Ever notice how some divers blow through their tank really fast, while others can stay down so long
you figure that they must be hiding a set of gills under their wetsuit? What gives? And more importantly,
what can we learn from divers who "sip" their air?
Air Consumption Above and Below Water
The typical diver, at rest, takes about 16 breaths per minute; that's about a cubic foot of air every
three minutes, so he would use up an 80-cf tank in about four hours. However, no matter how leisurely
a dive might appear, we are still working harder than at rest, so a leisurely dive would drain a 80-cf
tank in approximately 80 minutes. However, this is still air consumption as measured at the surface.
What happens when we dive, and start to breathe compressed air?
The deeper one goes, the denser the air one breathes; thus, the tank supplies more compressed air per
breath. A typical diver will draw twice the air out of the tank at 33 feet than at the surface, three times
more at 66 feet, etc. Therefore, a tank containing enough air for an 80-minute dive at the surface might
last about 40 minutes at 33 feet, or 27 minutes at 66 feet.
Measuring Carbon Dioxide Levels -- and Our Breathing Clocks
To understand air consumption as we dive, we also need to examine the two main respiratory
drives in our bodies that make us breathe. One system self-regulates the level of carbon dioxide (CO2)
and oxygen (O2) in our blood. The higher the CO2 level -- or the lower the O2 level -- the faster we
breathe. (Because CO2 is poisonous to the body, the main chemical drive for respiration is the CO2
level in the blood.)
The second self-regulation system acts like a clock in our brain -- we breathe a certain number of
times per minute. The clock ticks faster when we exercise or are nervous, and slows down when we are
relaxed or sleeping, but tends to bottom out at around 11 breaths per minute when asleep.
What happens during a dive? If you descend to 33 feet, your lungs now contain twice as much air as
at sea level -- the air has been compressed to twice its density, but the volume remains the same, because
your lungs didn't collapse. At 66 feet, there are three times more air molecules than at the surface. There
are also three times more O2 molecules that your lungs can use to power your body before the O2 level
runs low. Equivalently, there are that many more molecules in the lungs into which the CO2 produced
by your body can diffuse before the concentration rises enough to force you to breathe. Divers should,
therefore, be able to breathe at half the sea level volume per minute at 33 feet, and one third the sea level
volume per minute at 66 feet. Breathing slower can accomplish this.
Dive Deeper, Breathe Slower
The trick is to convince our bodies to allow the chemical sensors to power our respiratory system,
and to ignore the ticking clock. That clock is so accustomed to telling us to regularly breathe at a rate of
at least 16 breaths per minute that it will continue to do so, regardless of our need. And this is where two things come into play -- how we are individually "wired," and whether we can teach ourselves
to breathe differently.
Some divers' respiration will be more "clock- driven" and firmly fixed, while others will be more
"chemically driven" and adaptable. Most of us are a combination of the two -- we can probably adapt
our respiratory rate, but to variable degrees. The goal is not to stop breathing, or even to slow down the
respiratory rate; rather, the objective is to breathe when your chemical receptors tell you to do so, and
not at some clock-driven time interval more appropriate for above-water air pressure.
|The objective is to breathe when
your chemical receptors tell you
to do so, not at some clock-driven
time interval more appropriate for
above-water air pressure.
Slowing down your respiratory rate takes a conscious
appreciation of your breathing pattern. Start
by lying down, in a warm environment, and have
someone count your breathing rate while relaxed.
Continue by consciously trying to slow down your
respiratory rate, but not to the point of "air hunger,"
feeling the overwhelming need to take a breath.
Additionally, pay attention to your breathing pattern
-- when the in, out and pause occur in the cycle. Become more conscious of your respiratory rate, depth
of respiration and pattern as you sit, walk, run, and do other activities.
The next time you dive, take the opportunity to find a nice patch of sand and rest quietly on the bottom.
Count your respiratory rate, note your pattern and record your depth. Repeat at varying depths,
again avoiding any air hunger, but feeling out what is driving your personal breathing pattern. Can you
"feel" the chemical sensors, and accustom your pattern to them, rather than the arbitrary clock ticking
in your brain? Are you relaxed enough to slow your respiratory rate? Can you comfortably allow it to
slow even more as you dive deeper?
Out, In, Medium Pause
When performing this exercise out of water, you'll notice that the normal respiratory cycle is this: in,
out, medium pause. The resting, or "pause" phase for our respiratory muscles is at the end of exhalation,
and this phase can last many seconds. While on land, where air is plentiful, there is little detriment
to this pattern, but as our respiratory rate increases with exercise, the pause phase shortens considerably.
Underwater, where the air supply must be conserved, a change in the standard pattern can further
diminish air consumption. The objective is to maintain your lungs in the inflated state for a longer portion
of the respiratory cycle, allowing for better absorption of the O2 you have breathed in, and a larger
space into which the CO2 produced by your body can diffuse.
When I dive, I change my standard breathing pattern to this: out, in, medium pause. As a result, the
average volume of my lungs is greater than when using the land-style pattern, increasing the efficiency
of gas exchange. This is not a recommendation for "skip breathing," essentially breath-holding to try to
conserve air, which can lead to complications, including death. At no point should you feel air hunger,
or try to avoid taking a breath to conserve air. Rather, breathe when your body tells you to do so; at no
point should you feel the urge to breathe and not do so.
On a recent dive trip, my buddies videoed me with their GoPros and later sent clips to me. As I
toured the reefs at 50 feet, magnifying glass in hand and oblivious to their recording, I was breathing
slowly, at an average of 8 breaths per minute. Watching the videos, it was easy to note my respiratory
pattern: out-in-medium pause. I have been diving with my buddies for decades, and they have also
adopted the two methods just discussed. As a result, we routinely consume far less air than most other
divers with whom we're paired, even though we are far from spring chickens. But then again, none of
us are full of hot air either.
Daniel Spitzer, M.D., has been a scuba diver and New York City-based neurosurgeon for more than 30 years. He wrote a more extended version of this article, which includes references and measurements, that you can read on
our blog at www.undercurrent.org/blog
Disclaimer: Diving, and breathing underwater while doing so, is the sole responsibility of the diver himself. Neither
Undercurrent nor the author of this article assumes any responsibility for either of these actions.