And you do it/ SEPTEMBER/OCTOBER 2019 / 83sides. Simply put, the sympathetic ner-
vous system activates the body and pre-
pares it for action, while the parasympa-
thetic nervous system calms it down.
Within the central nervous system, ni-
tric oxide (NO) is excreted by various cells
and used to suppress the activity of the
sympathetic nervous system and widen
blood vessels. Under normal conditions a
fraction of these NO molecules will bind
with molecules of oxygen that are by-
products of cell metabolism. These mol-
ecules are called reactive oxygen species
(ROS). Remaining unbound NO molecules
are meant to stay unbound and available
to bind with other molecules to control
the sympathetic nervous system.
If we are exposed to elevated par-
tial pressures of oxygen, excess oxygen
molecules enter our cells and may bind
with all available unbound NO molecules,
leaving none to suppress reactions from
the sympathetic nervous system. When
sympathetic activity becomes extreme,
the body releases enormous quantities
of adrenaline and noradrenaline in a re-
action called an adrenergic storm. This
storm can cause seizures, narrowing of
the arteries and a spiking of blood pres-
sure and heart rate that may lead to cap-
illary damage, compromised gas ex-
change and cell or tissue death. Seizures
that occur underwater are extremely
dangerous, and death by drowning is al-
most inevitable when they occur.
WHO IS AT RISK?
Any diver breathing a gas with elevated
oxygen content should be familiar with
oxygen toxicity. Rebreather and techni-
cal divers are at particular risk because
of the lengths of their dives and the po-
tential for inadvertent exposure to pure
oxygen, but recreational divers face the
risk as well.
Fortunately, analyzing your nitrox mix
before every dive, properly planning your
dive and diving your plan can help you
avoid this condition entirely. Because
even a slight infringement can get you
into trouble, it is imperative that every
diver breathing nitrox is able to proper-
ly calculate the depth and time limits for
their chosen gas mixture, then respects
those limits—it’s not just a training rec-
ommendation, it’s critical for safety.
For more information on oxygen toxicity,
visit dan.org/health or contact DAN
directly at dan.org/contact.
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