NAUI Nitrox Diver
38 The Physiology of Diving and Nitrox
Appearance of any sign or symptom of oxygen
toxicity is reason to terminate the dive. But, because
precursor symptoms are highly variable–as well as subjective–
and they may not occur, it is doubly important
that divers keep their PO2 exposure within an acceptable
limit. In one study, convulsions were the first noticed
manifestation in 40% of the subjects studied. In another
study, nausea was the most common first manifestation,
followed by muscular twitching and vertigo.
If a convulsion were to occur underwater, there is
little that can be done until the active phase of the seizure
is over and the muscles relax. Muscle contraction
may cause the diver to lose the regulator, but the victim
also ceases to breathe during the active phase as the
vigorous, uncontrolled muscle contractions interrupt
breathing and the tongue blocks the airway. No attempt
should be made to surface victims of an “Ox-Tox Hit”
at this time because they are effectively holding their
breath. Because the convulsion was precipitated by
breathing a high partial pressure of oxygen, and oxygen
tensions in the body are therefore high, the person
remains well oxygenated during the convulsion, and
hypoxia is not a problem. Carbon dioxide levels will
also become very high because the muscles are exercising
heavily while the victim is not breathing. When
the post-convulsive, resting phase begins, the muscles
relax, and the victim remains unconscious. At this
point, the victim can be taken to the surface and first
aid care begun.
MANAGING OXYGEN EXPOSURE
The best way to avoid oxygen toxicity problems is
to stay within correct oxygen exposure limits. As stated
above, the generally accepted limit for diving is 1.4 ata
PO2, with 1.6 ata PO2 as a contingency. These limits
are not lines drawn with a sword in the sand–see the
casino comments above. Many divers have dived to 61
meters (200 feet) and even considerably deeper on air,
and most have returned none the worse for wear. But
prudence should be part of all dive planning, and if
you have reason to dive beyond recreational limits, you
should definitely be preparing for and taking a NAUI
trimix training course as well as one in decompression
techniques. In deep diving, trimix reduces not only your
oxygen exposure but also your nitrogen exposure. (If
you want to safely dive deep and remember what you
saw and did down there, try trimix.)
The oxygen exposure limits described in this book
carry an extremely low risk of oxygen toxicity. They are
well below any levels that might reasonably be expected
to cause problems.
NOAA OXYGEN EXPOSURE LIMITS
PO2
(atm)
Maximum
Single Dive
Limit (minutes)
Maximum
24-Hour
Limit (minutes)
1.60 45 150
1.55 83 165
1.50 120 180
1.45 135 180
1.40 150 180
1.35 165 195
1.30 180 210
1.25 195 225
1.20 210 240
1.10 240 270
1.00 300 300
0.90 360 360
0.80 450 450
0.70 570 570
0.60 720 720
FIGURE 3-6: NOAA OXYGEN EXPOSURE TIME LIMITS. THE TABLE GIVES
THE LIMITS FOR A SINGLE DIVE EXPOSURE TO VARIOUS
PO2 LEVELS AND FOR EXPOSURE DURING ANY 24-HOUR
PERIOD. (NOAA DIVING MANUAL, 4TH EDITION, 2001).
OXYGEN PARTIAL PRESSURES BETWEEN 1.4 ATA AND
1.6 ATA ARE SHADED.