Aeromedical Forum: February 2015
Pulse oximeters: How blue are you?
Back in last June, we talked about new training options for learning about hypoxia and your own specific response to low blood oxygen. As an alternative to an extended cross-country flight to an altitude chamber, the FAA has introduced the 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. Responding to the article, one of our faithful readers pointed out that it’s great to know how it feels to be on the verge of conking out while cruising along at FL 230. But shouldn’t we be a little more proactive and monitor our blood oxygen levels at altitude before hypoxia sets in?
The answer is, of course yes. And 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 you 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.
I’m not sure how that will affect your rudder work or headset seal however. Probably 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 a 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 percent at age 18 to 95 percent at age 70. Generally speaking most clinicians use 95 percent 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 percent and will drop further with increasing altitude. More to the point, at what SpO2 do you start getting goofy, have vision or mental impairment? When do you have to start worrying about your oxygen supply?
Again this depends on the individual but I would definitely start looking at increased supplemental oxygen at a SpO2 of 90 percent 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 not, 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 cell 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.
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!
As always, comments, questions and suggestions are welcome: firstname.lastname@example.org.