How to Breathe Underwater

Or really, how to master the “dive deeper, breathe slower” approach

Ever notice how some divers blow through their tank really fast, while others can stay down so long you figure that they must have a set of gills hidden under their wetsuit?  What gives? And, more importantly, what can we learn from divers who “sip” their air?

To do so, first we need to discuss some aspects of respiratory physiology (i.e., how people breathe) in straightforward terms.  Please bear with me as I give you some simple numbers and equations that will let you personalize the numbers to fit how you breathe.

The Basics (rounded off to simplify)

* 1 cubic inch = 16 cubic centimeters
* 1 cubic foot = 1,728 cubic inches
* Depth at 33 feet of seawater is 1 additional atmosphere (ATM) of pressure, so total pressure is 2 ATM.  A depth at 66 feet of seawater is 2 additional ATM, so total pressure is 3 ATM.

The typical diver, at rest, takes about 16 breaths per minute. Assuming the diver weighs 175 pounds, each breath has a volume of about 550 cubic centimeters (cc) or, divided by one cubic inch (16 cc), that equals 34 cubic inches.  Multiplying that by 16 breaths per minute equals 550 cubic inches of air, or about 3.1 minutes per cubic foot of air (cf).  So a diver at rest with an 80-cf tank would allow 80 cf x 3.1 minutes, equaling 248 minutes of respiration – that’s four hours!  (I did get over 90 minutes out of a 63-cf tank one time, without coming close to draining it, but four hours from an 80 would be extreme for all but free divers moonlighting in scuba gear.)

No matter how leisurely a dive might appear, we still work our muscles above the resting state, and thus breathe more air to power these muscles.  A rate of 7 Metabolic Equivalents (METs), which means seven times the standard resting energy expenditure, has been quoted for scuba diving, or the equivalent of “moderate intensity energy expenditure.” I feel this is high for recreational diving, so I’ll instead use 3.2 METS – closer to a brisk walk – as the energy expended over the course of a dive.

Multiply our energy expenditure , and thus our air consumption (16 breaths per minute equaling 550 cubic inches of air) , by 3.2 METS, and you get 1,760 cubic inches per minute, or a little more than one cubic foot per minute. The typical 80-cf tank now allows approximately 80 minutes until fully drained. However, this is air consumption as measured at the surface.

Calculating Your Personal Air Consumption

While breathing underwater, a  diver’s respiratory volume is about the same as it would be if he worked at the same rate on the surface.  The consequence of this is that a tank containing enough air for a 100-minute dive at 1 ATM would last about 50 minutes at 2 ATM  (33 feet) or 20 minutes at 5 ATM (130 feet) for dives with the same energy expenditure. So the deeper one goes, the denser the air one breathes, and thus the tank supplies more air per breath, and the pressure in the tank drops faster. A typical diver will draw twice the air out of the tank at 33 feet (one atmosphere) than at the surface, three times more at 66 feet, etc.  Thus a typical diver at a depth of 33 feet will use 2 CF per minute, and an 80-cf tank will last a maximum of 40 minutes till bone dry. Assuming the diver uses some discretion and surfaces with 400 PSI, and thus uses 69 cf for the dive, again at 2 cf/minute, the dive duration is 35 minutes.

On a typical dive trip, I calculated my air consumption over a series of dives as follows:

An 80-cf tank, with starting pressure of 3000 psi, my ending pressure is typically 1400 psi, and my average depth is 40 feet. Divide 1600 psi used by 3000 psi start, and multiply by 80 cf, and I used 43 cf on a dive. The duration of my dive is 50 minutes, so I used

0.86 cf per minute.

Note that this is less than half the air consumption predicted by the standard model, and approximately half what many divers use.  Possible explanations include my young age (though I am 58), my small size (but I am 5’10” and  174 pounds) or my movie-star looks and physique (not).  So there must be some other reason I consume about half the air of many other divers.

Measuring Carbon Dioxide Levels and Our Breathing Clocks

Let’s look at the two main respiratory drives that make us breathe.  One system measures the level of carbon dioxide (CO2) and oxygen (O2) in the blood. The higher the CO2 level, or the lower the O2 level, the faster we breathe.  Incidentally, CO2 is poisonous to the body; we tolerate low O2 levels much better than we can tolerate high CO2 levels.  So the main chemical drive for respiration is therefore the CO2 level in the blood.

The second system is 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 when asleep.

What happens as we 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 (your lungs didn’t collapse).  So there are two times more air molecules in your lungs at 33 feet, and three times more at 66 feet, than there were at sea level. There are therefore two (or at 66 feet, 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.

The “Dive Deeper Breathe Slower” Approach

Just based on absorption of O2 from the lungs and diffusion of CO2 back into the lungs, divers should 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 faster, as well as breathing deeper, can accomplish this.

The trick is to convince our body 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 breathe at a rate of at least 16 BPM that it will continue to do so, regardless of need. And this is where two things come into play – how we are individually “wired,” and whether we can teach ourselves to breathe differently.

Along those lines, some divers’ respiration will be more “clock driven” and firmly fixed, while others will be more “chemically driven” and adaptable.  Most  of us are probably a combination of the two – we can all probably adapt our respiratory rate, but to variable degrees.  The record voluntary breath-hold is 23 minutes, but this amazing feat was performed after pre-breathing 100 percent O2, something no diver can do for fear of developing oxygen toxicity and seizures.  Even without O2 pre-saturation, the record is nearly 12 minutes,  all the more astonishing since conventional wisdom holds that brain damage starts after three minutes.  Clearly, the typical recreational diver is unable to slow their respiratory rate to that degree, nor is that the objective. The goal is not to stop breathing, or even to slow down the respiratory rate; rather, the objective is to breathe when our chemical receptors tell us to do so, and 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 BPM 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. (More on this below.) Become more conscious of your respiratory rate, depth of respiration and pattern as you sit, walk, run, and do other activities.

Finally, 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 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 is limited and thus is best 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 better absorption of the O2 we have breathed in from the tank, and a larger space into which the CO2 produced by our 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 — that can lead to complications, including death. At no point in the respiratory cycle should your epiglottis  at the back of the throat  be closed.  Neither should you feel air hunger, or try to avoid taking a breath in an attempt to conserve air.

Rather, I recommend you breathe when your body tells you to do so; at no point should you feel the urge to breathe and not do so. What I’m suggesting is a reversal of the normal pattern of respiration, with the normal pause phase simply being moved to the full rather than the empty phase of the cycle.

I’ll end on a interesting (at least to me) note. On a recent dive trip, my buddies carried 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 BPM.  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. A a result, we all routinely consume far less air than most other divers whom we’re paired with, even though none of us is particularly young, small or sporting a movie-star physique.

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Daniel Spitzer, M.D., has been a scuba diver and neurosurgeon for over 30 years. This is an e extended version of the article which appears in our July, 2015 issue.

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.