Does Your Pulse Oximeter Mean What it Says? (Does Your Spouse?)

By Dr. Daniel L Johnson

Originally Published in Soaring Rx, Jun 2012 (Section titles added by Mountain High)

How Useful is a Pulse Oximeter?

FAR 91.211 requires supplemental oxygen after 30 minutes above 12,500 feet MSL and continuously above 14,000 feet and for pax above 15,000 feet MSL. A pulse oximeter can show you why.

Finger pulse oximeters are mainly useful to reassure you that blood oxygenation is OK (90+%), or that you should use oxygen (85% or below). However, oximeters often report results which are less than accurate. The fact that they can have readings that are often inaccurate or misleading does not mean that they are useless but that if you get a reading that seems faulty you should take heed of its warning and then troubleshoot.

Felipe’s Flight: A Sample Scenario

Felipe enjoys flying high –  to look down at mountains is always inspiring, so he’s happy to have today’s 15,000-ft-high cloud bases. And he’s always wondered whether those FAR oxygen requirements are realistic, and how much oxygen he should really use at any particular altitude. Does his automatic Mountain High system really give him what he needs?  If he keeps oxygen flow down to a minimum, then the tank will last longer, and he won’t have to cut short a flight from running out. But what’s the minimum?

Last week he bought a pulse oximeter, and brought it with him today. He plans to answer these burning questions. It’s a bit cool, so he tucks his gloves in the side pocket where he can get at them after he’s done checking out the oximeter. Field elevation is 5,000 msl –  while he waits for the tow plane, he puts on the oximeter: it flickers 94, 93, 95%. He feels fine. He runs through his checklist, he gets hooked up. The tow plane tugs hard, and he’s off.

Right away he realizes that keeping the oximeter on his finger is going to be hard, because he has to use his hands to fly, and it seems to dislodge a little too readily. Maybe he should have taped it in place. And the numbers are always pointed away from him when he’s got a grip on any control. When he lets go to read it, he realizes that the numbers are a little hard to read in the direct sunlight under the canopy.

As he thermals up, he keeps glancing at the numbers on the oximeter. He remembers being told that the number should be over 90%. He’s forgotten just why that’s a special number, but obviously it’s to keep the brain functioning. In his mind his wife chuckles and says, “Will you be able to tell the difference?”

He’s a little surprised that the number starts flirting with 89% before he gets to 9000 msl! But he feels fine, and the FARs don’t require oxygen until he gets to 12,500. He will wait to turn on the oxygen –  he really wants to know what his numbers will be without it, and how much and how fast they’ll pick up when he turns the oxygen on.

At 12,500, the oximeter readings are hovering around 84%. The numbers fluctuate a little more than he expected. He feels fine. He’s actually feeling a bit euphoric.

At 14,000, the oximeter reading often drops below 80%, and the numbers are less stable than they were. He reaches for the oxygen-system switch, then pauses. “Deep breaths should give me more oxygen – maybe I’m just not breathing enough.”

He takes a series of deep breaths. Sure enough, the readings bump up, into the mid-80s. It’s like descending a couple thousand feet! He notices a little shortness of breath, turns on the oxygen, and waits to feel better.

Now he’s past 15,000 ft. Each time he inhales, the system gives a reassuring soft snort into his nostrils. The oximeter’s readings climb above 90, and he keeps breathing deeply to see just how high he can carry the reading. This is interesting.

He’s under a cloud, near cloud base now, and out of the sun, he gets chilly. Despite the oxygen and deep breathing, the numbers are mostly in the 80s%. He’s puzzled, and a little worried. The oxygen bottle and gauge are behind him, and he can’t turn far enough to read the gauge. He tries to remember the last time he filled the bottle.

After a few turns under his growing cloud, he actually begins to shiver. The dew point is less than 40 dF, and in the shade, with the vent open to keep the canopy from getting fogged, this is definitely cool. The numbers on the oximeter are now dipping into the 70s. The oxygen cannula is still giving a little puff with each inhalation, but with these low readings, something has to be going wrong with his oxygen system.

He starts feeling a little dizzy. He gets more short of breath. A metallic taste is in his mouth. The oximeter is in the mid-70s. Something is wrong, definitely!  He’s feeling a weird mix of euphoria and anxiety.

He lets doubt nourish good sense and puffs the spoilers. Experiment over! On the way down, he starts to warm up. Out beyond the cloud, into the nice warm sun, and into sink, he quickly descends toward home, where he can troubleshoot this thing on the safe, hot airport apron.

OK, What Happened to Felipe?

Every electronic measuring device will display numbers if powered up. The question is: what is the relationship between reality and the displayed number? This is the main question of the science of metrology, the science of measurement, which is really the science of measurement error.

Those of us who aren’t schooled in this are sometimes deluded into confusing precision (the number of digits after the decimal) with accuracy (the connection of the number to reality).  Three seconds’ sober reflection is enough to realize that when the bathroom scale says 183.6, this is probably closer than the carnival shill giving cheap prizes for fooling him, but not many of us weigh a package on the bathroom scale and bother to complain at the post office when that scale differs.

With home medical care, there are three devices beside the bathroom scale that have more precision than accuracy: the blood pressure cuff, the glucose monitor, and the oximeter.

The simple truth about blood glucose and oximetry is that both of these instruments get less accurate when the readings are more important – lower than normal. This is because an instrument designed to detect something has a harder time finding it when there’s less of it to detect.

It’s the same problem faced back in the days of AM radio: a weak signal is hard to separate from the electronic noise that’s all around, so there’s a lot of static with weak stations. This adds to the romance of late-night radio, but simply creates doubt about low blood sugars and oxygen levels. A low signal-to-noise ratio is the bane of meteorologists.

Were Felipe’s Readings Accurate?

First, What Was the Quality of His Oximeter?

If you search the internet, you’ll see them offered from about $25 to about $250. The over-$200 meter is a Nonin pulse oximeter, the first and the gold standard. Are you paying mostly for the name? Or do you get what you pay for in the inexpensive meters? I’ve never been able to find research comparing them. The most useful page I found by Googling site: squidoo. com  “Finger Pulse Oximeter Reviews.” (Nonin’s competitive meter is the Nonin G02 for $88 with an LCD display that reads fine in direct sunlight without reading glasses.)

This web page shows that manufacturers claim readings are+/- 2% or +/-3% in the range of 70-100% saturation. This means that when the oximeter reads precisely 87%, your actual blood oxygen saturation is probably in the range of 84-90% for the less accurate, 85-89% for the more accurate, in the best laboratory conditions, which your cockpit is not.

As an aside, the fact that oximeters are reasonably accurate only to 70% saturation indicates it’s hazardous to the “guinea pig” to go lower than that, so ethics prevent manufacturers from using volunteers to calibrate their units to a lower level.

Second, What Were the Conditions of Measurement?

He was mostly using this in bright sunlight. Sunlight can “overpower” the unit’s own spectrophotometric light source. Some models are shielded better than others.

Felipe is Hispanic. Pigmented skin yields lower saturation readings when actual values are in the 80%s. He moved his hands a lot, as we normally do in flight, and when he got cold he shivered. Simple fidgeting degrades accuracy of these units significantly, saturation numbers dropping by 5-20% (that is, with a true O2 saturation of 95%, the meter may read 75-90%.) Some professional units are specifically designed to mathematically filter out the effects of tremor. I couldn’t determine whether any fingertip units do.

A reason that tremor degrades accuracy is that these things are pulse oximeters – measurement is limited to the pulsatile light transmission, to eliminate the absorption by bone, skin, tissues, & skin pigment.

A challenge is to differentiate between the pulsation of a wobble and a heartbeat: what’s the difference mathematically? Some oximeters have more successful algorithms than others. In any case, movement can quickly drop the “measured” saturation.

This month’s jargon treat: the device is a spectrophotometer that measures absorption of light at 660 nm and 940 nm. At 660 nm deoxyHgb absorbs lOx as much light as oxyHgb; at 940 nm dem: yHgb absorbs less light than o>.yHgb. The oximeter estimates functional Hgb by comparing amounts of oxy and deo>.y Hgb; a fast Fourier transform generates Fourier spectral peaks, representing the pulse, the pulse harmonics, noise artifact, and any noise and artifact harmonics. (Jargon courtesy of Respiratory Care, January, 2002, Vol47 No. 1, p. 59)

The oximeter is measuring the oxygenation of the blood in the fingertip, but what really matters is the oxygenation of the blood flowing through the brain. The fingertip’s blood can easily be different from that of the blood flowing through the brain. The oxygenation of the finger is interesting only to the extent that it serves as a proxy for our brain. As we begin to cool, before we ever feel cold, arterial blood supply to fingers and toes is constricted. In some people, veins dilate. Arterial constriction means that there’s less pulsing blood to measure. Venous pooling, if it occurs, means that more of the blood available for measurement is desaturated.

The bottom line is that if the fingers are cool or cold, we simply can’t take the finger reading as an indication of what the brain is getting, and it’s falsely low.

It’s quite easy to get a falsely high reading. The most important cause of this is carbon monoxide. Real men don’t soar with engines running, but this matters for smokers, tow pilots, and touring motorgliders. In smokers, up to about 10% of hemoglobin may be locked to carbon monoxide, which is useless for oxygen transport, and is read by the oximeter as 10% desaturated and 90% saturated. This falsely raises oximeter saturation readings. When the tow pilot prangs the prop, perhaps the crack in the exhaust bracket is the real cause.

The Impact of Hyperventilation

Last, let’s consider the effects of hyperventilating. (See the December, 2011 edition of Soaring Rx for details.) Hyperventilation increases peripheral oxygenation while decreasing brain oxygenation.  At 14k msl, your saturation should be about 85%. If you hyperventilate, you can bring your oximeter reading up to 96%, but your brain oxygen saturation falls to about 55%, which rather decreases your ability to be an interesting, well-rounded, skilled pilot with really good judgment. Felipe was feeling hypoxia-induced euphoria when he hyperventilated to raise his oximeter reading.

Then there are the annoying symptoms of hyperventilation, most importantly shortness of breath. Ultimately, this is what drove Felipe home. A good decision, but avoidable if he’d known How Things Work. Playing around with hyperventilation, plus the several factors leading to an erroneously low pulse oximeter reading, led him to suspect that a perfectly good oxygen system had malfunctioned in some mysterious and unknowable way.

Since it’s your brain we’re most concerned with, why measure saturation in the finger at all? Purely for convenience. We pass on better sites because we can see the meter better (unless you’re one of those special mothers who can see her own ears).

The Takeaway

The bottom line is that finger pulse oximeters are mainly useful to reassure you that your oxygenation is OK (90+%), or that you should don oxygen or troubleshoot the situation (85% or below.) The fact that they are not extremely accurate does not mean that they are useless!

FAR Sec. 91.211 Supplemental Oxygen. (a) General. No person may operate a civil aircraft of US. registry –  (1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration; (2) At cabin-pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes; and (3) At cabin-pressure altitudes above 15,000 feet MSL) unless each occupant of the aircraft is provided with supplemental oxygen.

Thanks to Patrick L. McLaughlin, who at an SSA dinner taught me much about respiratory physiology. It’s not his fault I wrote this, but his conversation energized me to do the research. And thanks to Paul Kran1 for reading the first draft critically. (I read the draft and see what I mean, he reads the draft and sees what I said. This is the great thing about having readers.)