How a Pulse Oximeter Works

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Dear COPD Coach, My husband has been diagnosed with COPD but has not yet had pulmonary function tests to determine the type/severity. We purchased a pulse oximeter to monitor his 02 levels (he was sent home from the hospital on 4 liters but is now reduced to 2). My question is, do you take the first reading that comes up when the pulse oximeter is put on or do you wait 15 seconds and use that reading? Also, I would like to say this is one of the most informative websites I have found on COPD and I certainly appreciate the support it provides.

—Curious about Pulmonary Function Testing

Dear Curious, A pulse oximeter can be a very useful tool for a COPD patient, providing you use it correctly and realize it has limitations.

Oximeter The pulse oximeter measures two distinct things: The first number that comes up is most often the pulse rate. Usually this number is marked by a small heart. The second number that comes up is the level of oxygen in the blood. Both numbers are needed to assess your present levels.

At this point, it is important to discuss the limitations of the device. The pulse oximeter only gives you a limited amount of information, and is in reality just a snapshot of your functioning at that particular time. For example, it does not tell you the concentration of CO2 (Carbon Dioxide, the waste product of your breathing and something that is not healthy in high levels) in your blood stream. This means that you could be retaining a large amount of CO2, which could be limiting the amount of usable oxygen in your blood.

In other words, the pulse oximeter is not a substitute for more extensive tests that give you and your doctor a better idea of your exact pulmonary function.

The second limitation is that all pulse oximeters “are not equal.” Less expensive units often do not have the accuracy of other more expensive units. It is never a bad idea to take your unit with you to your doctor’s appointment and compare the readings with his unit. I was at a trade show one time and a company there was selling very inexpensive pulse oximeters. I tried 6 different units and never once was able to get a reading! In order to get an accurate oxygen saturation reading, the device must sense every beat of a fairly strong pulse – if your pulse is irregular, weak or not being sensed for some reason, the pulse oximeter may show an oxygen reading, but it is not necessarily accurate. It is also important to keep in mind that many things can affect the reading, such as nail polish, cold or heat, or even which finger you are using it on. If your hands are cold, warm them before taking a reading. If you get a suspicious reading, take it again, or maybe use another finger.

Now for the question of understanding the readings. Ideally, a reading should show a relatively normal heart rate (between 60-100 beats per minute) and an oxygen reading in the middle to high nineties. If the heart rate is higher than usual, but the oxygen number is normal, it means that the heart is working harder to keep your saturations high which could indicate a problem if it remains that way on subsequent readings. If the heart rate is high and the oxygen reading is low, it could also indicate a problem. The important thing is to not panic! Wait a few minutes, and take another reading. If the abnormal readings continue, and you are feeling out of breath, contact your doctor.

To get more information on pulse oximeters, check out our educational materials, which has a lot of good information that can really help you! Also, please call our COPD Information Line at 866-316-COPD (2673). A trained patient or caregiver associate can answer any questions you might have!

Thanks for writing, and be sure to stay in touch!

-The COPD Coach

Ask the Expert is aimed at providing information for individuals with COPD to take to your doctor, and is not in any way intended to be medical advice. If you would like to submit a question to the Coaches Corner email us at We would love to hear your questions and comments. You can address your emails to any of the following: COPD Coach, Caregiver Coach, COPD Doctor or COPD RT.


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  • Hi. Well, I"m not technical but I have been using a pulse oximeter for a decade now. When I am exercising or out of breath if my sats fall below 90 I stop what I'm doing and start pursed lip breathing. I continue this until I reach 90 again. I have had very severe COPD for a decade now, since I was 55.
    I purchased mine for forty dollars from a pharmacy. When I go to the doctor it is alway very close in numbers.
    Best advice ever. Keep exercising and build your muscles to help you breathe. I know, you say I'm out of breath. So was I, but the more I work on it, the better it becomes.
    Good luck and keep on keeping on.
    • Use PLB all the time and you'll find you may not go below 90 at all.
    • Hi Jean, I have the same problem,my oxygen drops if I am exercising or running around like a crazy woman,which is always. Question is when it falls to 89 or 88,do i need to put on oxygen or just do pursured breathing. It always comes back up in a couple of seconds/ minutes.Thank you
  • In answer to the question originally posted above: "My question is, do you take the first reading that comes up when the pulse oximeter is put on or do you wait 15 seconds and use that reading?" , yes you should wait for a brief time for it to settle ... and yes 15 seconds sounds reasonable if rested. If your husband has been moving around just prior to taking the reading, there can be quite a time before you see his 02 level come back up on resting. Remember your pulse ox reading is always playing catch up - eg. if I jump up and down for 10 seconds and immediately take a reading it can look like my usual rested reading for 30 seconds or so before it dips for a few seconds before climbing back up again - hope that makes sense ;) From your question though I would say you likely already have a good idea of how to use it :)
  • A detailed look at how a pulse oximeter works.

  • Thanks to the COPD coach for a very informative explanation. I like the way you explained the heart rate verses the saturation. Sometimes my heart rate is high and my oxygen is in the 90's but I am having hard time breathing. This helped me understand better, really like this board.

  • I love you question and the information provided. I tried buying an OTC pulse ox meter & had to return it due to poor accuracy. Do the research & compare with your doctor's for accuracy. Better to spend a little more on a better unit that's more accurate.
  • I am not promoting a certain very popular shopping website that starts with A and often proceeds the word "jungle," or "river," but I found that a good place to shop for a pulse oximeter. Not only do they have a variety to choose from, but you can also see which ones have been highly rated by other purchasers.

    I got one from there for under $30 and it seems to provide results very similar to the doctor's office.
  • I got one on e-__y very resonable
  • I do think waiting 15 seconds sounds like a good average rule of thumb. Typically I only check when exercising. Soon as I see the same one or two numbers repeating I go with that result. I look for the consistency. For example when I first put in on, it might ready 97 or 98... I wish:) Within a couple of seconds I might see it alternating between 93 and 94 or 91 and 90. As I breathe (big plb breathing) I will see the number change, I typically see two repeating numbers rather than one steady number. I wait for a consistent result and go with that.

    Also make sure the lanyard is not between your finger and the light, I've done that more than once and wondered why my O2 was so incredibly low.
  • Thanx for the discussion. I took particular not of the "pulse" #'s I frequently see a pulse of 120 w/O2 either in the mid 90's or near 88. But like you said, take another reading and/or wait for the pulse to settle down. I also find that PLB helps normalize both #'s.

  • My resting pulse/ox never gets above 96. With minimal movement or exertion, like walking around the house, fixing a meal, or washing dishes, it drops to around 87. Walking to the car from the house, it drops to around 84. I take a breathing treatment with my Nebulizer before I leave the house when going to appointments or to the store for groceries, etc.
    • ...and I meant to say -- this is on 4 liters of continuous oxygen.
  • Pleit, I think if you use pursed lip breathing from the beginning of your exercise or whatever you're doing, you'll find that your O2 sats won't fall as quickly and you won't have to stop so much. If you know it's going to fall, start the activity with it on, then you won't have to worry. You won't do yourself any harm, and you won't have to stop what you're doing either.
    • I should have said, start the activity with your O2 on, and then you won't have to start, stop, etc.

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