treatment of decompression illness, but the last 30 years has seen them used also as pressurized living habitats for divers working at great depths.

The hyperbaric chamber environment and saturation diving

As atmospheric pressure is increased in the chamber there is adiabatic heating and the atmosphere warms. This heat does disperse once a stable pressure is obtained, but during decompression the atmosphere cools, often to the extent that water condenses out and a fog appears. Gas density increases with atmospheric pressure; the partial pressure of the various gases in the atmosphere rises proportionately and their various toxic effects become manifest. At pressures over three atmospheres (more than three times the typical barometric pressure at sea-level) the nitrogen in air exerts its anaesthetic properties, with gradual loss of mental ability and co-ordination, until at 10 atmospheres and above loss of consciousness occurs. The increased gas density can also cause under-ventilation of the lungs, and carbon dioxide build-up in the blood. For these reasons the use of air in hyperbaric chambers is limited to six atmospheres, and usually three atmospheres are not exceeded. At higher pressures, chambers are usually compressed using helium, which is very much less narcotic and also less dense, although it does distort speech to the extent that special electronic equipment is required to make the speech to the divers comprehensible. As pressure is increased further with helium, the thermal capacity and thermal conductivity of the gaseous environment increase; conductive and convective heat loss increase, and the body loses the ability to control its temperature. At about 30 atmospheres the body is effectively poikilothermic (the body temperature changes with that of the environment). Chamber temperature under these conditions needs to kept at around 30°C to avoid hypothermia in the chamber occupants, and much higher temperatures can cause hyperthermia.

The partial pressure of oxygen also gives cause for concern. At partial pressures much over 0.3 atmospheres, oxygen affects the lung, and at over 0.6 atmospheres it is frankly toxic, causing oedema and inflammation, although this takes some time to happen and the early effects are entirely reversible. At higher partial pressures, oxygen is also toxic for the brain, causing convulsions, and disturbs most other body systems. In hyperbaric oxygen therapy, not more than three atmospheres of oxygen are ever used, and then only within very strictly controlled schedules.

The increased pressure of oxygen and nitrogen increase the fire hazard in pressure chambers. Ignition temperature falls as the partial pressure of oxygen rises, and the rate of spread of fire increases as the partial pressure of nitrogen rises, since the atmosphere conducts heat more efficiently. In order to speed decompression schedules, and in medical applications, 100% oxygen is frequently breathed in pressure chambers. This is accomplished in small monoplace medical chambers by simply flushing the chamber with a high flow of oxygen. In large, multiplace chambers, the presence of high levels of oxygen in the atmosphere is considered to pose too great a fire risk, and the oxygen is dispensed from special breathing equipment, which allows the exhaled gas to be vented from the chamber. The problem of escape from the chamber and the rapid rise of pressure caused by the increasing temperature makes fire safety a primary concern in hyperbaric chamber operation.

The hyperbaric chamber is an enclosed space and so, as the occupants consume oxygen the level of this gas falls and the products of metabolism such as heat, carbon dioxide, water vapour, and other gaseous wastes build up. Compressed air chambers at low pressures can be ventilated, but as pressure increases this becomes more difficult because of the amount of gas required. Chambers compressed with helium, which is expensive, are not ventilated. Oxygen can be added to the atmosphere and carbon dioxide removed using a chemical absorbent. If the chamber is to be occupied for some time then an entire atmosphere conditioning unit or life support system is required, which also controls temperature, humidity, and other contaminants. Oxygen levels are kept at or below 0.4 atmospheres. Needless to say these chambers are equipped with showers and toilets appropriately modified for the environment. Such systems are used for ‘saturation’ diving techniques, so-called because the partial pressure of inert gas in the divers' tissues equilibrates with that in the pressurized atmosphere. The divers live in a chamber, on a vessel or offshore structure, which is held at the same pressure as their underwater work site for periods of up to four weeks. They commute to and from their work in a pressurized diving bell. The time wasted by compression and decompression, and the associated dangers, are thus minimized, since the diver may only have to be compressed and decompressed once during the spell at pressure. Decompression, however, takes days rather than the minutes taken using conventional diving techniques.

Hyperbaric chambers in medicine

In parallel with the industrial use of hyperbaric chambers went attempts to develop more general medical applications. Between the years 1834, when Junod used compressed air to treat various pulmonary ailments in France, and 1928, when Cunningham constructed the largest pressure chamber of all in Cleveland, Ohio, compressed air therapy achieved some degree of popularity with the public, but never with any demonstrable medical benefit. Cunningham's chamber was six stroreys high and 64 feet in diameter. It had all the facilities of an hotel, including carpeting, dining rooms, and a smoking room. Cunningham was unable to validate compressed air therapy for the general medical conditions that he treated, such as syphilis, diabetes, and cancer, and it was condemned by the American Medical Association and the chamber forced to close in 1930. Compressed air is no longer used as a medical treatment.

Nevertheless, hyperbaric chambers are still quite justifiably used in medicine to allow the administration of very high partial pressures of oxygen — so-called hyperbaric oxygen therapy. This technique has largely replaced recompression in air as the treatment of decompression illness, since it allows treatment in the absence of inert gas, which is accordingly excreted very much more quickly and effectively. Oxygen breathing at high atmospheric pressure increases the body's oxygen stores, and because of this it is possible to stop the heart, or cut off the blood circulating to an organ, for very much longer than otherwise possible. At three atmospheres pressure, enough oxygen can be dissolved in the plasma to support the oxygen demands of the body at rest in the absence of haemoglobin. Because of these effects, hyperbaric operating theatres were developed in the 1950s and 60s to enable various cardiac and circulatory operations. However, since the development of the cardiac bypass apparatus this application has dwindled. Hyperbaric oxygen therapy can also stimulate new blood vessel growth and improve healing in hypoxic wounds. At present the generally accepted medical indications for hyperbaric oxygen therapy are acute carbon monoxide poisoning, gas embolism, gas gangrene and similar infections, and in the treatment of tissue damaged in the course of radiotherapy for cancer. There are a number of other less well-accepted indications and there is a thriving interest in the use of hyperbaric oxygen in alternative or complementary medicine for a number of conditions. These include multiple sclerosis, and unfortunately patients are treated without any real or objective demonstration of clinical efficacy.

Hyperbaric chambers are simple devices, but their operation can be far from straightforward either from the technical or physiological point of view. It is essential that they are well maintained and operated within the statutory guidelines and regulations by trained personnel.

John A. S. Ross

See also decompression sickness; diving; oxygen.