Respiration: What Pilots Need To Know (But Aren't Taught)
To most of us who fly, aviation is as important as breathing. Or, at least it seems that way. We take our flying very seriously, and spend countless hours receiving ground and flight instruction, and reading every book and magazine article and accident report we can get our hands on, trying to learn everything we can to make us better, safer aviators. Yet even after 35 years of being a pilot, fight instructor, aircraft owner and aviation information junkie, I never fail to be amazed at how much more there is to learn. It's one of the things that makes aviation such a lifelong fascination.
I recently stumbled across a subject of paramount importance to me as a pilot, one about which my years of aviation study and experience had left me completely ignorant. Ironically, that subject is what doctors call respiration and the rest of us call breathing.
Now, it's obvious that if we want to fly, we have to breathe. But, so what? We've been breathing successfully from the time we were born, haven't we? Inhale. Exhale. Lather, rinse, repeat. It doesn't take rocket science. In fact, breathing is so easy that we don't even have to think about it.
It's no different in the cockpit, right? Except that, as all pilots are taught, as our cabin altitude climbs higher, the amount of oxygen available to breathe goes down. So if we climb high enough in an unpressurized airplane, we have to use supplemental oxygen to ensure that we aren't impaired by hypoxia. The FAA says we need to use oxygen whenever the cabin altitude is above 14,000', or whenever it exceeds 12,500' for more than 30 minutes. That pretty much covers what pilots need to know about breathing, right?
That's what I thought, too. Boy, was I ever wrong!
Feeling fine while flying high?
As someone who does quite a lot of high-altitude flying in an unpressurized turbocharged airplane, I've long had more than a passing interest in hypoxia, and I've long had the feeling that there was a lot more to this subject than what's in the FARs and the AIM. Although I have always scrupulously followed FAA guidelines for supplemental oxygen use, I've long been aware that my physical reaction to altitude is extremely variable.
Most of the time, I feel just fine at the end of a long high-altitude flight. But sometimes, I develop a headache by the end of the flight. Once in a while, I've experienced even more distressing symptoms during high-altitude flights ranging from nausea to joint pain. A few times over the years, I felt so lousy that I decided to land short of my intended destination. At the time, usually I blamed lack of sleep or something I ate. In retrospect, however, I'm sure I was experiencing some sort of altitude sickness.
It seemed to me that there had to be a more scientific way to deal with the physiology of high-altitude flight. So I became very excited recently when Nonin Medical introduced their Model 9500 Onyx finger pulse oximeter -- the first oximeter small and inexpensive enough for pilot use -- and I immediately started flying with one. At last, I had a precise way of monitoring my hypoxia level, and determining precisely how much supplemental oxygen was needed to avoid impairment. But, the first time I went flying with this new gadget, I discovered something truly weird.
Unexplained O2 oscillations
At sea level, the pulse oximeter showed the oxygen saturation of my arterial blood to be normal (97% to 98%). And just as I expected, I could see my O2 saturation gradually decline toward 90% (roughly the onset of measurable impairment) as the airplane climbed through 6,000 or 8,000 feet. But then, as I continued to climb higher, I noticed something weird and quite unexpected. As the oximeter readings decreased into the high 80s, they started to get erratic. At first, I thought they were just jumping around randomly. But by the time I reached 11,000 feet, it became clear that the oximeter readings were oscillating up and down in a predictable fashion, about three or four times a minute. As I continued climbing to 12,000 and then 13,000 feet, the oscillations became more and more pronounced, with readings that varied from 90% (barely hypoxic) to 80% (dangerously impaired).
"I don't think this instrument is working properly," I told Dr. Brent Blue, the Senior Aviation Medical Examiner with whom I had been consulting on evaluating pulse oximeter use in the cockpit. "When I get above 10,000 feet, the O2 saturation readings are jumping all over the place." Brent and I agreed to meet the following week to investigate the situation further.
Dr. Blue arranged to borrow two different clinical pulse oximeters from a local hospital, one a suitcase-sized unit that cost about $5,000, and the other a smaller model that cost around $2,000. We set up the big oximeter in the back seat of my Cessna T310, the smaller one between the pilot and copilot seats, and the tiny Nonin Onyx in my shirt pocket. We also brought a notepad and digital camera to record our findings. And then, we went flying.
As soon as we were out of the traffic pattern and the airplane was trimmed for cruise-climb, I engaged the autopilot, set up a 500 FPM climb, and donated my right hand to science. I clipped the Onyx to my index finger, and Brent clipped the other two pulse oximeter probes to my middle and ring fingers. Within ten seconds, all three units were displaying my pulse rate and oxygen saturation. All three O2 saturation readings agreed within one percentage point. As we climbed, all three readings gradually declined. As we passed 10,000 feet, all three started to oscillate. The readings from the three pulse oximeters remained in almost precise agreement, and the oscillations on all three units were perfectly synchronized.
Clearly, there was nothing wrong with my Nonin Onyx oximeter. The oscillations in oxygen saturation of my arterial blood were apparently real. Something weird and quite unexpected was going on in my body! Is something wrong with me?
As an additional cross-check, we transferred the three pulse oximeter probes from my hand to Brent's and waited a few seconds for the instruments to lock onto his pulse and the readings to stabilize. It was immediately apparent that Brent's O2 sat readings were oscillating up and down, almost exactly like mine had been doing. Whatever strangeness was going on here, at least it wasn't unique to me.
Physiologically, Dr. Blue and I are about as different as two people can be. I live on the California coast at about 300' MSL, while Brent lives in Jackson Hole, Wyoming, at an elevation of about 6,600' MSL. I'm 54 years old, considerably overweight, and definitely out of shape. Brent is younger, thinner, and in considerably better cardiovascular condition. If both Brent and I experienced these strange oscillations at altitudes of 10,000' and above, I could only assume that most other pilots react the same way. But why?
Prime suspect: Cheyne-Stokes breathing
Back on the ground, Dr. Blue and I puzzled over possible explanations for the oscillating O2 saturation readings we saw. Brent was just as astonished at the phenomenon as I was, and theorized that the most likely cause was a respiratory anomaly called Cheyne-Stokes breathing. This is an involuntary and unconscious waxing and waning of respiration in which a person at first breathes more deeply than usual, then breathing gets progressively more and more shallow (and in some cases stops altogether), after which the cycle repeats itself over and over again. While Cheyne-Stokes breathing is most often associated with serious medical problems like cardiac failure and brain stem damage, it has also been documented in healthy mountain climbers during sleep periods at high altitude. However, an online search of the medical literature failed to turn up any studies of Cheyne-Stokes breathing in the context of aviation.
A quick review of a standard physiology textbook revealed that the underlying mechanism of Cheyne-Stokes breathing is well-understood. Suppose you breathe more rapidly and/or more deeply than usual. Such hyperventilation flushes carbon dioxide out of your lungs, and the reduced CO2 level causes the blood flowing through your lungs to become slightly alkaline (increased Ph). Some seconds later, this alkaline blood reaches the brain, where the respiratory center in the lower brain stem starts to inhibit respiration. As your breathing becomes more and more shallow, the level of CO2 in the lungs gadually increases and your pulmonary blood becomes more acid. Some seconds later, this acid blood reaches the brain stem, where the respiratory neurons detect it and stimulate respiration. Your breathing becomes deeper and the cycle repeats over and over again.
In a normal person at low altitude, the feedback of the brain stem's respiratory center mechanism is sufficiently damped to prevent Cheyne-Stokes breathing under ordinary conditions. If you purposely overbreathe for a minute or two and then let your involuntary respiratory control mechanism to take over, you'll generally first go into a brief period of apnea (no breathing) and then go through one or two highly damped cycles of Cheyne-Stokes breathing before your respiration returns to its normal steady state.
However, reduced oxygen at altitude stimulates an oxygen-lack-chemoreceptor in the brain stem's respiratory center, greatly increasing the system's feedback gain and allowing Cheyne-Stokes oscillation to occur spontaneously. In fact, oxygen therapy is the standard clinical procedure for suppressing Cheyne-Stokes breathing.
Confirming the Cheyne-Stokes theory
Dr. Blue's theory that the oscillating oxygen saturation readings we had seen at altitude were due to Cheyne-Stokes breathing was an appealing one that made a lot of sense. On the other hand, I remained skeptical that I could be breathing in such an anomalous and cyclical pattern without being aware of it. It was also hard for me to believe that such an obvious phenomenon could occur at moderate altitudes like 10,000' MSL and yet not be discussed in the AIM or aeromedical texts. Well, it would be easy enough to find out for sure.
On my next cross-country flight, I filed for 13,000' and clipped my Nonin Onyx pulse oximeter to my finger. As I climbed through 10,000 feet and my O2 saturation fell below 90%, the oscillations started. By the time I leveled at 13,000 feet, my O2 sat readings were cycling like crazy between 80% and 88%. I donned a nasal cannula and turned on some supplemental oxygen. Within seconds, the oximeter reading climbed to the mid 90s and the oscillations stopped completely. When I shut the oxygen off, the O2 sat dropped into the 80s and the oscillations started again.
Next, I tried to take voluntary control my breathing rhythm. I started breathing deeply and slowly, about six breaths per minute (10 seconds per breath). Within seconds, the oscillations in the pulse oximeter readings stopped. Even more surprisingly, the O2 sat reading climbed steadily to 92% and stayed there. The altimeter showed 13,000 feet, but my blood had the oxygen saturation that I'd have expected to see at 6,000 feet. All I was doing was breathing differently.
Interestingly, I found it moderately difficult to breathe deeply and slowly like that. It took all the concentration I could muster, and it definitely felt strange. At one point, my concentration was interrupted by a call from ATC. I keyed the mic, read back the handoff instructions, dialed in the new frequency, and checked in with the next controller. By the time I was finished and returned my attention to the pulse oximeter, by O2 sat was back in the low 80s and oscillating. My involuntary breathing reflex had taken control, and I was back in Cheyne-Stokes mode.
On subsequent flights, I experimented with different breathing patterns (slow/deep vs rapid/shallow) at various altitudes, with and without supplemental oxygen. I found that any conscious, rhythmic breathing would supress the oscillations in pulse oximeter readings. But, I also discovered that slow, deep breathing resulted in substantially higher O2 saturation readings than rapid, shallow breathing. This turned out to be especially true when using supplemental oxygen up at the Flight Levels.
Another visit to the physiology textbook helped explain why.
Respiratory volume and "dead space"
The capacity of the human lungs varies greatly from one person to another. An average young male adult has a total lung capacity of about 5.8 liters. A large, trim, athletic man might have substantially greater capacity, and a small, fat, sedentary man would have considerably less. Females generally have about 25% less lung capacity than males. In addition, maximum lung capacity can only be achieved in the upright position -- capacity is substantially reduced when lying down, and reduced even more while sitting. Not all of this capacity is useable. After expelling as much air as possible, a substantial residual volume remains -- about 1.2 liters for a young male adult. This leaves some 4.6 liters as maximum vital lung capacity that can be inhaled and exhaled during maximum exertion.
However, normal breathing utilizes only a small fraction of this capacity. The average tidal volume of a young male adult while breathing is normally only 1/2 liter (500 ml.) or so. Even breathing deeply while in a seated position with seatbelts on (as in the cockpit) produces a tidal volume of only 1 liter or so. To breathe much more than that, you need to be standing up.
Furthermore, not all of that tidal volume reaches the alveoli of the lungs where it can oxygenate your blood. When you inhale, a good deal of the new air must first fill your nasal passageways, pharynx, trachea and bronchial tubes before any reaches the alveoli. This so-called dead space volume amounts to roughly 200 ml. Thus, when you take a normal 500 ml. breath, only about 300 ml. makes it to the alveoli where it can do any real good.
This is particularly important when you're using supplemental oxygen. If you "breathe normally" while using an oxygen mask or cannula, roughly 40% of the O2 you consume never gets beyond your dead space. What a waste!
Now let's consider total alveolar ventilation, which is the total volume of new air that reaches the alveoli each minute. Normal breathing averages 12 breaths per minute, and 500 ml. of tidal volume per breath, of which only 300 ml. actually reaches the alveoli. So total alveolar ventilation averages 12 x 300 or 3600 ml. per minute.
On the other hand, suppose you make a conscious effort to breathe slowly and deeply: say 6 breaths per minute and 1000 ml. per breath. Allowing for 200 ml. of dead space once again, total alveolar ventilation averages 6 x 800 or 4800 ml. per minute. If you're using supplemental oxygen, only 20% of it gets wasted in dead space.
A better way to breathe?
You can see why breathing slowly and deeply provides far more efficient respiration at altitude, particularly when supplemental oxygen is being used. The question is: does this have any real practical value? Can a pilot learn to change his or her breathing habits?
Frankly, the jury is still out on this. To my knowledge, neither the inefficiencies of normal breathing nor the aggravating effects of involuntary Cheyne-Stokes oscillations at altitude have been investigated in an aviation context. I am aware of some documented attempts to teach emphysema patients to breathe more efficiently, and those trials were reportedly not particularly successful. On the other hand, those patients did not have the benefit of the "biofeedback" provided by a pulse oximeter.
In the few weeks that I have been investigating this issue, I've been successful in optimizing my breathing for 10 to 20 minutes at a time, producing very dramatic improvements in my arterial blood oxygenation as measured by a pulse oximeter. I've proven to my own satisfaction that I can lower my physiological altitude by 8 to 10 thousand feet and eliminate oscillations in oximeter readings, purely by modifying the rate and depth of my breathing. However, doing so requires considerable conscious effort, and distractions (such communicating with ATC) disrupts the desired breathing pattern. This drastically limits the practical value of this technique. My hope is that, with the help of my new pulse oximeter, I can teach myself to breathe more slowly and deeply while aloft without conscious effort. It's way too soon for me to tell whether or not this can be done.
On the other hand, I remember many years ago when I first learned to drive a stick-shift automobile. At first, the mechanics of shifting and clutching consumed my full attention. With practice, however, it became virtually automatic, requiring no conscious effort at all. Later, when I learned to fly "on the gauges," I had the same experience -- scanning the gauges required enormous conscious effort at first, but became automatic and effortless with practice. So perhaps there's hope after all. Time will tell.
NOTE: The Nonin Onyx pulse oximeter may be purchased from Dr. Brent Blue's company Aeromedix.com either online or by telephone (888-362-7123).
In the interests of full disclosure, I should point out that I serve as a paid consultant
to Brent's company, and have been involved in the selection and evaluation of most of the
products he offers for sale through Aeromedix.com.