Pulse Oximetry and O2 Saturation


This may seem simple… but it is my constant battle: valuable but so difficult.


This is one simple vitals measurement with one tiny tool. There can’t be that much to it…

Let’s go for a ride down the river.

The first day at your new job, there are lots of jobs for your team to do, but there just aren’t enough people showing up for the jobs. If these people don’t show up, you will still all get in your boats and go down the river to work, but there just won’t be enough people to get all the jobs done. You will work your butt off and feel exhausted, but the work just won’t get done.

The second day, all your team members finally showed up, but the boat company gave you fewer boats based on yesterday’s attendance. So now you have all the people to do the work, but you can’t get everyone down the river to work to do the jobs. Again, you will work so hard, be exhausted, and the jobs will not get done.

The third day, there is a storm. There are trees fallen across the river. You have your boats and people this time, but it is really difficult to get down the river to work because there are so many tress in the way and the path is very narrow. So the people who get there will work really hard, feel exhausted, and the jobs still won’t get done.

The fourth day, a new company moves in and builds a dam upstream. They will only let so much water flow through the dam and its just not enough to float your boats down the river, so you have to walk to work. You’ll get there, slowly, but you’ll lose a lot of time and you’ll be exhausted and the work won’t get done.

This is how our patients feel in different states of oxygen deficiency.

People = Oxygen Molecules
Boats = Hemoglobin
Trees = DVTs/atherosclerotic plaques/arterial calcification
The River = Cardiac Output

There is a whole set of considerations to using pulse oximetry. SpO2 only measures the saturation of oxygen in hemoglobin. But what if there isn’t enough hemoglobin? You can have them 100% saturated but still have a very breathless patient who need critical intervention (a transfusion!). And what if there isn’t enough cardiac output, literally not enough blood leaving the heart (aka Ejection Fraction) to go to the body? Now your patient doesn’t even have enough breath to walk to the bathroom and your oximeter is reading 100%. What if you have enough output, but the vessels are obstructed due to PVD or PAD or a DVT? Pulse oximeter still reads 100%, if you can get a reading at all.

The story of the people and their boats is how I explain each of these deficient states to my patients depending on what state they are in.


Ho-Ly-Crap…

Yep. That’s how I feel about pulse oximetry. I NEED to know when my patient is desaturating. I need to know how much. I also need to know if they are too saturated! And, in the home environment, or in the gym at my SNF, I have pulse oximetry to give me those answers. The unfortunate thing about pulse oximetry is how unreliable it can be and how difficult it can be to use. But, like I said, I NEED it!

If you are regularly using a fingertip pulse oximeter, there are a more than a few things you need to know:

  • You need to simultaneously palpate or auscultate the patient’s heart rate/pulse rate during use. If the heart rate doesn’t at least somewhat match from manual palpation to the device reading, you cannot trust the oximetry reading.
    • These devices require a regular and relatively strong heart rate to provide a proper reading.
    • Most oximeters have a heart rhythm bar that goes up and down with each beat. If that bar is not going up and down in a regular fashion, the oximetry number is not correct.
  • If your patient has any cardiovascular comorbidities, odds of a pulse oximeter working are reduced. You may be thinking, “Well shoot, Doctor B, that’s who I need to be using this with.” And you’d be right.
    • Does your patient have A-Fib? Definitely won’t work. (see above)
    • Do they have PVD? Almost definitely won’t work. (see above)
    • Do they have peripheral neuropathy? Probably won’t work.
    • Do they have Raynaud’s Phenomenon? Probably won’t work.
    • Do they have essential tremor? Definitely won’t work.
  • If your patient has no fingers or toes, I don’t even know what else to tell you. There are different types of pulse oximeters that can be taped to different body parts like ears, feet, hands, arms, wherever you can get good contact. They are regularly used in NICU settings. But, I don’t carry one of those in my bag and my SNF doesn’t have one. So…
    • Unfortunately, these other devices come with their own flaws. Performing isometric contractions of the muscles where they are located can drain the local oxygen supply and show falsely depleted oxygen saturation. (Death gripping a walker handle will impair fingertip oximeter readings.)
    • They are also more prone to interference from body movement if you have to perform any other type of activity with the body part it is attached to.

So, what the heck do I do with this thing?

Correlate clinically.

Your pulse oximeter shows desaturation to 84% but your patient is happily talking and exercising with you without shortness of breath and they have no pulmonary pathology. Obviously a false reading.

However, your patient is sweating, panting, and slightly dusky around their mouth while walking with you. You know they recently had COVID-19. Your pulse oximeter says 96%. Obviously a false reading.

If things aren’t making sense, pick a different finger and make sure that finger is warm before putting the oximeter on it. If that doesn’t work, change your batteries.

That’s Troubleshooting 101, 102, 103, 401, and 700. Make sense?

Hint = SpO2 is oxygen saturation measured with an oximeter. PaO2 is the partial pressure of oxygen in the blood measured with an ABG.

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So what’s normal?

Even debilitated patients who probably have a lower ejection fraction shouldn’t significantly desaturate with normal activity unless they have severe comorbidities, like end-stage COPD. If they are, they need supplemental oxygen to perform these tasks. A normal decrease in saturation with activity is 1-2% even if someone has been immobilized. SpO2 should be around 97-99% in healthy individuals.

However, if your patient desaturates more than 4%, or desaturates to less than 90% SpO2, with activity, this is a problem (unless they are a CO2 Retainer). If this happens, you should hopefully see some compensations in heart rate or blood pressure, or both, and it should recover with rest.


What if SpO2 doesn’t recover with rest?

If your patient’s SpO2 is dropping with basic household mobility, your patient is struggling to function, and their vitals show hypoxia, you either need to get them supplemental oxygen, or you need to titrate the supplemental oxygen they already have.

WHOA!!! Hold up, wait a minute!

I know, I know. You are thinking, “Oxygen is a drug, I can’t titrate it without a doctor’s order!” Yes, you are absolutely right. But are you going to let your patient go in to acute respiratory distress in the meantime? I think not.

While you are helping your patient recover from their desaturation with rest, breathing, and increased O2, get the doc (or their nurse) on the phone, let them know what you found and what you did. Ask them for some “titrate to” orders to help you out so you don’t have this problem anymore. I’m sure they will be happy to oblige you. Team work makes the dream work.

So what about COVID-19?


In the rush to open critical beds and move people out of the hospital as quickly as possible, rehab and nursing providers may not get the full opportunity to assess a patient’s response to activity, specifically their oxygen response. When you start seeing these patients in the post-acute settings, like home care, inpatient rehab, sub-acute rehab, or skilled nursing settings, you may find they weren’t quite ready to be off supplemental oxygen all together just yet. It is very possible your patients still may need supplemental oxygen with activity, even if they are well saturated at rest.

How do you know when a patient is orthostatic or just deconditioned as opposed to needing supplemental oxygen to prevent hypoxia with activity? The symptoms can be similar: shortness of breath, increased respiratory rate, fatigue, lightheadedness. So how do you know? Here’s how:

Take baseline vitals at rest (like really rest) in supine (HR/PR, BP, O2, RR)
Sit up and take another set of vitals
Perform an ADL or UE activity from seated and take another set of vitals
Stand up and take another set of vitals
Walk and take another set of vitals

If blood pressure drops, heart rate rises, and oxygen stays the same on the BOLD lines, you have orthostasis.
If blood pressure and heart rate elevate, and oxygen drops on the ITALICIZED lines, you have activity-induced hypoxia.

With orthostasis, blood pressure changes happen in response to changes in position with gravity. Blood pressure drops and the heart attempts to compensate by rising, but O2 saturation should stay the same, even in a deconditioned person. We will talk more details on this in another post.

With activity-induced hypoxia, blood pressure elevates and heart rate elevates in response to low oxygen levels (the body is trying to get more oxygen-rich blood out to the tissues to complete whatever work needs to be done), but the diffusion of oxygen across the alveolar membrane isn’t happening at a rate that can match the demand (problem with V/Q matching) due to the alveolar destruction caused by COVID-19 (or any other pulmonary pathology). This patient needs some supplemental oxygen. They could also benefit from some hyperinflation techniques if they have recently been ventilated, experienced pneumothorax, or any other condition that would result in collapsed alveolar clusters (aka restrictive conditions).

I get pretty high on my vitals soap box sometimes, but this is why. These numbers are where your decision making NEEDS to come from. You can’t base your treatment on how the patient looks or how they say they feel. You have to see the numbers for yourself and know the numbers are correct.


What’s next?

Start exercising! Left ventricular remodeling is important to get cardiac output up and helps to improve ejection fraction over time. Alternating intervals helps to alternate lung volumes which helps with reinflation of collapsed segments. These happen with exercise!

In the acute and immediate post-acute phase, you should be targeting a 5 on the Borg RPD scale (moderate) or whatever corresponds to moderate breathlessness on the scale you use. You don’t really want to go higher than that because of the severe pathology this patient is still fighting.

In the sub-acute and ongoing settings, start working them harder. If they begin to be more limited by deconditioning than breathlessness, switch to a Borg or Borg Category Ratio scale instead and target 7-8/10. Use heart rate calculations if you can (Call up your old college buddies, Tanaka and Karvonen) to be more precise in your prescription with 70-80% HRmax. Make sure you are utilized standardized measures to establish baseline function and demonstrate improvements over time!


Hey, Doctor B, I think you got a little carried away here…

Yea, probably. Thanks for sticking with me.

I will tell you that I’ve had all of these patients in the home and SNF settings. I’ve ordered supplemental oxygen at a home PT eval. I’ve explained a 15% ejection fraction and why this patient needed to wear oxygen even though his SpO2 was 97%. I’ve called the doc and requested a CBC to check hemoglobin levels (and I’ve been right). I’ve titrated oxygen for recovery and requested “titrate to” orders more times than I can count. I’ve read ABGs and found new CO2 retainers. If I’ve encountered it, you will, too. So I want you to be ready and understand what you need and why you need it.


That was a lot. If you want to know more, ask me in the comments!

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Reference:

Hillegass, E. (2020). Vital Signs, oxygen, & exercise prescription: How are these impacted by COVID-19? PACER Project. Cardiovascular and Pulmonary sections of the APTA. Retrieved from https://www.youtube.com/watch?v=lj716KWNcig

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