Minnesota Flyer - Serving Midwest Aviation Since 1960

By James D. Lakin PhD MD FACP CFI
FAA Senior Aviation Medical Examiner 

Stayin' Alive with Pulse Oximeters

 


Last month we talked about the dangers of hypoxia and some procedures to avoid the potentially fatal effects of low blood oxygen levels.

Symptoms of hypoxia vary from airman to airman so it is a good idea to take a ride in a hyperbaric chamber to personally experience the real thing. Learn more at https://www.faa.gov/pilots/training/airman_education/aerospace_physiology/.

The problem is not everyone can fly down to the FAA's chamber in Oklahoma City. As an alternative you can take a trip in a so-called "portable reduced oxygen training enclosure" or PROTE. This can be transported to an FBO near you and provide a great real life experience of hypoxia under carefully controlled conditions. Google "FAA PROTE" for a current schedule.

Of course, you would hope that a prudent pilot would never have to put to use his experience of hypoxia. To be a bit more proactive it's a great idea to monitor blood oxygen levels at any altitude where the danger of hypoxia exists.

Happily, current technology provides a relatively inexpensive and effective tool to accomplish this-the pulse oximeter. You can buy these things for $40 to $60 at most of the aviation supply websites. It's a gizmo that rapidly measures how much oxygen is being carried by your red blood cells to the organs of your body.

It does this by shining light at two wavelengths-red and infrared-through a part of your body that is more or less translucent and has good blood flow.

This usually is your finger tip although if you're wearing nail polish, a toe or earlobe will do. That might cramp your rudder work or headset seal, however. So it's best not to wear nail polish and use your finger! The ratio of red to infrared light that passes through the finger depends on the percentage of hemoglobin, the oxygen carrying protein in your red blood cells. By measuring this ratio, the percentage of hemoglobin carrying oxygen is calculated.

This is reported on the instrument's read-out as the percentage of oxygen saturation of hemoglobin or SpO2. So what is normal? Like so many things, it depends.

At sea level, values range from 97% or above at age 18 to 95% or above at age 70. Generally speaking most clinicians use 95% as the cutoff. However, as we all know, when altitude increases, atmospheric pressure decreases. So does SpO2. So "normal" at 8,000 ft. MSL is around 92% and will drop further with increasing altitude. More to the point, at what SpO2 do you start getting goofy, have vision or mental impairment-signs of hypoxia? Again this depends on the individual, but I would definitely start looking at increasing supplemental oxygen at a SpO2 of 90% or below. If you get that low while on O2 check your oxygen delivery system. A kinked delivery tube or a loose connection might be the cause of your problem.

If that's not the problem, try increasing the flow rate of your delivery system. Also consider changing your delivery system.

In a non-pressurized cabin a nasal cannula works pretty well up to FL 180. Above that you'll need a full face mask. Don't have one? Ask ATC for lower!

As you might imagine, there are a number of things that can screw up the readings from a pulse oximeter. We mentioned nail polish. Likewise darker skin leads to overestimation of oxygen saturation. A big "gotcha" is carbon monoxide. It displaces oxygen on hemoglobin but still keeps the blood cells looking rosy red. Thus your oximeter may show a "normal" SpO2 when you are about to keel over from CO poisoning from your faulty heater.

Also, cold fingers will have less blood flowing through them and lead to an erroneously low figure or even no reading at all. Bright light in the cockpit can fool the oximeter into thinking the SpO2 is lower than it is.

So realize the limitations of the pulse oximeter. Use it properly and you will gain a powerful tool to assure safe flight at altitude. Fly wisely. See you next month!

 

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