Ask any pilot in the lounge about oxygen and flying and youll get, Well, youve got to have oxygen on above 14,000 feet, and above 12,500 feet if youre going to be there for more than half an hour. Ask the same pilot how much oxygen and youre likely to get a puzzled look.
The FARs outline when to use oxygen and the Airmans Information Manual says cannulas may be used up to 18,000 feet. But other than that the aviation bibles are mute on the issue.
Pilots are left to guess how much oxygen they need as they climb into the oxygen altitudes.
In addition, lower fliers can be left wondering what to make of conflict between the regulatory oxygen altitudes and evidence that hypoxia symptoms may begin to appear half that high.
A 1997 Civilian Aeromedical Institute study of pilot decision making in the altitude chamber showed an increase in errors in the 10,000-foot environment, and a significant time to recovery. Night vision can start to go at 5,000 to 7,000 feet.
If you have supplemental oxygen, how do you set the regulator? If you dont, how can you tell when youre in danger of hypoxic symptoms? Why arent there any guidelines?
A review of federal research on oxygen administration makes it quite clear that the often cited FAR guidelines are broad strokes. Any AME can tell you that the oxygen demands of an individual are quite variable, from day to day and from person to person.
That is precisely why CAMI hasnt published guidelines. On a population basis, there simply isnt enough information to do this.
I asked another CFI recently how to tell how much oxygen to use and after some thought he replied, Until that darned headache goes away and I can see color again.
Well, hes on to something but still a long way from the right answer. Another one replied that hed use enough oxygen. Hes a bit closer, but the question is how much is enough?
The Oxy Train
To understand the dynamics of oxygen use, you may need a short refresher on physiology. Imagine if you will that each red blood cell is a little train car on its way through the rail yard known as the lungs. As it chugs by it gets exposed to the oxygen loader. If everything works perfectly, none of the train cars emerges without a near full load. Of course, nothing ever works perfectly, so some cars do leave empty.
Down the rail yard a ways, all the cars get merged back together in the arterial blood. Even a few unloaded cars really drags the average content of the blood down.
Next is the distribution system. Some parts of the body, especially the brain, need a lot of oxygen. The freight cars need to be full and they need to come by quickly to feed the need of the cells. If the cars come too slowly or are not full enough, the time for trouble is near.
Thats why carbon monoxide poisoning is so deadly. Carbon monoxide replaces the oxygen in the train cars and puts great strain on the other cars, which must keep up with the demand for oxygen. The brain may starve because the train cant carry enough of its oxygen cargo to the critical areas.
Finally, the cars must be unloaded properly at the destination. Some oxygen in the red cells cant be unloaded – thats the part of the content below the pressure required to drive oxygen into the bodys cells. People who live at high altitudes adapt to them by increasing the tendency of the blood cells to unload oxygen at the delivery platform.
Enter the Blood Stream Fuel Gauge
The oxygen guessing game has gone on since pilots first started climbing to five-digit altitudes, but a cheap new device promises to change all that. For about $400, you can equip the airplane with a battery-powered device that quickly (and painlessly) measures your blood oxygen level. This device, the pulse oximeter, enables you to get most of the answer.
A pulse oximeter is basically a spectrophotometer, which measures the adsorption of a pair of light beams. Remember that oxygen-saturated blood is red and unsaturated blood is blue.
The device compares optical density at both wavelengths to a built-in set of standards and returns a percent saturated value.
It manages to ignore the adsorption of skin, fingernails, nail polish, etc., by canceling the value of anything which doesnt pulsate. This tells you, in aggregate, what percentage of all the capacity of the red cells has been loaded with oxygen.
Some things stack the deck against you from the start. Smokers lose up to 10 percent of their train cars from carbon monoxide tying up the hemoglobin.
People who are anemic have fewer red blood cells available. If you get dehydrated the blood can be moving too slowly and the train cars just cant contain enough oxygen to keep up with the demand.
Under the best conditions, most healthy people maintain about a 95 percent saturation at sea level. Blood returning will be about 75 percent saturated, in aggregate.
As you climb higher and higher, the oxygen pressure in the air also decreases until, at about 12,500 feet in healthy people, theres not enough oxygen pressure left to keep the loading station output at 95 percent. Your brain and other organs respond by returning the red cells less and less saturated. The question then is how saturated should the blood be in the first place?
In the hospital environment, a patient is not even considered likely to be of sound mind unless their oxygen saturation is above 90 percent. Given that, you must decide whether you would want any less for yourself in the air.
Pilots will frequently tell you that theyve been at 16,000 feet or 17,000 feet for short periods without adverse consequences, but that simply isnt so. They were just unable to recognize their impairment.
What Is Enough?
About eight years ago I took part in a video production in which a professional pilot, spotted by an oxygen-wearing copilot and by a third crew member, was videotaped at 18,000 feet without oxygen.
After less than 60 seconds at that altitude, his blood saturation had dropped to 86 percent and he began missing radio calls. That was the end of the video as oxygen was rapidly reapplied.
The human brain appears to protect consciousness to the last, diverting blood away from the frontal lobes in which judgment resides. You might be awake, but youre only partially there and the judgment to realize your handicap has been shut down.
On the FAA decompression chamber ride, in which FL 250 is attained in 10 seconds, AME pilots grade each other, trying to spot the signs of hypoxia in the first minutes of decompression. Invariably the fellows wearing oxygen spot more signs and symptoms than the hypoxic AME, despite the fact that the hypoxic doctor is trained to recognize the signs and is expecting the decompression. Sometimes the chamber staff has to go and physically put the oxygen on a clearly impaired AME pilot.
Because judgment is sacrificed early and consciousness is preserved as long as possible, the role of the pulse oximeter must be to serve as a warning, before the selective shutdowns begin, of the impending sacrifice of the various parts of thinking.
Once you get below the judgment impairing value, you are unlikely to remedy the situation no matter how loud the alarm. In my view, theres no reason to accept an oximeter value below 90 percent saturation.
Though some people say you can reasonably tolerate 10 points less than on the ground, I specifically dont like that notion.
Theres a physiological reason for not accepting anything less than 90 percent saturation, and it has to do with the nonlinear binding of oxygen to the blood. Below 90 percent saturation, very small changes in pressure result in large changes in blood oxygen content. Its a slippery slope, and too much is at stake to accept less than 90 percent.
If youre flying between 12,000 and 25,000 feet in an unpressurized general aviation airplane, its because you are planning to use the full envelope of the airplanes ability to fly over terrain or weather, or because youre trying to get maximum range.
Any flight that attempts to maximize performance requires good planning and judgment. In this environment, you cannot accept conditions in which your capacity for judgment is not a certainty.
That Was Then, This Is Now
Keep in mind that your personal tolerance to altitude varies from day to day and over time. About 10 years ago, I was flying at 11,500 feet over the Colorado Plateau and experienced nausea in rather severe clear air turbulence. I set up a VFR climb from 11,500 to 13,500 feet. I was only marginally conscious when my spouse got my attention by screaming into my right ear, and I set up a descent. I mistook the nausea as a consequence of the turbulence, but actually it was an unrecognized harbinger of hypoxia.
Shortly thereafter I was flying a non-turbocharged Cessna from Grand Junction to Flagstaff. At 11,500 feet, near Navajo Mountain, I began to notice that the environment was hazy. I asked Denver Center if there were any reports of reduced visibility over the Four Corners area, and they replied, Visibility out there is supposed to be 50 miles.
By this time my vision had deteriorated to the point that I could only see my basic flight instruments. I was below the Centers radar coverage and climbing was not an option because the oxygen system was in the baggage area.
There was lots of cumulo-granite around, so I flew an NDB approach in VMC to Bluff, Utah. At about 8,800 feet my vision returned to 50 nautical miles. I concluded that I was still not quite right after suffering from a cold about a week earlier.
From that moment forward, my aircraft has always had a fully charged oxygen cylinder aboard and a forward regulator that is controllable from the pilot seat. The system is preflighted for every cross country trip.
If an oxygen system requires the hypoxic, confused pilot to leave the PIC seat to energize it, its next to useless. Just try crawling to the rear of the cabin in turbulence, ice or IMC to get to the valve that you forgot to turn on.
My personal aircraft has been equipped with a pulse oximeter for about five years, and at present my personal minimums are:
• During the daytime, pulse oximetry must indicate more than 90 percent. The oxygen is on at and above 10,000 feet, with flow sufficient to maintain 90 percent saturation using oxymizer, or reservoir, cannulas.
• At night, pulse oximetry must indicate more than 90 percent. The oxygen is on at and above 8,000 feet, with flow sufficient to maintain 90 percent saturation. Color vision at night is usually impaired early (but not reliably early) in the brain shutdown sequence.
• At or near 18,000 feet, the reservoir facemask gets used in place of a cannula because cannula flow becomes excessive to maintain saturation.
• Pulse oximetry continuously above FL20.
Using these guidelines, my 50 cubic foot tank will supply about 29 man-hours of flow for flight at between 16,000 and 18,000 feet. I also insist on having more time in my oxygen tank than in my fuel tanks.
When faced with the prospect of descent into icing conditions, for example, most pilots are tempted to stretch the oxygen supply in order to maintain altitude as long as possible. This is very ill advised, because the pilot will become very, very stupid. Useful pilot time at 18,000 feet is about five minutes.
Theres another consideration when determining tank requirements. The FBO at the terminus may not be able to refill your bottle.
That means you may not be able to proceed on the next leg without figuring out a solution. Carry a second tank. My backup is a 19 cubic foot affair and has a second regulator. With a single regulator, a regulator failure also signals the end of the trip.
Measuring Blood Saturation
Pulse oximeters take the guess work out of setting the oxygen flow, helping you ensure youre getting enough while still maximizing the time in your bottle. For those who fly below the oxygen altitudes, the oximeter can warn of impending hypoxia that can become a problem far lower than the current FARs suggest.
Were not talking about carrying along another briefcase here, either.
Nonin Medical Products makes a compact fingertip version that goes for about $400. It runs about 12 hours on a pair of AAA cells and is good for about 1,000 spot checks. Siemens Medical also has a tiny one powered by two penlight batteries.
My older version takes six penlight batteries and is the size of a checkbook. It will run 100 hours on a set of batteries.
All cost less than anything that bears the name King or Garmin on the faceplate and can be worth much more when the chips are down. The devices have historically been sold by prescription only, but I cant imagine a local AME turning you down when you request an Rx for one; I honor all such local requests.
Nonin says it recently began selling its tiny Onyx without a prescription to pilots who planned to use it for non-commercial use only.
A pulse oximeter doesnt require a medical degree to use, either. Pop the sensor on a finger and wait a few seconds for the pulse rate and saturation level to display.
Bear in mind that the pulse display is required if you want to believe the oxygen saturation figure, because the device needs to be locked on to a pulsating component of your finger – the blood.
Its amazing what you see over the years on the ramp and in the pilot lounge. Ive seen pilots load up scuba tanks to go flying, mistakenly believing that theyre doing themselves a favor.
The tanks are filled with air, 80 percent nitrogen and 20 percent oxygen. As the gas exits the tank, it assumes ambient pressure and becomes just more ambient air. Except for one very expensive system, scuba regulators are not set up for 100 percent oxygen.
In the scheme of flying, pilots spend thousands to monitor the equipment and back up important components with a second one in case the first one fails. Think of a $400 pulse oximeter as an annunciator for the most important computer aboard the plane – and the one for which most pilots never have a backup.
-by Bruce Chien
Bruce Chien is a physician, CFII, MEI and Airman Medical Examiner, and owns a Piper Seneca II.