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Let’s Talk Pulse Oximetry

We have all seen them – either the sticker or clip on your finger attached to the vitals machine. The elusive pulse oximeter. There are also small portable ones that are the size of a snack-sized snickers bar.


What does a pulse oximeter do?

A pulse oximeter measures 2 different things- heart rate and oxygen saturation in the blood. The heart rate part is pretty straight forward. It picks up on the lub-dub of your heart and in turn shows the number on the screen. The oxygen saturation reading indicates the percentage of hemoglobin molecules in arterial blood which are saturated with oxygen. It works by shining a light through your finger and the sensors detect how much oxygen is in your blood based on the way the light passes through your finger. Since the oximeter detects the saturation peripherally on a finger, toe or earlobe, the result is recorded as the peripheral oxygen saturation, known as the SpO2.

Nowadays pulse oximeters are readily available to purchase both online and in most pharmacies to purchase over the counter. Back when I started my career in Respiratory Therapy many years ago I had to show my license in order to be able to purchase one without a prescription. That’s not the case anymore.

Limitations of pulse oximeters

There are limitations to pulse oximeters, the biggest one being that not all pulse oximeters are created equal. Oximeters that are less expensive than the hospital-grade ones often do not have the accuracy that those in in the hospital or that your respiratory therapist (such as myself) has in their pockets.

There are also many things that can affect the reading and give a false one, such as nail polish, poor circulation to the extremities, movement (or shakiness), and cold hand, which are all things to take into consideration when getting a reading. Also, if your pulse oximeter has a display that shows a waveform, you want to see that the waveform is uniform with the hills & valleys equal as it goes across the screen while giving a reading.

Here is a really good graphic that shows a good pulse oximeter reading vs an inaccurate one:


Understanding pulse oximeters

Oxygen saturation is not a good indication of how bad your asthma flare-up is. Asthmatics are notoriously good oxygen compensators and can have near normal or completely normal oxygen stats during the midst of a severe asthma attack. Asthma is a disease of the airways, not the little air sacs (alveoli) where the gas exchange itself takes place. So even if your airways are very constricted and tight, often enough oxygen is able to pass through to your blood and therefore show a normal or near normal oxygen saturation.

Now if you have been working really hard to breathe for a time (which can be days if you’re in the middle of a bad flare up) your body will eventually tire out as you won’t be able to compensate anymore and your oxygen levels will start to rapidly drop.

If your oxygen levels are in the normal range (textbook normal range for oxygen saturations is 92% to 100%) it definitely does not mean you aren’t suffering nor having a potentially severe asthma attack. It just means that you are still able to oxygenate your body. When an asthmatic’s oxygen levels start to trend downward is when immediate medical attention is required (if you aren’t already at the hospital or doctors office) as it can be a sign of impending respiratory failure.

Pay attention to your symptoms

I want to really stress that symptoms trump numbers. I’ll say it again: SYMPTOMS TRUMP NUMBERS. If you are having difficulty breathing, consider that before the number you see on a screen. I teach new respiratory therapists, nurses, and interns that you always treat the patient and not the numbers you see.

While pulse oximeters are great & can be a handy little tool, knowing exactly what they do and their limitations can be helpful.

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This article represents the opinions, thoughts, and experiences of the author; none of this content has been paid for by any advertiser. The team does not recommend or endorse any products or treatments discussed herein. Learn more about how we maintain editorial integrity here.


  • seaturtle08
    2 years ago

    What you describe is exactly my experience over several extremely bad flare ups. The first and third times I was intubated for asthma, my pulse ox was still in the mid-90s, but the dr’s acted based on my symptoms and rising CO2 levels in my ABGs and intubated proactively. The second time, however, I was in the ICU for 2 days prior to being intubated, fighting to breath and being told it was in my head, because my O2 sats were normal. That time, I went into respiratory failure…

    Even with that experience under my belt, I still struggle to be taken seriously in the ER at times. It seems to just depend on the nurse and RT who are treating me; sometimes it’s no issue, sometimes I’m told I’m faking it or that I’m not really in that much distress. I’m going to print this off and keep it in my bag, so I have something to show them if they try telling me that I’m fine because my sats are fine. Do you have any suggestions for other articles or research that I could have handy as well?

  • CourtneyC
    2 years ago

    Wow! I am 31 years old and have had moderate to severe asthma for nearly my whole life (diagnosed when I was 3 years old). I have been in and out of emergency rooms many times, and they always treat the pulse ox reading as if it were the gold standard in deciding how seriously to take my case. In fact, just last year, I went to an urgent care clinic when I had such trouble breathing that I couldn’t even speak without taking a gasping breathe between each word. When they took me back, the doctor looked at my pulse ox and, seeing that it was in the high 90s, immediately suggested that my asthma attack was not as serious as I thought it was. I was baffled at how I could get a normal reading even with such severe symptoms. Now I know why! Thank you for this information, I now know to advocate for myself strongly in ERs and urgent care clinics, even if my pulse ox reading is normal.

  • Leon Lebowitz, RRT moderator
    2 years ago

    Hi CourtneyC and thanks for letting us know just how well this article resonated with you. It’s always gratifying for us to hear our community members find value in our published material – practical application of the science!
    Wishing you the best,
    Leon (site moderator)

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