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Hyperbaric means "relating to, producing, operating, or occurring at pressures higher than normal atmospheric pressure."  As early as the 1600s, practitioners varied atmospheric pressure in attempts to heal. Using a system of organ bellows, a British clergyman named Henshaw could adjust pressure within a sealed chamber called a domicilium . The simplistic principle behind its use was that acute conditions would respond to elevated atmospheric pressures, whereas chronic conditions would benefit from reduced pressure.
As time passed, air-compression devices evolved in appearance and function. It also was discovered that the use of compressed air could facilitate other methods. For example, a French surgeon named Fontaine created a mobile chamber that took advantage of a basic law of physics (Henry's law), which states that the solubility of a gas in a liquid is proportional to the pressure of the gas over the solution, provided that no chemical reaction occurs. By raising the atmospheric pressure within the chamber, Fontaine was able to increase the amount of oxygen carried by the patient's bloodstream during the administration of nitrous oxide anesthesia. This prevented blood oxygen levels from falling too low as typically happened with surgically acceptable depths of anesthesia .
In the early 1900s, Cunningham observed that patients with cardiovascular disease who dwelled at high altitudes fared less well than comparable patients living closer to sea level. Suspecting that altitude-dependent changes of atmospheric pressure were responsible, Cunningham hypothesized that raising pressure beyond a normobaric level would confer even greater benefit. He successfully treated a young colleague with influenza who was near death from lack of oxygen secondary to restricted lung function. With that success bolstering his confidence, he developed a cylindrical hyperbaric chamber approximately 3 meters in diameter by 27 meters in length, which could be used to treat many conditions .
Cunningham's fortunes took another upturn following the recovery of a patient afflicted with kidney disease. Ascribing his dramatically improved health to hyperbaric therapy, the grateful patient built for Cunningham a chamber fit for a king. This chamber -- built in Kansas City in 1921 -- was a hollow steel ball of approximately 20 meters in diameter and equipped with a smoking lounge, dining facilities, rich carpeting, and private quarters .
As grand as it may have been, the largest and probably the most ostentatious hyperbaric chamber in history met an undignified end. Its continued survival depended on demonstrable successes. Cunningham postulated that anaerobic bacteria (bacteria preferring low oxygen environments) were responsible for cancers, high blood pressure, and many other conditions. Based on this, he predicted that all would resolve at elevated atmospheric pressures that increase blood oxygen levels. Unfortunately, medical authorities did not find the results compelling, and Cunningham's hyperbaric "hospital" was closed and demolished for scrap metal.
In 1670, Robert Boyle observed how the eye of a snake could express a gas bubble visible through the cornea (the transparent outer membrane at the front of the eye). He concluded that tissues undergoing rapid decompression could express bubbles of previously dissolved gas. His conclusion is embodied in Boyle's law, which states that at a constant temperature, the volume and the pressure of a gas are inversely proportional. In other words, a gas will compress proportionately to the amount of pressure exerted on it.
Boyle's law helps explain what happens when a bottle of warm soda is opened. Under pressure, a large volume of carbon dioxide (which gives soda its fizz) dissolves in the beverage in accordance with Henry's law. When the cap is removed, the pressure on the liquid is relieved and the fluid cannot hold as much gas in solution. The gas forced out of solution rapidly forms bubbles. During deep-sea diving, inert gas breathed at relatively high pressure dissolves and accumulates in body tissues. As the diver returns to the surface, the gas may form bubbles that interfere with normal physiological processes. For example, blood circulation can disrupted by bubbles that clog small blood vessels, and pain can result when bubbles attempt to expand within the closed spaces of joints. The condition in which this occurs is called decompression sickness or "the bends."
Many years passed before Boyle's discovery was put to practical use in humans. In 1845, Triger wrote about symptoms in coal miners consistent with decompression sickness . Compressed air was used to force water from the tunnels. Like Boyle's snake, the miners apparently suffered no ill effects while under pressure. However, muscular pains and cramps occurred after they left the pressurized regions of the mine. In 1854, Pol and Watelle wrote that decompression was necessary for symptoms to develop and -- perhaps most important -- that recompression reduced symptoms . In 1876, Bert reported that nitrogen bubbles formed in tissue during rapid decompression . Nitrogen was thus implicated in the "Grecian bend," a term articulated by workers constructing the piers of the Brooklyn Bridge. The bent posture of afflicted individuals approximated the Grecian bend, a fashionable posture assumed by women of the period. Decompression sickness later became known as "the bends."
Workers constructing the tunnel beneath the Hudson River were also harmed by compressed air. Approximately one quarter of them apparently died from decompression sickness. When Moir treated the affected workers with \recompression and relatively slow decompression, the death rate dropped dramatically . This worked because pressure forced the bubbles to redissolve and gradual reduction in pressure permitted nitrogen to emerge in bubbles small enough to circulate to the lungs, which disposed of them through exhalation.
Through subsequent decades, scores of therapeutic recompression/decompression protocols were devised. These developments were spearheaded by the military, who eagerly exploited the advantages of the hyperbaric submarine environment.
Studies in the 1930s suggested that supplementary oxygen could play an important role in treating decompression sickness. However, because oxygen could be explosive, three decades passed before equipment was developed that could safely handle its administration. Oxygen breathed under pressure forcibly washes nitrogen from tissues. Treatment protocols using hyperbaric oxygen therefore require substantially less time to complete than do those using only compressed air. Thus, hyperbaric oxygen remains the frontline instrument in the treatment of decompression sickness.
Hyperbaric oxygen first entered land-based medicine in the Dutch surgical realm with the concept of drenching tissue, which required pressurization of the entire operating theater . Within a few years, hyperbaric oxygen proved effective against anaerobic infections (those caused by bacterial organisms that thrive under conditions of low oxygen), most notably gas gangrene . Hyperbaric oxygen also showed promise for treating carbon monoxide poisoning .
Fueled by early medical successes employing hyperbaric oxygen, the clinical community began deploying hyperbaric chambers at additional sites. Meanwhile, the academic community commenced high-profile international meetings, and reputable scientific medical bodies -- such as the National Academy of Sciences -- embraced hyperbaric medicine. Through these efforts, the fundamental principles governing the physiological effects of hyperbaric oxygen were derived, and practical issues pertinent to engineering and hyperbaric medicine came to the fore .
During the 1970s, the practice of hyperbaric oxygen therapy experienced hard times because (a) more effective therapies such as cardiac surgery became available, (b) efforts to use it for various medical conditions proved unsuccessful, and (c) rogue physicians who were high-profile HBOT advocates damaged its reputation. However, respectable practices have been defined and established through further research, the development of textbooks and scholarly journals, and the activities of professional associations such as the Undersea and Hyperbaric Medical Society. Insurance programs now cover hyperbaric therapies in approved medical conditions, and systematic training and certification of health-care professionals have proliferated. In the United States, strict standards related to construction and safety of hyperbaric chambers and facilities have developed and are enforced. In March 2000, the American Board of Medical Specialties approved undersea and hyperbaric medicine as a subspecialty of both emergency medicine and preventive medicine.
Dr. Morrison is a resident in emergency medicine at Sinai-Grace Hospital in Detroit, Michigan. Dr. Kirkby is Professor of Medical Neuroscience and Executive Director, Master's Program of Undersea and Hyperbaric Medicine at the University of Sint Eustatius School of Medicine, Netherlands-Antilles.