Breathing Basics —O2 Saturation and Oximeters

Today’s Breathing Basics review is all about Oxygen Saturation and Oximeters.

If you have lung disease of any kind, no doubt you’ve come across the words “Oxygen saturation” or “O2 Sats” , “Pulse-Oximeter”?
I use those terms on my blog all the time, but do you really know what they mean ? I suppose you could Google a definition, but I think I can probably do a better job of explaining it in a way that makes more sense to the average person, and hopefully in a way that won’t bore you to death.

O2 Saturation is simply a numerical representation ( expressed as a percentage) of how much oxygen your blood is carrying at any given moment. This is a useful number to know when someone with a respiratory disorder is ill, because it indicates how well the lungs are functioning and/or how much oxygen is available to the vital organs of the body.

Before I get into how we measure oxygen saturation, let me explain what the term “oxygen saturation” actually means. If you remember some basic human biology, then you already know that we have these little donut shaped cells in our blood called “Red” blood cells or RBC’s for short. Inside these red blood cells is an iron-rich protein called “Hemoglobin” also known as Hb. Well, when hemoglobin comes in contact with oxygen ( in your lungs) it soaks it up like a sponge. It’s actually the combination of oxygen and hemogloblin that gives blood it’s bright red color. The more “saturated” the hemoglobin is, the brighter red the blood becomes. When the Oxygen saturation of the hemoglobin is low, ( usually below 75%), the blood actually gets darker…almost black in color(such as the blood that’s carried in your veins). Each hemoglobin molecule has a certain capacity for oxygen in which it can bind with, and it’s this capacity for oxygen that we are actually measuring when we do a oxygen saturation check… ie. O2 sat. So, if I check my O2 sat and the reading is 95%, this tells me that my hemoglobin is saturated to 95% of it’s carrying capacity. So basically, the more oxygen that gets into the blood stream, the higher the saturation should be.

There are several factors that effect how much oxygen our hemoglobin can hold, but that’s beyond the scope of this discussion… at least for now. The important thing to understand, is what oxygen saturation is, and how it relates to your particular type of lung ailment. For asthmatics, knowing your O2 sat is probably less important than someone with CF or pulmonary fibrosis or emphysema. That’s because asthma is not a disease that normally effects our ability to oxygenate. Asthma is a disease of the airways, not the alveoli where gas exchange takes place. So unless your airways are totally blocked ( which is rare unless you have some sort severe anaphalatic reaction), air still manages to squeeze by the obstruction. What this means, is that you can suffer a pretty nasty asthma flare and initially have normal or near normal O2 sats. Significant desaturations usually don’t usually occur in asthmatics unless the person is extremely ill, has a secondary lung problem or has a pneumonia brewing.

Now that we know what O2 saturation is, how do we measure it? Well, that’s where the handy pulse-oximeter comes in. Hard to believe that this device has only been around for a couple of decades. When I first started working as an RT back in the 1970’s we didn’t have such a luxury. Back then, to measure someones O2 saturation, we had to actually take a blood sample from an artery.. i.e. an ABG ( arterial blood gas), then run it through a special machine that would measure the amount of oxygen dissolved in the plasma, and then hand calculate the results with a slide rule. Talk about a long and drawn out process… not to mention painful.

A pulse ox works like this; when you place the probe on your finger, the top of the probe contains a transmitter that emits a beam of light that penetrates through the finger. That beam of light is then absorbed by the hemoglobin in the blood cells as they pass through the the tiny blood vessels in your finger. The receiver at the bottom of the probe then measures the difference in wavelength and calculates the O2 Saturation,which is then displayed on the meter. Because the oximeter needs to be able to differentiate the light absorbed by arterial blood from other interference, it looks for your pulse. Arterial blood is pumped by the heart, so it pulsates and fades with each heartbeat. The oximeter subtracts the trough from peak levels, and the difference is the light absorbed only by the arterial blood. This is why it’s important that you verify that the pulse reading on the oximeter is correct. If your skin to too cold or your blood pressure to low, then it becomes very difficult for the oximeter to sense your pulse. Pretty cool huh?
PCO2( carbon dioxide in your blood) is actually more of a concern than O2 saturation when you have an obstructive disease like asthma. We’ll talk about PCO2 the next time around.

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52 thoughts on “Breathing Basics —O2 Saturation and Oximeters

  1. So you're saying that if you have asthma, and have an asthma attack, it's normal (or at least not abnormal) for your sats to be totally normal or very near normal? All my doctors seem to be trying to tell me that if my O2 level is at 99 or 100%, that there's absolutely no way I can have asthma, no matter how tight and short of breath I am. (ok, I guess I should add that for them 99-100% O2 sat + no wheeze = no asthma, but the no wheezing is another topic I guess)

  2. That's correct. It's not uncommon to have a severe asthma flare and still have normal sats or just slightly abnormal sats, though I admit, 99-100% is rare , even if you're not flaring. I could see how a Dr would be a little suspicious.

    What often happens is that some people confuse a panic attack or hyperventilation syndrome( which can make your sats really high) with an asthma attack. A skilled physician or RT can usually tell the difference, but not always.

    The severity of an asthma attack should only be based on the patients history, their physical appearance ( accessory muscle use , etc.) PFTs ( Pfs and FEV1), breath sounds and possibly ABGs. They should never rule out asthma just because your sats are normal or you're not wheezing. The fact is, most asthmatics don't desaturate until they're on the brink of respiratory failure and some of the sickest ones don't wheeze at all. If your sats are under 92% during a flare, you are in bad shape.

    If someone is telling you that you don't have asthma just because your sats are normal, you need to go somewhere else. That just goes to show how under educated some physicians are when it comes to treating acute asthma patients(though Ive found this to be less common in emergency rooms)

      1. Depends what their normal sat is. I would expect to see a sat of 93-94 in someone who is having a pretty rough time and would want to keep a close eye on them, but in no way would I consider a sat of 93-94 to be critical. Again, sats are only a small part of the overall picture with a sick asthmatic. In some of the hospitals Ive worked at, you could be sick as a dog, but if your O2 sat was above 90%, they would not admit you too the hospital. Thankfully , that protocol is rarely used anymore.

        1. my son sits low but he gets rushed into hospital,i now type this in hospital his sats go from 99% and when sleeping drop down below 90% when o2 is off or he bumps it off,i wondered ,what was safe when he was sleeping,is anything above 94% ok
          3year old boy with c,l,d…Chronic Lung Disease

  3. The same thing happened to me! Doctor didnt beleive I have asthma because my sats were between 97-98%, and then what happened was that he undertreaded me, and then a week later I had a severe attack, and ended up on Bipap. I dont understand doctors, one minute they tell me im in bad shape , the next they say im fine. what gives?!

    1. Hi i have bronchiectasis copd grade2 and a 50% lung reduction right side.
      my sats are usually 96% average,at the monent sat readings over the last three days 94%,and i feel rough rough rough

  4. Very cool. I was trying to explain to my little one how the machine could check her oxygen…but she's 7…she just told me she didn't get it, and as long as her machine worked, it was fine. Her doctors are often baffled by her…that she'll desat spontaneously, with no apparent cause…but will have a total white out X-Ray, and have sats of 99 or 100. (we play the O2 Sat game at our house…we all take our sats, the winner gets a little treat.)

    1. O2 sats readings can be wildly erroneous if the probe is not detecting the persons pulse accurately. This happens all the time in children, as their heart rates are much faster and are much harder to pick up on the oximeter. I would never rely solely on a pulse oximeter.

      1. Hi Stephen:
        My sats never drop during an asthma flare-up…and yes, some docs seem to think you're not too bad if you don't have lower sats. Yet I can be very bad shape during these acute episodes. The general public, too, seems to think an asthmatic must have low O2 sats during an attack. So I appreciate your explanation. I do wheeze a lot, however, I have been so tight at times that there's little or no wheezing. Ironically, I do desaturate to the 80s, but from sleep apnea, not asthma! (I've now used a C-PAP for several years).

        1. Yea, thats a shared frustration. Asthma IS NOT a disease that effects ones ability to oxygenate unless it gets really severe. The symptoms of asthma are caused my narrowing or constricted airways, nothing else. Granted, if your airways clamp completely off or stay severely constricted for a long enough time, eventually your o2 sat will drop, but the biggest concern is that your CO2 ( waste product gas) will increase.

      2. I don’t rely on it solely at all. We use it when she turns blue…so that her pulmo has a number on record, or if she’s actively flaring. I’m actually going to return the one they are renting us back to the respiratory equipment co, because they didn’t give me a pedi probe…so I would have to do several checks to get an O2 reading along with a pulse that was accurate…if the pulse isn’t accurate, I don’t even look at the O2 reading. I ended up purchasing a pediatric monitor that is FAR more accurate…and much more portable.

        I hope that you get to feeling better soon, feeling that elephant on your chest is such a terrible feeling.

  5. I always wondered about pulse oximeters. What happens if you are wearing nail polish – wouldn't that affect the color spectrum analysis? Plus, I have always wondered what happens if your pulse rate goes really high, how does it manage. Sounds like, from your explanation, that it does make a difference. Thanks for a very information posting.

    1. You\’re correct about fingernail polish, it can make it difficult to obtain an accurate reading.
      The way to tell if a pulse ox reading is correct, is to check the pulse manually and see if it jives with the pulse reading on the oximeter. Most meters will not display a sat reading unless there\’s a good steady pulse.

  6. Apparently my sats do drop – my Doc just Rx'ed O2, prn. O.o I feel very weird staring at the machine in my house. I am a weird asthmatic, Stephen. Maybe I really do need to do SARP.

  7. I'm the asthmatic with the low SATS! But you knew that!
    On air I run at around the high 80s. On 2L O2, about 94/5 % I use O2 overnight on 3.5L and wake up around the 96% mark. In the day I use my O2 PRN but 24/7 when my asthma is exacerbating-so i'm pretty much on it all the time now, but because I've had a bad lung infection, I'm only sitting around the 92/3% mark.

    Great topic and great comments though! xx

  8. I should also point out that O2 saturation can dip from time to time without any problems.Most people desaturate ( a couple points) when they sleep. It's only becomes a concern when youre having breathing problems at the same time. Here in the USA , a saturation of 92% or above is considered more or less OK. In fact, most insurance companies, including medicare, will not pay for home oxygen unless your room air saturation is 88% or lower.

  9. 99-100% is a little high, even in a non-asthmatic, 92-95% is more realistic. But yes, sats can be normal during an exacerbation. Asthma is not a disease of the alveoli ( where oxygen exchange takes place) , it\’s a disease of the airways. O2 saturations are usually the last thing to drop during a severe flare. A sat dropping below 90% in an asthmatic patient would be a true emergency.

  10. hi ive been too A and E this week due to asthma. i had a terrible experience. i couldnt breathe despite my inhalers and called an ambulance. the paramedics were judgemental saying they thought it was a panic attack because my oxygen sats levels were 100 percent. my peak flow was down. however they put me as a low priority and it was awful. in the end the doctor said she didnt know if it was anxiety, asthma or a chest infection and put me on antibiotics. im so angry i have suffered with asthma my whole life and because of my sats levels they assumed it was panic. i am going to complain

    1. So sorry to hear that you were treated so poorly. This happens all the time and I hope you complain. It is rare that an O2 sat would be a 100% during a severe flare, but it is possible. O2 sat is usually the last thing to drop during an attack. It\’s also possible to have asthma and anxiety at the same time, One can cause the other. Do you have a resp consultant? You should have them write a letter on your behalf. You might consider joining my Facebook asthma group .https://www.facebook.com/groups/severeasthma/ Hope you feel better soon.

    2. Hi all

      I had EXACTLY the same problem. I was admitted to hospital and had 98% SAT’s and the Dr’s were a nasty bunch, basically told me to do one and that it was a panic attack.

      Cue me feeling like dirt wondering why I was suddenly starting to have panic attacks with no ihaler and time off work because I could do anything without being short of breath.

      I am now suffering again with my Asthma, really bad for me this time 3 weeks in and it’s hurting to breathe in – I also have the lovely chipmunk voice!

      My white blood cell count is high, so I’m poorly, but they dont know what with and yet every time I have that sat reading, even when they do the reading before the nebuliser I am at min 96% and after nebs 98 – 100%.

      It’s not right that so many people have to suffer and feel as though it’s kinda in their head. I had to have am appointment to be signed back to work after I was hospitalised and it was my Dr that told me that in fact I had a chest infection and had had a nasty asthma attack. I got the meds I needed a week after I was admitted.

      So all I can say to those out there that think a good SATS reading means we are hypercondriacs is GRRRRRRRRRRRRRRRRR

  11. I just wanted to add, I used to be 99-100% on room air when healthy and would rarely desat. In 2006 I caught a nasty bug in Paris on vacation and my O2 started slipping. Since 2010’s severe episode, my sats are much more sensitive and a minor asthma flare up that doesn’t require anything other than an albuterol inhaler dose might push me down to 97% (one happened to coincide with a doc visit which is how I know).

    My guess is the wear and tear of the asthma on my lungs is showing as I age.
    M

  12. I wouldn’t get too fixated at on O2 sats. Normal values range from 96-100% while breathing room air and these numbers also decrease as we get older.

  13. What happens if the paramedics determine that your O2 Saturation is at 90% due to an asthma attack? Just how serious does asthma become after that point?

    I recently helped a victim who developed an asthma attack and it was almost frightning to see the person struggle for air even after multiple uses of the inhaler and drinking liquid benedryl straight up. I’ve got to admit I was afraid the victim was going to go into cardiac arrest. But then, firefighters on a fire truck came and helped; followed by the EMS.

  14. So, should I be concerned if my o2 sats were just recorded at 89-90% at the clinic, while feeling relatively no S.O.B? I have asthma, and have a virus right now (which flares my asthma), and had a relatively bad coughing fit this morning before the appointment. The appt. was for something else, but they checked my sats and the nurse didn’t like the reading, tried both hands and it was the same. The doctor didn’t even mention it though.

    I recall when I was first diagnosed with asthmatic bronchitis, I had pneumonia and didn’t know it, made a regular clinic appointment for what I assumed was just my normal bronchitis, and my 02 was like 78. I walked into the room and the nurse took one look at me, left the room to get the pulse ox, came back and measured it. Got that level, and went to get the oxygen tank. I was actually terrified. I had noticed I was out of breath a lot, and that I needed to sit down after walking across the room or up the stairs, etc. But I didn’t think it was that bad! They also nebbed me that visit.

    Since then I’ve had a methacholine challenge done and it was a pretty strong positive for asthma. But I still only notice it when I’m sick.

    I wonder if it’s time for me to move on from the rescue inhaler to a neb at home for when I’m sick? I’ve been hitting the inhaler a lot because it allows me to get a deep enough breath to be able to cough out what I feel that I need to cough out. So I guess really I *am* somewhat short of breath.

    In short, I’m just a bit concerned that my reading was that low today, but I guess if I had an attack this morning it’d be normal. I don’t know.

    1. I wouldn’t be too worried about your sat. Hopefully it will bounce back when you get over your virus.

      Asthmatics rarely desaturate from an asthma flare itself, unless they’re on the verge of respiratory failure or a resp arrest.

      If your sats readings continue to be low you should probably have a complete work up done.

  15. I have been on home oxygen for 4 months following pulmanary embulisms. The doctors expect me to get off it eventually…but don’t know when. The process was slowed down by fluid around my lungs. I am 72. I still work full time…(except for the last 4 months). My job is very physically demanding (for an over weight old lady)… lots of walking….short distances. I also have asthma and COPD…I don’t need oxygen for either. I want to go back to work. I just don’t. Seem to be able to get my oxygen to stay high enough to do that. I sat in my chair yesterday with no oxygen for about 6 hours…but couldn’t walk in the hallway because it would drop too low. This morning I tried to walk and the same thing. I can sit here with no oxygen and sats are between 96% and 98%. Put my canula on with two litters of. Oxygen and Go into the hallway to do my walking and it goes down to 94% to 92% and. sometimes 90%.. I have been told not to let it go below 92. I keep wondering if there is something I can do. Doctors say its just time. Any ideas?

    1. Hello, It’s very difficult for me to tell you what’s going on with your oxygenation based on what little I know about you. I can tell you though, that it’s not uncommon for people with COPD and asthma to desaturate when they exert themselves, especially if you’re still recovering from an exacerbation, or as you said, fluid around your lungs. While exercise is definitely good,you should probably be using your O2 when you’re doing it. I wish I could tell you that your sats will return to normal again, but I just dont know.

      I wish you the best!

  16. i posted wrong,will re post to fix,

    December 1, 2012 at 4:47 pm
    my son sits low but he gets rushed into hospital when sick,i now type this in hospital his sats go from 99% and when sleeping drop down below 90% when o2 is off or he bumps it off,i wondered ,what was safe when he was sleeping,is anything above 94% ok
    3year old boy with c,l,dโ€ฆChronic Lung Disease
    his o2 is at 1 ltr ,trying to get him off it,
    i need to know if when anyone sleeping with Chronic Lung Disease

    does it drop and what is safe o2 readings,what is not .

    1. Im afraid I dont have enough information to provide you with a good answer. In general though, a saturation above 92 % is considered acceptable. Most people desaturate a little bit when they sleep.

  17. Stephen:

    I recently came down with a cold and then couldn’t breathe right so I went to the doc and my sat was 89% she gave me a nebulizer treatment and it went to 97. Is this considered severe asthma? She said there was no sound in my chest before treatment. Why didn’t I know that I was that bad?

    1. Without knowing more of your medical history I couldn’t tell you if you have severe asthma, but from the symptoms you’re describing it doesn’t sound so. The fact that your sat returned to normal so quickly is a good thing ๐Ÿ™‚

  18. Thanks … this makes a lot more sense now ๐Ÿ™‚ wasn’t always sure what the cut off was. I knew the last time i was in and I was only sitting at 90-1 that wasn’t good … but was a little to busy to ask questions at that time ๐Ÿ™‚

    Kelly ๐Ÿ™‚

    1. Yeah, Sat readings are a poor indicator for flare severity, unless your numbers are super low. Unfortunately, a lot of ER folks don’t know that. The first and foremost observation should be, is the patient struggling to breath, are they moving air, are they displaying signs of impending respiratory failure? Once that’s established, then other vital signs, including sat, and ABGs if needed…In that order. Ive treated a lot of acute asthmatics in the ER and most them sat in the mid to low 90’s. The ones that desat below 90 are usually the ones who end up intubated or on bipap or ended up arresting.

  19. My grandson diagnosed with pneumonia. No chest x-ray. Saturating 98% in office. Was put on Z-Pak. Had very bad night. Still having trouble breathing. Saturations today running 95 to 93. I know it takes 48 hrs for Z-Pak to kick in. Should I call doctor back or take to the ER or just give it another day?

  20. Thank you for this clear and concise description on this. I have suffered from CHF for over 10 years now and never really understood what pulse ox was. I recently was in the hospital and when they drew for blood gases my O2 sat was 47%. This was not mentioned to me at all on discharge. Is this something I should be concerned with? Maybe not knowing is sometimes a good thing???

  21. I have copd and blood clots in my lung and I am on oxygen not all the time at night most of the time.My oxygen level is about 95 sometimes 94 I am on predisone 10mg every day I still fight to breath .

  22. Hi , I had a Pulse oximetry on my doctor visit and read 98 % ! Then I read that if u wear nail polish (pink color ) is could not be accurate ! She had no problem getting an accurate reading ! So is a reason to doubt the reading ? Thank you !

  23. Hi,actually i was trying to figure out the duration it will take for a normal adult for the oxygen to saturate?.generally our breathing rate will be 12-20 breaths/min so for per second its around 0.333 breaths/sec.so for oxygenation and deoxygenation its half of that na?its almost for 1 breath it will take 3 secs…so how could it be possible for pulse oximeter to show spo2 level always grater than 95% ..it will take almost 1.5sec for complete saturation of blood

    1. Pulse oximeters dont measure oxygen saturation directly. They have an emitter and receiver that measures how much light is absorbed in capillaries , above and below the finger where the unit is placed, and then an algorithmic is applied which estimated saturation. The sat is calculated over time based on the frequency and strength of the pulse reading… hence pulse oximeter.

  24. Brittle asthmatic with spo level of 93-94% crackle in chest taking doxcycycline and 40mg of prednisolone not overly concerned as yet but what stage would be cause for concern (been asthmatic for many years but just recently used spo meter and just curious when infection free normally get 99% give or take 1

  25. Hi I sufferry with C.o.p.d I’ve bought a oxymeter and don’t understand the reading the top number was78 and the bottom was 95 is this ok

  26. I went in with asthma attack today and have an upper respitory infection. My pulse of was 88 upon arrival and now are around 92 after breathing treatments and steroids

  27. Sat at rest room atmosphere 97%, with activity lowers to 88%. I do not feetll out of brearh, but lightheaded. Diagnosed few days ago with Brinchiolitis. Will my O2 Sat get to normal level so that I can resume work and ADLS? ?? Or will I have t o use a O2 for ever ???

  28. There are many types of bronchiolitis, some worse than others. Without knowing more about your overall condition I cant really answer that. But in general, you should only need supplemental oxygen if your sats are consistently under 90% . As far as it effecting your ability to work, it probably depends on the type of work.

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