3
38 NAUI Nitrox Diver
gas breathed under pressure can cause narcosis, and the
malady is more correctly called inert gas narcosis. The
narcotic potency of an inert gas is related to its solubility
in lipid (fatty) tissues. Because helium has minimal
narcotic potency, it is used as a diluting gas in trimix
when diving to depths where nitrogen narcosis becomes
a serious concern or incapacitating.
The effects of narcosis can be measured at shallower
depths, but they become more pronounced when
the partial pressure of nitrogen is approaching four
atmospheres, which is about 40 msw (132 fsw) when
breathing air. Symptoms increase with increasing
depth. Other factors also affect the degree of narcosis.
Anxiety and stress, fatigue, cold, hard work, high carbon
dioxide levels in the body, and alcohol have all been
shown to enhance narcosis. On the other hand, positive
motivation seems to reduce its effects. Some divers report
acclimatization following repeated exposures, but studies
have shown that any adaptation is largely subjective.
Narcosis itself is not the danger, but the impaired
judgment, loss of orientation, and reduction in problem
solving capabilities are, and the “narked” diver is at
increased risk. It may become difficult for a diver to
monitor time, depth, and air supply, remember the
dive plan, or concentrate on the task at hand. Perhaps
the most insidious thing about nitrogen narcosis is that
divers may not be aware that they are impaired.
At first glance, you might assume that you should be
less subject to nitrogen narcosis when diving with nitrox
because you have replaced some of the nitrogen with
oxygen. However, there is no hard or definitive evidence
to support this, and it is safer to assume that there is no
appreciable benefit to breathing nitrox.
As you will remember from your entry-level scuba
course, nitrogen narcosis is reversible. An ascent to a
shallower depth is typically all that is required when
operating within recreational limits. The symptoms
disappear as you ascend. It could also be that the stress or
anxiety that you are experiencing at depth is as much due
to psychological factors of being outside your personal
“comfort zone.” In this case, the obvious solution is also
an ascent to a shallower, more familiar depth.
Decompression Sickness
You learned about the basics of decompression sickness
(DCS or “the bends”) – what causes it, its signs and
symptoms, how to avoid it, and how it is treated – in
your entry-level scuba course. A brief review will be
useful, and we can now use some of the concepts you
learned in earlier chapters of this book. Again, if you
want to learn more about decompression theory, the
NAUI Master Scuba Diver course will greatly increase
you knowledge and understanding.
DEPTH
DIVE
#2
TIME
DIVE
#1
Represents Residual Nitrogen
DIVE
#3
Figure 3-2 Tissue nitrogen levels increase
during dives and decrease during
surface intervals.
During a dive, our bodies are exposed to increased
pressure. While we are underwater, the increased
partial pressure of the nitrogen in the air (or gas) we
are breathing forces additional nitrogen into solution
in the tissues of our body (Henry’s Law). Passing out
of the lungs through the walls of the alveoli, dissolved
nitrogen enters the blood and is carried to all parts of
our body. There, the pressure gradient between the
nitrogen dissolved in our blood and the nitrogen in the
surrounding tissues causes dissolved nitrogen to move
into the tissues. The greater the pressure (depth) and the
longer we are submerged, the more nitrogen will dissolve
in our bodies until eventually the gas tension (partial
pressure) of the dissolved nitrogen in our tissues reaches
equilibrium with the partial pressure of the nitrogen in
our breathing mixture at that depth. This may take more
or less time depending on many factors, including the
type of tissue and the circulation to it. Because nitrogen is
metabolically inert, it simply remains in our tissues while
we are at depth (Figure 3-2).
When we ascend properly from a dive, the reverse
occurs. The partial pressure of nitrogen in our lungs is
now reduced (to 0.79 atmosphere at the surface), and
what is now excess dissolved nitrogen migrates from the
areas of higher nitrogen tension (tissues) and diffuses
into the free phase nitrogen within the blood stream
(that is, nitrogen is already present as micro-bubbles)
(Figure 3-3). 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.