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