Extremely deep recreational dives: the risk for carbon dioxide (CO(2)) retention and high pressure neurological syndrome (HPNS).
Clear differences between professional and recreational deep diving are disappearing, at least  when taking into account the types of breathing mixtures (oxygen, nitrox, heliox, and trimix) and  range of dive parameters (depth and time). Training of recreational deep divers is conducted at  depths of 120-150 metres and some divers dive to 180-200 metres using the same diving  techniques. Extremely deep recreational divers go to depths of more than 200 metres, at which  depths the physical and chemical properties of breathing gases create some physiological restrictions  already known from professional deep diving. One risk is carbon dioxide retention due  to limitation of lung ventilation caused by the high density of breathing gas mixture at great  depths. This effect can be amplified by the introduction of the additional work of breathing if  there is significant external resistance caused by a breathing device. The other risk for deep  divers is High Pressure Neurological Syndrome (HPNS) caused by a direct compression effect,  presumably on the lipid component of cell membranes of the central nervous system. In deep  professional diving, divers use a mixture of helium and oxygen to decrease gas density, and  nitrogen is used only in some cases for decreasing the signs and symptoms of HPNS. The same  approach with decreasing the nitrogen content in the breathing mixture can also be observed  nowadays in deep recreational diving. Moreover, in extremely deep professional diving, hydrogen  has been used successfully both for decreasing the density of the breathing gas mixture  and amelioration of HPNS signs and symptoms. It is fair to assume that the use of hydrogen will  be soon "re-invented" by extremely deep recreational divers. So the scope of modern diving  medicine for recreational divers should be expanded also to cover these problems, which previously  were assigned exclusively to professional and military divers.
National Centre for Hyperbaric Medicine, Institute of Maritime and Tropical Medicine in Gdynia, Medical University of Gdansk, Gdynia, Poland. jkot@gumed.edu.pl
Jacek
J
Kot
D002245:Carbon Dioxide / Q000378:metabolism*; D004242:Diving / Q000009:adverse effects*; D006801:Humans; D009422:Nervous System Diseases / Q000453:epidemiology / Q000209:etiology*; D011044:Poland / Q000453:epidemiology; D011998:Recreation; D018570:Risk Assessment; D012309:Risk-Taking; D013577:Syndrome
D002245:Carbon Dioxide