|
57
law). From there it is gradually taken up by the blood and
delivered to the body’s various tissues (Henry’s law). The rate
of inert gas uptake in the blood and other tissues depends on
several variables. Key among them are speed of compression,
type of inert gas breathed and its related solubility
coefficient, body temperature, inherent tissue perfusion and
level of exercise or work load.
In recreational diving, nitrogen uptake essentially ends once
the diver begins his ascent to the surface. I say “essentially”
because the body’s “slower” tissues — those that are less well
perfused or are supported by simple diffusion, for instance
—may continue to take on nitrogen during the early and
intermediate stages of ascent if their nitrogen pressures
remain lower than the blood’s. Thus, nitrogen in the blood
will continue to transfer into these tissues until such time
that blood nitrogen levels fall to the level of those tissues. It is
at this point that slower tissues will begin offgassing. This is
why it is important that ascents be mostly direct and largely
linear. Divers who slowly meander back to the surface may
accumulate levels of nitrogen in certain tissues in excess of
those assumed by decompression tables. In this way, repetitive
dives can lessen the protective capabilities of the table in use.
If a diver remains at depth more than 12 to 18 hours (in
a seafloor habitat or a commercial oilfield saturation-diving
complex, for example), all of their tissues — fast, intermediate
and slow — will re-equilibrate with nitrogen (or helium) at
the new depth. This is called saturation diving. Except for tiny
variations that may occur with body-temperature fluctuations,
it is physiologically impossible for any additional inert gas to
be taken up without further change in depth. When a diver
ascends from a saturation dive, inert-gas elimination occurs in
the same manner as it does at the end of a recreational dive, as
described by Henry’s and Dalton’s gas laws.
Once a diver has returned to the surface, regardless of
whether the dive was a short recreational dive or a long
saturation dive, all tissue inert gas in excess of normal
atmospheric pressure will be eliminated over the following
12 to 18 hours (i.e., his body’s tissue nitrogen levels will be
re-equilibrated to the ambient atmospheric pressure). After
that period, no additional nitrogen above normal atmospheric
(sea-level) pressure will remain in the body.
Residual nitrogen is never “trapped” in the body, so there
is absolutely no basis to treat divers for chronic nitrogen
saturation. It is a misunderstanding at best, and a hoax
perpetrated on divers at worst. Don’t fall for this.
—Dick Clarke, President, National Baromedical Services
DAN
Cares
Together, we save lives.
Whether it is assistance provided
over DAN’s 24/7 diving emergency
hotline, an important discovery through
our continued research, or application
of emergency first aid – all of these
elements directly impact diver safety.
With your help, we continue to invest
in these initiatives because we care.
Together, we make a difference.
All contributions are tax deductible and benefit the DAN Foundation, a 501(c)(3) organization, Tax ID # 56-1696689.
DAN.org/DANCares
