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Chapter 3 The Physiology of Diving and Nitrox 41
Manifestations of hypoxia include impaired mental
performance and defective memory, blueness of the lips
(cyanosis), fatigue, visual disturbances, and dizziness.
For most persons, symptoms begin to appear if the
partial pressure of inspired oxygen falls below about 0.16
atmosphere. If the partial pressure of inspired oxygen
falls below about 0.10 atmosphere more severe symptoms
leading to unconsciousness will occur. Because the
breathing stimulus is largely controlled by carbon dioxide
levels rather than oxygen levels, a hypoxic diver rarely
feels any warning of impending loss of consciousness.
Hypoxia is not a problem for divers breathing either
air or oxygen-enriched air, but it must be monitored in
some rebreather situations or deep diving applications.
Extremely deep dives may require low percentages of
oxygen in the “bottom mix” (the breathing gas used at
depth), and mistakenly breathing the bottom mix at
shallow depth can lead to hypoxia. Although extremely
rare (only one reported death since accident records
have been kept) hypoxia has also occurred from internal
rusting of a steel cylinder stored over many months with
some water inside, which supported additional rusting
that consumed the oxygen in the stored air.
Physiological Effects of High
Oxygen Levels
Oxygen is essential to us, and it also plays an important
role in the treatment of diving maladies such as
decompression sickness. Divers planning stagedecompression
dives will breathe high concentrations of
oxygen during decompression stops in order to offgas
nitrogen more quickly, but safely. As nitrox divers, we
use oxygen-enriched air to safely prolong our dive time
or to increase our nitrogen safety margins (Figure 3-6).
However, because we are utilizing an oxygen-enriched
mixture, we must control and monitor the inspired
partial pressure of oxygen we are breathing as well as
pay attention to the other parts of dive planning. If we
did not do so, we could easily get into trouble by diving
to depths that allow the oxygen partial pressure in our
breathing gas to become dangerously high.
With oxygen and nitrox, it is entirely possible to
get “too much of a good thing.” There are two types of
oxygen toxicity.
Central nervous system toxicity has a wide range of
signs and symptoms, the most dramatic being epilepsylike
convulsions. CNS toxicity can result from relatively
short exposures to high partial pressures of oxygen.
The other type is called pulmonary toxicity or
whole body toxicity. Pulmonary toxicity results from
prolonged exposure to elevated partial pressures of
oxygen (above about 0.5 atmosphere). As the name
implies, its most pronounced effects are on the lungs,
producing signs and symptoms such as chest tightness,
Figure 3-6 As nitrox divers, we use oxygen-enriched
air to safely prolong our dive time or to
increase our nitrogen safety margins.
breathing discomfort and pain, shortness of breath, and
coughing. Development of pulmonary toxicity requires
long-term exposures such as encountered in saturation
diving, certain military and commercial diving, or
recompression chamber treatment.
Pulmonary toxicity is not a concern of the normal
recreational nitrox diver; CNS toxicity is a concern.
CENTRAL NERVOUS
SYSTEM (CNS) TOXICITY
Susceptibility to CNS oxygen toxicity varies greatly
between individuals and is affected by other factors
and conditions as well. One’s oxygen tolerance has also
been shown to vary from day to day. It is impossible to
predictably relate CNS oxygen toxicity appearance to
any definite PO2 and time exposure. Nevertheless, it is
certain that the greater the oxygen partial pressure and
the longer the time of exposure, the more likely it is that
symptoms of CNS oxygen toxicity will develop. In setting
oxygen exposure limits, it is best to err on the side of
safety.
Beginning with its first presentation of oxygenenriched
air for scuba diving in the second edition of
the NOAA Diving Manual (1979), NOAA recommended
a maximum oxygen partial pressure of 1.6 ata. In the
fourth edition (2001), they add the cautionary comment:
“A slightly lower level provides less oxygen exposure
risk.” Diving to a PO2 higher than 1.6 ata has been
likened to knocking on the door of a casino. Once you go