NAUI Nitrox Diver
34 The Physiology of Diving and Nitrox
into the free-phase nitrogen within the blood stream
(that is, nitrogen is already present as micro-bubbles).
The interplay between the dissolved phase and the
free phase (bubbles) determines the effectiveness of safe
nitrogen elimination as well as possible damage to the
body. Ideally, the free phase nitrogen is carried via the
blood to the lungs where it passes into the alveoli and
is exhaled. This elimination occurs over time until the
nitrogen tensions in our body’s tissues are again in
equilibrium with the partial pressure of nitrogen in the
atmosphere. This absorption and elimination of nitrogen
are termed ingassing and offgassing.
However, if we dive too deep, for too long, and
then ascend too fast, the pressure gradient between the
dissolved nitrogen and the ambient pressure is too great
for proper, controlled offgassing, and excess released
nitrogen diffuses into the free-phase bubbles within the
body. Once the bubbles enlarge and begin to aggregate
they cause mechanical injury and also block the blood
flow to tissues, and the trauma presents as decompression
sickness.
The signs and symptoms of DCS vary greatly
depending on the location of the injury and the severity
of the case. They can range from simple excessive
fatigue to weakness and tingling in the limbs, limb
and joint pain and dysfunction, skin rashes and itching,
vertigo, loss of sensation, paralysis, and even death. Divers
once referred to “niggles” (from the word “niggling,”
meaning troublesome or irritating), mild symptoms of
DCS that would probably (or hopefully) resolve themselves
and go away over time. But in reality, anyone with a
suspected case of DCS should be administered 100%
oxygen as immediate first aid, examined by a trained
physician, and if decompression sickness is suspected,
receive recompression chamber treatment. If in doubt,
treat! Untreated, even the mildest case of decompression
sickness may worsen over time, and permanent injury is
a more likely outcome.
You have probably heard that “all dives are decompression
dives.” This is a truism, in that; any reduction in
pressure is consistent with the literal definition of decompression.
Every diver ascending from depth is experiencing
decompression in this sense. Divers, however, usually
use decompression to mean the controlled reduction of
pressure on ascent in order to control bubble growth and
prevent decompression sickness. It is the rate of ascent
that will determine level of risk with regard to bubble
growth in accordance with Boyle’s Law calculations. As
divers, we plan our dives to prevent excessive nitrogen
loading during the dive and to control bubble growth on
ascent in order to avoid “getting bent.”
FIGURE 3-2: DISSOLVED CARBON DIOXIDE IS INVISIBLE IN A SEALED
BOTTLE OF SODA BUT APPEARS AS BUBBLES WHEN THE
PRESSURE IS SUDDENLY REDUCED
FIGURE 3-3: BUBBLES IN RIGHT ATRIUM AND VENTRICLE OF AN
EXPERIMENTAL SUBJECT FOLLOWING DECOMPRESSION
Image courtesy of Neal W. Pollock, Ph.D.; Center for Hyperbaric
Medicine and Environmental Physiology, Duke University Medical
Center