I first became acquainted with pulse oximeters the hard way -- in a hospital Emergency Room a few years ago. My wife and I were in the midst of a brief mini-vacation in the California High Sierras when I fell ill. After I spent a horrible sleepless night in the hotel, feeling nauseous and dizzy, my wife made a command decision and drove me to the ER, despite my protestations that I really didn't need to see a doctor.
When I arrived at the hospital, the ER doc took one look at me, told me to lie down, and clipped some sort of probe onto my finger. The probe was connected by an electrical cable to a box with a digital readout on it, and the physician noted some readings from the instrument onto my chart. Then he stuck a cannula up my nose and started administering oxygen therapy.
The doctor told me that I was profoundly hypoxic -- he classified my condition as cyanotic (meaning I was actually starting to turn blue) -- and said that the instrument connected to the finger clip probe was a pulse oximeter that was reading the percentage of oxygen saturation in my arterial blood. Readings of 95% to 100% are normal at sea level, he explained, and even at the 8,000' elevation of Mammouth Lakes, the oxygen saturation for a flatlander like me should still be 90% or so. But when he first hooked me up, the reading was 75% -- a severely hypoxic state that was literally life-threatening if it had been left untreated for a few more hours.
After listening to my lungs with a stethescope, the ER doc diagnosed HAPE -- High Altitude Pulmonary Edema -- which is a form of altitude sickness in which fluid starts collecting in the alveoli (tiny sacs) of the lung tissue, interfering with the lungs' ability to oxygenate the blood. The only remedy was for me to be transported immediately to lower altitude (while continuing to breathe supplemental oxygen) and stay there for 24 to 48 hours until the fluid was reabsorbed and my lungs started to function normally once again.
My wife drove us down to Bishop, Calif., at 4,000-foot MSL and I checked into the Bishop Hospital ER, where I was hooked to another pulse oximeter and pronounced out of danger. I was instructed to stay overnight in Bishop, remain on supplemental O2, and return the next day for another check. When I returned the next morning, my oxygen saturation readings were in the 90s and I was given a clean bill of health.
Although my encounter with HAPE was scary and messed up my vacation, I was certainly fascinated by the pulse oximeter technology that had been used to measure the oxygen saturation of my blood. As a pilot who does quite a lot of high-altitude flying in unpressurized airplanes, I was intrigued by the notion of carrying such an instrument along in the cockpit and using it to monitor my hypoxia level and that of my passengers. Such instrumentation would let me know precisely when I needed to start using supplemental oxygen, and precisely what O2 flow rates were necessary to prevent impairment or discomfort. I decided that I wanted one of these instruments for my airplane.
I did some checking with medical supply houses to see whether it might be possible to purchase such an instrument for in-flight use. The results were not encouraging. The least-expensive pulse oximeter I could find was rather bulky -- about the size of a battery charger -- and cost about $2,000. Furthermore, the distributors were not comfortable selling such a device to anyone but a doctor or hospital, and told me that at the very least I'd require a doctor's prescription.
While discussing my HAPE scare with Dr. Brent Blue -- a good friend and Senior Aviation Medical Examiner based in Jackson Hole, Wyoming -- I mentioned the idea of using a portable pulse oximeter in the cockpit. Brent thought it would be an excellent idea, and offered to write me a presecription if I wanted to buy one. But $2,000 seemed awfully steep, so I shelved the idea.
Then, a few months ago, Dr. Blue sent me a "bingo card" from one of his medical journals that advertised a new, micro-miniature pulse oximeter from Nonin Medical, Inc., in Plymouth, Minnesota. (In case you were wondering, the company's name is short for "non-invasive.") Through the magic of state-of-the-art custom integrated circuit technology, Nonin Medical has managed to squeeze the pulse oximeter electronics into the finger-clip probe itself, creating a self-contained finger pulse oximeter called the Nonin Onyx . The Onyx is incredibly tiny (1.3" x 1.3" x 2.2"), weighs only two ounces, and is powered by two AAA alkaline batteries. Best of all, the Nonin Onyx sells for less than $400, clearly within the realm possibility for individual pilot use.
My interest was definitely rekindled. I contacted Nonin and told them of my interest in using their little pulse oximeter for in-flight monitoring of flightcrew hypoxia levels. Surprisingly, they seemed uninterested in talking to me. It seems that oximeters are FDA-approved medical devices, and Nonin's policy is to offer them for sale only to hospitals and physicians. Interestingly enough, the company does sell their oximeters to veterinarians for use on animals. Pilots, however, are not on their radar screen. According to Nonin, the U.S. Food and Drug Administration (FDA) dictate these silly restrictions.
When I told Dr. Blue that I wasn't getting anywhere with Nonin, he volunteered to call them on my behalf. A few weeks later, a Nonin Onyx showed up on my doorstep. I was anxious to put it through its paces and see how it worked in the cockpit.
I started out by familiarizing myself with the unit on the ground. Operation of the Onyx the ultimate in simplicity. Unlike the big hospital units I'd seen before, the Onyx has no switches or controls. You simply clip the unit to the fingertip of your choice and the thing automagically turns itself on. After a few seconds, the "perfusion display" LED starts blinking in sync with your pulse. The color of the blinking LED is green, yellow or red, indicating whether the unit is detecting good, marginal or inadequate pulse amplitude. (If the indication is yellow or red, simply reposition the clip or change to a different finger.)
After a few heartbeats, the two numeric LED displays light up. The top number -- labeled "%SpO2" -- shows the percentage of oxygen saturation of your arterial blood, normally a figure between 95% and 100% at sea level, and progressively less at higher altitudes. The bottom number -- labeled with a little heart symbol -- shows your pulse rate in beats per minute.
You can monitor yourself continuously in flight, or if you prefer you can conserve batteries by doing periodic spot-checks throughout the flight. A pair of AAA alkaline batteries is good for 12 hours of continuous operation, or about a thousand 45-second spot checks. The unit shuts itself off automatically ten seconds after you remove it from your finger, so there's no chance of accidentally running down the batteries because you forgot to turn the thing off. If the batteries get low, the numeric displays start to flash once per second to warn you, but the unit continues to function normally for quite a while after that.
My initial flight tests with the unit produced some encouraging results, but also some puzzling ones. At sea level, the readouts 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 to 8,000 feet.
The instruction manual had cautioned that the oximeter measurements might be affected by "excessive or rapid movement" and "fluctuating or flickering light." So I tried intentionally subjecting the Onyx to in-flight vibration by putting it in contact with various portions of the aircraft (the glare shield, the windows, etc.), and to expose it to various light levels. Nothing I did seemed to have any effect on the readings, however, and I concluded that only slow, rhythmic movement or light pulses (that the unit could falsely interpret as a pulse) would be a problem.
I found the LED digital displays to be somewhat difficult to read in bright, direct sunlight. However, there was no problem reading them while my hand was in my lap, on the throttle, or anywhere else that was shielded from direct sunlight by the instrument panel or glare shield. I also did some night flying with the Onyx, and found its displays to be very pleasant to read (and not distractingly bright as I had feared).
The puzzling results came as I climbed above 10,000 feet into "hypoxia territory." The oximeter's saturation readings decreased into the 80s, as expected. But they also started to vary quite a bit, especially when I climbed to the 12,000 and 13,000 foot levels, at which point the oscillations became quite pronounced. My initial assumption was that the instrument was not functioning correctly at these higher altitudes and lower O2 saturation levels.
To find out for sure, I made arrangements with Dr. Blue to borrow a couple of 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. Brent and I set up these units in my Cessna T310 -- the big one in the back seat and the smaller one between the seats -- and the two of us took off on a test flight to compare results from the two hospital oximeters with those from my little Nonin Onyx. Shortly after takeoff, I put the airplane on autopilot and hooked my right hand to all three pulse oximeters, the three finger-clip probes clipped to three different fingers. (Boy, did that feel weird!)
We step-climbed all the way up to 17,000 feet, leveling off at each 1,000-foot altitude and noting the readings of the three oximeters. All three O2 saturation readings consistently agreed within one percentage point, indicating that the inexpensive little Onyx was every bit as accurate as the big, expensive hospital units. Interestingly enough, as we climbed above 10,000 feet, all three started to oscillate -- the readings from the three pulse oximeters remained in very close agreement, and the oscillations on all three units were perfectly synchronized.
NOTE: We later discovered that the oscillations were not an instrumentation artifact, but a genuine oscillation of my blood oxygen saturation level at altitude. This fascinating and potentially important finding is the subject of a companion article: Respiration--What Pilots Need To Know (But Aren't Taught).
As we climbed above 14,000 feet, Brent and I donned conserving cannulas and turned on the flow of supplemental oxygen. After leveling at 17,000 feet, we started experimenting with various oxygen flow rates (using a calibrated vernier flowmeter) to determine the impact on oxygen saturation. We found that without supplemental oxygen, O2 sat readings decreased into the mid-70s (extreme impairment), but that saturation could be brought up to a very acceptable level (low 90s, equivalent to a physiological altitude of 6,000 or 8,000 feet) by using an extremely low O2 flow rate (0.5 liter/minute or less). Turning up the flow rate had no significant beneficial effect whatever.
Based on our oximeter results, we concluded that it would be prudent to start using supplemental oxygen at altitudes well below what the FARs require. O2 sat levels definitely drop to those associated with measurable impairment at cabin altitudes as low as 10,000 feet, and supplemental O2 is probably warranted at altitudes as low as 6,000 or 8,000 feet if you're a smoker or if you have respiratory congestion (e.g., a cold) or emphysema or any number of other conditions that can reduce the efficiency of your respiratory system. Using a conserving cannula and a vernier flowmeter, together with a pulse oximeter, it's often possible to get by with drastically lower flow rates of supplemental oxygen than those that are normally used. On the other hand, there are times when the O2 flow will need to be turned up. The oximeter makes it possible to administer precisely the required amount of O2 needed for your altitude and physical condition.
All in all, the Onyx performed astonishingly well alongside the large and expensive hospital oximeters. This test flight gave us great confidence in the accuracy of the Nonin instrument, despite its tiny size and relatively low cost.
About my only gripe with the Nonin Onyx is the fact that its digital readout is designed for attended-care monitoring of a patient, and is oriented "upside down" for self-monitoring. In order to monitor your own readings, you must either (1) turn your palm up and bend your instrumented finger toward your palm, or (2) learn to read the display upside-down. In my early trials with the Onyx, I used the finger-bend method, but after using the instrument for awhile, I found that the upside-down-reading method was just as easy. If the use of pulse oximeters in the cockpit becomes popular, perhaps we can persuade Nonin to produce a model with an inverted display. In the meantime, it's not a serious problem, just a mild annoyance.
The technology that makes possible the non-invasive measurement of the oxygen saturation of arterial blood is quite fascinating -- simple in concept, but complex in execution. Pulse oximetry takes advantage of the fact that blood changes color depending on whether the hemoglobin in the red blood cells are oxygenated or deoxygenated. Oxygenated blood is bright red, while deoxygenated blood is dark red in color, bordering on purple. It is therefore possible to deduce the degree of oxygen saturation of blood from its color. The basic idea behind a pulse oximeter is to shine a red light through a vascular bed (such as a fingertip or earlobe) and measure how much of the red light is absorbed. Dark-red or purple deoxygenated blood will absorb most of the red light, while bright-red, oxygenated blood will allow the red light to pass right through.
Unfortunately, there lots of other things that affect how much red light is absorbed: finger thickness, skin thickness, skin pigmentation, bone thickness, fingernail thickness, the presence of nail polish, etc. To account for these things, the pulse oximeter shines a second light beam of different color through the finger (actually, an infrared beam) and measures the differential attenuation of the two wavelengths. Extraneous sources of light attenuation are thus cancelled out, allowing accurate determination of red light absorption by the blood relatively unaffected by finger-to-finger variations.
This still leaves the problem of how the pulse oximeter can distinguish between arterial blood (which is what we want to measure) and venous blood (which is always deoxygenated). Here's where the "pulse" part comes into play. The oximeter takes advantage of the fact that arterial blood flows in pulses, while venous blood flow is steady. By locking onto the pulse and measuring only the differences in red light absorption between the high and low points of pulse fluctuations (systolic vs. diastolic), the oximeter is able to cancel out the effects of steady venous flow and measure only the color of the pulsating arterial blood.
All this magic is done through complex digital signal processing by a tiny microprocessor and sophisticated software, using elaborate exponential smoothing and empirical approximation algorithms to come up with a reading that correlates very closely with the results of laboratory blood gas analysis over a wide range of conditions.
The technology involved is really cool. The fact that this technology can now be reduced to a two-ounce gizmo that slips into your shirt pocket is nothing short of remarkable.
From my perspective as a pilot, perhaps the most important limitation of pulse oximeters is that they will not detect carbon monoxide (CO) poisoning. When CO binds with the hemoglobin in your blood, the cells turns bright red...just as if they had been oxygenated. The resulting molecule (carboxyhememglobin) is incapable of carrying O2 to your cells, but is indistinguishable in color from oxygenated blood so far as the pulse oximeter is concerned. Consequently, it's important for pilots to carry a CO detector, especially when flying single-engine aircraft that utilize an exhaust-muff-type cabin heat system.
NOTE: AVweb has completed an extensive investigation into CO monitors for aircraft use. We do not recommend the inexpensive chemical-spot detectors available from most pilot supply stores.
By the same token, a pilot or passenger who smoked a cigarette shortly before takeoff will tend to be more hypoxic than the pulse oximeter readings indicate, because smoking causes CO poisoning which the oximeter cannot detect.
A second limitation is that pulse oximeters tend to become inaccurate at extremely low levels of oxygen saturation (below about 75%). This is not a serious problem for in-flight use, because accuracy at those levels is not important. Who cares whether your O2 sat is 76% or 73%? If it's below 80%, you're in big trouble, and you better crank up the O2, get down to a lower altitude, or both...fast!
A third limitation is that the pulse oximeter depends on the presence of a good pulse. People with unusually low blood pressure or impaired blood flow to the fingers may have difficulty getting valid oximeter readings. Conditions that cause constriction of the blood vessels in the extremities (e.g., cold temperatures, profound hypoxia) can also interfere with oximeter readings, as can drugs that are vasoconstrictors or vasodilators (e.g., nitroglycerine), or drugs that affect blood color (e.g., sulfonamides). In most such cases, the oximeter will warn of inadequate perfusion (a yellow or red LED in the case of the Nonin Onyx). Once again, these problems are not often seen in the cockpit.
It's one thing to comply with the FARs, but it's quite another thing to fly safely and comfortably. Hypoxia is insidious and highly physiologically variable from pilot to pilot...and in the same pilot from day to day. The FAA-mandated requirements for supplemental oxygen use may be too liberal or too conservative, and there is no objective way for a pilot to know without using a pulse oximeter to measure the actual level of oxygen saturation. An overweight, out-of-shape, middle-aged smoker will become hypoxic at a far lower altitude than a young, athletic non-smoker.
At sea level in a healthy person, the oxygen saturation is typically 95% to 100%. At 6,000 feet, the normal oxygen saturation drops to the 90% to 95% range, and continues to decrease as one goes to higher altitudes. As O2 sat decreases into the 80s, measurable impairment of cognitive and physical performance begins. Those changes don't occur immediately, but vary with the speed of ascent and the duration of exposure.
When oxygen saturation levels drop, bad things happen that are rarely perceived by the victim (at least in the early stages). Visual changes occur, including "tunnel vision" and a marked decrease in night vision. Other common symptoms of hypoxia include headaches, anxiety, panic sensation, inability to perform mathematical problems accurately, inability to program equipment such as a GPS, dizziness, nausea, headache, and confusion. Symptoms are different for each person, and can occur at altitudes far lower than most people would predict. (An excellent way to get in touch with your own hypoxic symptoms is to schedule a altitude chamber "ride" at the FAA Civil Aeromedical Institute in Oklahoma City, or at an Air Force base near you.) Symptoms of hypoxia generally remain consistent for a particular person, but the altitude at which the onset of impairment occurs is highly variable from day to day.
One of the earliest physiological effects of hypoxia is a change in respiration from steady to cyclical. This change in involuntary breathing patterns interfere with respiratory efficiency and exacerbate the hypoxic effect of high-altitude flight. For more information about this fascinating subject, see the companion article Respiration--What Pilots Need To Know (But Aren't Taught).
The availability of small, low-cost pulse oximeters suitable for use in the cockpit provides an enormous leap forward in detecting and dealing with in-flight hypoxia. Although not perfect, the pulse oximeter which can be worn on a fingertip by both pilot and passengers gives an almost instantaneous oxygen saturation reading. Oximetry has now become so important in the hospital setting that doctors often refer to it as "the fifth vital sign." (The first four are pulse rate, blood pressure, respiratory rate, and temperature.)
Dr. Blue offers the following guidelines are offered with respect to pulse oximeter use in the cockpit:
Based on our recent in-flight testing and research, Dr. Brent Blue and I have come to the conclusion that the use of a pulse oximeter in the cockpit is an essential safety measure for any pilot who flies at cabin altitudes of 10,000 feet and above. We also believe oximeter use is prudent -- regardless of cruising altitude -- for any pilot who smokes, flies with a cold or cough, or suffers from any other respiratory condition. In short, we think that most pilots would be wise to carry a pulse oximeter in their flight bag.
Because of the difficulties I encountered in obtaining my oximeter (medical companies don't want to talk to you if you're not a doctor or hospital administrator), I asked Dr. Blue to set up a distributorship called Aeromedix.com for the purpose of making the Nonin Onyx pulse oximeter (and other relevant medical products) easily available to pilots at discount prices and with the FDA-required prescription thrown in at no charge. Aeromedix.com has now set up a web page where you can order the Onyx oximeter online at a discounted price, significantly less than the $425 or so that you'd pay at a medical supply house (assuming you had a doctor's prescription so they'd sell one to you). You can also order 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.