O
xygen accounts for 21 percent of the air we
breathe and is critical for our survival. When
we inhale, oxygen enters alveoli (air sacs) in
the lungs, where it crosses a thin membrane
into the capillaries and binds to the hemoglobin of red blood
cells. The blood then transports oxygen to the rest of the
body. Administering oxygen is essential first aid for people
experiencing symptoms of decompression illness (DCI).
Dive-accident victims benefit from the use of oxygen in two
fundamental ways. First, breathing pure oxygen speeds the
washout or elimination of inert gas (i.e., nitrogen and, for trimix
divers, helium). Second, areas of the body with reduced oxygen
supplies due to compromised blood flow may receive enough
oxygen to minimize or prevent tissue injury.
Understanding Partial Pressure
A helpful concept for better understanding the use of oxygen
in diving and dive emergencies is partial pressure. The partial
pressure of a gas is the fraction of that particular gas in a gas mix
multiplied by the ambient pressure. At sea level —which is 1
atmosphere absolute (ATA) of pressure — the oxygen partial
pressure (PO
2
)
of air is 0.21 (21 percent oxygen x 1 ATA), and the
PO
2
of 100 percent oxygen is 1.0. At a depth of 66 feet (3 ATA),
the PO
2
of air is 0.63 (21 percent oxygen x 3 ATA), and the PO
2
of 100 percent oxygen is 3.0. A PO
2
that is either too low or too
high can be dangerous to humans. Loss of consciousness due to
hypoxia is likely at a PO
2
below about 0.16. A PO
2
in excess of
about 1.5 puts a diver at risk of central nervous system (CNS)
oxygen toxicity, which can lead to convulsions and drowning.
Means of Administration
Devices used to deliver oxygen include demand valves,
nonrebreather masks (NRBs), bag-valve masks, manually
triggered ventilators and nasal cannulas. A demand valve is
similar to a second-stage scuba regulator in that it delivers
gas only when the patient inhales. With a well-sealed mask,
a demand valve can deliver about 95 percent oxygen. It is
designed for conscious, alert patients whose respirations are
strong enough to engage the flow of oxygen.
Nonrebreather masks provide a constant high flow of
oxygen. The flow rate of an NRB is manually adjustable,
and the rate is usually set between 10 and 15 liters per
minute. A high flow rate does not necessarily equate to
more effective treatment, however. To avoid wasting
oxygen, set the flow rate just high enough to prevent the
reservoir bag from collapsing fully when the patient inhales.
Take care to ensure the mask maintains a good seal for as
long as it’s used.
Bag-valve masks and manually triggered ventilators are
used to administer positive-pressure ventilations of oxygen
to people who are not breathing on their own. Caregivers
accomplish this with a bag-valve mask by squeezing a gas
reservoir bag and with a manually triggered ventilator by
pressing a button that delivers a safe volume of gas.
Nasal cannulas consist of two small plastic prongs that
fit into the nostrils and continuously blow oxygen. Inspired
fractions of oxygen are elevated only minimally compared
to breathing air, so this method of oxygen administration
provides little therapeutic benefit to injured divers.
B y L a n a S o r r e l l , E M T , a n d N i c k B i r d , M . D . , M M M
48
|
FALL 2012
RESEARCH, EDUCATION & MEDICINE
//
S A F E T Y 1 0 1
Tips
for
Better
Oxygen
Administration
Emergency oxygen administration
remains the cornerstone of
treatment for acute DCI.
Opposite, from left: A good seal
increases the concentration of
oxygen the patient breathes.
The flow rate of a nonrebreather
mask should be set just high
enough to prevent the reservoir bag
from collapsing fully when
the patient inhales.
STEPHEN FRINK