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T
here is an appeal to being able to
venture underwater using a minimum
of equipment. The desire to explore
unencumbered has led to increasing
interest in breath-hold (apnea) diving,
also called freediving. There are,
however, potential hazards associated with this activity that
should be reviewed. Understanding the basic components
of the human respiratory drive can help reduce the
potential for life-threatening or life-ending complications.
Human respiration is primarily an autonomic function,
requiring no conscious effort. We can exercise control
over our breathing, but only for short periods. Most of us,
at some time in our lives, have attempted to determine
how long we can hold our breath. Regardless of the length
of time or circumstance, there is a point at which the urge
to breathe is overpowering, and we end the breath-hold.
With practice and observation we may have discovered
techniques for increasing our breath-hold time.
One common technique is taking multiple breaths in
rapid succession just before holding the last inspiration.
This is known as hyperventilation: effectively,
ventilation in excess of metabolic need. Some think that
the multiple breaths increase the level of oxygen (O₂) in
the blood, but the increase is actually fairly trivial.
With the exception of a few chronic medical
conditions (which are generally disqualifying for diving),
respiration is driven by the increase of carbon dioxide
(CO₂) in the blood. The concentration of CO
2
in the
blood is not maintained at the same concentration
as air but at much higher levels to provide proper
acid-base balance. The normal concentration of CO
2
is 140-160 times greater than that of air. Because of
this, hyperventilation can create drastic drops in blood
CO
2
levels. The reduced CO
2
present at the start of
breath-hold prolongs the breath-hold time necessary to
accumulate enough CO₂ to trigger the urge to breathe.
While the urge to breathe is delayed, the trivial
increase in O₂ content means that O
2
levels can fall too
low to maintain consciousness before there is an urge
to breathe (see the figure on the next page). Reduced
O₂ is known as hypoxia, and excessive hyperventilation
alone is sufficient to cause blackout. When combined
with a vertical dive, the decrease in ambient pressure
during ascent further hastens the drop in O
2
, increasing
the risk of loss of consciousness.
This latter phenomenon is commonly known as
“shallow-water blackout,” since loss of consciousness
is most likely to occur during the final stage of ascent,
where the decline in relative pressure is greatest, or after
surfacing before the freshly inspired oxygen reaches
the brain. Taking a maximum of two full ventilatory
exchanges prior to breath-hold is probably a reasonably
safe level of hyperventilation, but great caution is
required since hyperventilation works by compromising,
if not obliterating, our natural protections.
Some advocates will claim to not rely on
hyperventilation but to instead employ “work-up
breathing” or “cleansing breaths” prior to breath-hold.
These are just different names for hyperventilation and
thus must be employed thoughtfully and cautiously.
The risk remains, regardless of what it is called.
Physical activity during breath-hold will increase O₂
consumption. Expected “safe” breath-hold times can
easily be overestimated.
RESEARCH, EDUCATION & MEDICINE
SAFETY 101
52
|
SPRING 2016
Hypoxia in Breath-
Hold Diving
By Marty McCafferty, EMT-P, DMT
STEPHEN FRINK
With proper training and
dedicated one-on-one
supervision, breath-hold
diving can be an enjoyable
pursuit.
Opposite:
Hyperventilating
before breath-holding
increases the risk of loss of
consciousness by delaying
the urge to breathe.