2000-10 An End to Decompression?
By Fred Bove, M.D., Ph.D.
If you have been a diver for any period of time you understand the importance of proper decompression. Research during the last 100 years has provided an understanding of the problems caused by inert gas breathed at increased pressure. Divers have dreamed for many years of methods to eliminate decompression but the basic need to fill the lungs with only the proper amount of oxygen requires an inert gas. Let's look at some possibilities that go beyond the conventional approach to decompression.
An artificial gill allows oxygen from the water to pass through a membrane and enter the blood directly. If a person had gills connected directly to the bloodstream, then only oxygen would be transferred and inert gas would not be required to fill a gas space. Attaching gills to a diver would solve the problem but the technology for such a device is far in the future. At least for the foreseeable future, a gill attached to the bloodstream is out of the question. However, the ability to absorb oxygen from the water through a membrane to deliver an unlimited supply for a diver is not. We may eventually see the equivalent of an artificial gill that would provide oxygen to breathe, but would still use an inert gas so that the lungs can be filled with an appropriate gas mixture.
This technology is actually developed, and has been used for failing lungs that require improved delivery of oxygen. An experiment done more than 30 years ago demonstrated that the mice immersed in water inside a hyperbaric chamber compressed to six atmospheres survived while breathing the water. Oxygen dissolved in the water is adequate in meeting the oxygen needs of the mice, and when they inhaled water, instead of drowning, the mice survived. One of the major problems in the liquid breathing experiments was that water dissolves only small amounts of oxygen, making this technology possible only at increased ambient pressure.
Scientists began searching for a liquid that could hold large amounts of oxygen and carbon dioxide and found liquid flourocarbon. In the 1970s, research on liquid flourocarbon was done to explore the possibility that a space suit using liquid breathing would allow exploration of planets with very high gravity. Liquid breathing would be necessary in high gravity exposures because the blood is pulled to the bottom of the lungs, and oxygen cannot get to the blood. If the lungs were filled with liquid, however, the pressure gradient in the lungs would match the gradient in the blood, therefore, distributing the blood normally. Studies to test the ability of liquid flourocarbon to prevent hypoxia under high gravity were also performed in the early 1970s and proved that this method could be used to counter gravity effects on the lungs.
This same concept was considered for diving with a liquid breathing scuba that would not require inert gas. In this scuba, the diver would breath liquid flourocarbon oxygenated from a portable oxygen supply. A CO2 scrubber would remove the CO2 from the flourocarbon. No inert gas would be necessary and no decompression would be required. This technology has not yet been explored in scuba, but has provided an important new therapy for certain types of severe lung disease.
Scientists working at the Naval Medical Research Center in Bethesda, Maryland, are testing a method to remove inert gas from the tissues by converting it to a different chemical compound.* The body cannot metabolize nitrogen, helium or hydrogen; they are transferred in and out of tissues purely on physical principles. The scientists found a bacteria (methanobrevibacter smithii) that converts hydrogen to methane and hypothesized that hydrogen used as an inert gas in diving could be converted to methane in the colon, which would be eliminated through the rectum. Using hydrogen as an inert gas is risky because of its explosive nature, but in gas mixtures where oxygen is less than five percent of the total, combustion will not occur. Such mixtures can be used in deep diving, and in theory, decompression from a deep dive using hydrogen and oxygen could be modified by using bacteria to metabolize hydrogen to methane. Mice were tested breathing a hydrogen-oxygen mixture at 300 feet. The bacteria were injected into their intestines prior to the dive. The scientists measured the incidence of decompression sickness in mice that had the bacteria and a similar group of mice that did not have the bacteria. They found that the risk for decompression sickness was reduced by about half in the mice that had bacteria in their intestines. Similar experiments were done with pigs, and researchers found that the risk for decompression sickness was reduced by about half in pigs treated with the bacteria compared to pigs that did not have the bacteria.
Biologic decompression is not yet ready for daily use. Bacteria that metabolize hydrogen may have a place in deep diving if hydrogen is used as the inert gas. Because of the severe flammability of hydrogen, however, its use is limited, and routine use of hydrogen would require complex safety systems.
Bacteria that metabolize nitrogen are not as available. There are bacteria in nature that metabolize nitrogen into chemical compounds. None are compatible with humans or other animals and could become a source of infection. With our ability to modify the genetic makeup of bacteria, it is quite possible that a human-compatible bacteria with modified genes could produce the enzymes needed to metabolize nitrogen.
Gills are not likely to appear on human divers in the near future, and flourocarbon breathing is likely to be used mainly for medical indications. Future demands for deep diving and space exploration might stimulate the development of usable liquid breathing systems, but until that time, it is not likely that a liquid breathing scuba will become available.
Biologic decompression, however, may become an important tool very soon. A hybrid bacteria could be developed that is compatible with humans, and at the same time, would metabolize nitrogen into other compounds. If such bacteria can be produced, decompression will undergo a substantial change, and might be eliminated altogether.
*Kayar, SR. 1998. "ecompression Sickness Risk in Rats By Microbial Removal of Dissolved Gas." American Journal of Physiology (275:R677).
You can find more about diving medicine at www.scubamed.com.