Page 47 - AlertDiver_Winter2013

and unknown individual and environmental factors may
increase the risk of oxygen toxicity — even within limits
for a given EAN mix.
Divers using mixed gases in open-circuit scuba diving
incur the risk of confusing the various gas mixes they carry
and of using too much oxygen during decompression.
Closed-circuit-rebreather divers run the risk of incorrectly
selecting the partial pressure of oxygen as well as mechanical
errors. These may include errors in gas mixing, displaying
erroneous oxygen values and excess carbon dioxide in the
breathing loop due to equipment malfunction.
CNS oxygen toxicity is the main form seen in diving.
Pulmonary oxygen toxicity is not of concern except in very
long technical dives (extended cave or rebreather dives, for
example) lasting six to 12 hours or more. Prolonged dives
within the safe limits for CNS oxygen toxicity may still
cause ocular toxicity, which manifests as a narrowing of the
visual field and myopia.
Divers suffering decompression illness may also be at risk
of oxygen toxicity during the treatment they receive. Pure
oxygen at sea-level pressure (normobaric oxygen), often
provided as first aid during evacuation, is well tolerated for
up to 24 hours. However, because divers will most likely be
treated with hyperbaric oxygen as well, air breaks should
be considered in extreme circumstances when access to
hyperbaric treatment is delayed. Generally, such air breaks
will occur as a matter of course as the injured diver eats,
drinks and goes to the bathroom. Note that these air
breaks are intended to forestall symptoms of pulmonary
oxygen toxicity; there is no risk of seizure or other CNS
manifestations in these circumstances. Air breaks should not
be used when transport times are shorter than a few hours.
During hyperbaric chamber treatment divers may be
exposed to much higher oxygen pressures than would
be safe to use while diving. The oxygen pressure used
during treatment represents a balance between the benefit
expected and the potential risk. In general, oxygen is
better tolerated in dry, resting conditions than in wet
conditions and while exercising as in diving. In addition,
there is no risk of drowning in a dry hyperbaric chamber,
which is the major concern with oxygen toxicity in diving.
Oxydative and Nitrosative Stress
At the root of oxygen toxicity are reactive oxygen species
(ROS), short-lasting forms of oxygen molecules that
occur as byproducts of normal metabolic processes and
play an important role in the body’s normal functioning.
With increased oxygen pressure, the amount of ROS
increases, and it may oxidize other molecules important
for biological processes and the integrity of biological
structures. Especially important is the interaction of
ROS with nitric oxide (NO), another important signaling
molecule in physiological processes.
ROS occurs everywhere in the body, particularly in
areas of high blood circulation and oxidative metabolism.
NO is present in every tissue of the body, but it may
play different roles depending on the specific tissue. The
interaction of ROS and NO exhibits the most dramatic
effect in the central nervous system and lungs, although
practically every organ and tissue may be affected. Recent
studies have revealed that chronic pulmonary oxygen
toxicity at low oxygen pressure is caused mainly by ROS,
while newly described acute pulmonary oxygen toxicity
is caused mainly by NO-mediated massive sympathetic
outflow from the brain.
Effects of Oxygen on the
Autonomic Nervous System
NO has an important role in oxygen toxicity through
modulation of the autonomic nervous system. The
autonomic nervous system consists of two opposing sides:
the sympathetic and parasympathetic, which regulate
function of every organ. In general, the sympathetic nervous
system excites the body, preparing it for action, while the
parasympathetic nervous system calms it down to promote
recovery and restoration.
In the central nervous system, NO decreases the activity
of the sympathetic nervous system, while in the periphery
it causes widening of blood vessels (vasodilation). When
the concentration of oxygen is increased at normal or
slightly elevated ambient pressure, ROS in the periphery
bind with NO, and thus arteries remain narrow, causing
hypertension. At this point, baroceptors, specialized neuronal
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Prolonged, repetitive breathing of gas with elevated partial
pressures of oxygen can lead to temporary nearsightedness
and narrowing of the visual field.
STEPHEN FRINK