5 ms·

I'm a student EMT and our instructor told us not to rely on a pulse ox reading if our patient has dark skin, or if they were in a fire, or are a lifelong smoker

I'm a student EMT and our instructor told us not to rely on a pulse ox reading if our patient has dark skin, or if they were in a fire, or are a lifelong smoker, or if they have any nail polish or dirt/grease on their fingertips. We just look for the 94-100% range and would only take action if we get a reading around 87% or lower, or see a sudden drop during treatment. There are a few alternatives designs that aim to mitigate this particular issue by either preventing crosstalk [1] or by using multiple LED wavelengths and angles - the second one is something a med device engineer told me about a few months ago, but I'm having trouble finding any papers or information on it. [1] https://www.hindawi.com/journals/jhe/2018/3521738/ https://www.hindawi.com/journals/jhe/2018/3521738/

Edmond 6y ago
The article notes that the devices inaccurately report higher oxygen levels for darker skin tones. Meaning if someone dark skinned reports a result that falls into the 94-100% range, they may in fact be at lower oxygen levels that put them at risk.
- PascLeRasc 6y ago
  That's true. There are other context clues that we can use, like a patient's medical history, how their breathing looks/sounds, or if their environment or mental status suggests CO poisoning (that also won't show up even on the perfect pulse ox, since it binds to hemoglobin the same way oxygen does). Overall we just need a better way to get information about oxygen levels.
  - sbierwagen 6y ago
    At first thought, a hemoglobin protein loaded up with CO molecules should have slightly less mass than an oxygenated hemoglobin complex, which could... theoretically... be measured, maybe with Raman spectroscopy. That effect is going to be tiny, though. Wikipedia is telling me a hemoglobin tetramer masses "about" 64,000 daltons, so a complex loaded up with 4CO will mass 64,111 vs 64,127 amu. 0.024% difference.
- treis 6y ago
  The bias is smaller than the actual measurement error. The measurement error is around +/- 1. So a 96 reading means actual blood O2 of 95-97. The bias introduced by skin color is around 0.5. So a 96 reading means 95-97 in a white person and 94.5-96.5 in a black person. The bias from skin color isn't significant because the measurement itself is pretty inaccurate.
m463 6y ago
here is one: https://web.archive.org/web/20200708064647/https://www.howequipmentworks.com/pulse_oximeter/ https://web.archive.org/web/20200708064647/https://www.howeq...
rscho 6y ago
Personally, I've never seen a case where the difference was clinically meaningful. So, meh...
- RcouF1uZ4gsC 6y ago
  Yeah, and if there was a real pressing need to get an more accurate number, you would get an arterial blood gas. Pulse oximetry is for monitoring change. When a critical value is needed for medical intervention, such as a decision to put in a breathing tube, tests like arterial blood gas has are used.
tomphoolery 6y ago
This device seems pretty flawed if it can't read fingers on like 80% of people...
rscho 6y ago
If people were in a fire it's true the value may be flawed, but we usually say that more because of potential carboxy- and methemoglobinemia than because of soot.
adrianmonk 6y ago
> We just look for the 94-100% range and would only take action if we get a reading around 87% or lower I'm not very familiar, so I'm not sure how to interpret this. Does this describe the normal procedure for when you can rely on a pulse oximeter, or does it describe the modified procedure for when you can't?
- PascLeRasc 6y ago
  That's the normal procedure, as far as I know. Taking a pulse ox reading is done at the same time as we'd get blood pressure, listen to breath sounds and watch chest expansion depth/rate, and talk to the patient about how they're feeling or what's wrong. It's just one factor that could tell us that they're short of breath and we should provide oxygen, but it's important to consider the patient as a whole. They like to say "treat the patient, not the data".
  - adrianmonk 6y ago
    Thanks. So in a case where you cannot rely on it, do you completely avoid using the pulse oximeter at all? Or do you use it but place less faith in the readings and interpret the readings differently?
    - PascLeRasc 6y ago
      Since I'm just a student I think it's best if I defer to the textbook: "If your service has a pulse oximeter, you should have a protocol describing when to use it. Generally this will include all patients complaining of respiratory problems or otherwise at risk for hypoxia. When used properly, the device can help you to assess the effectiveness of artificial respirations, oxygen therapy, and bronchodilator (inhaler) therapy."
godelski 6y ago
Just so you have some extra information (I actually build a heart rate monitor and pulse oximiter) skin color and transparency makes a big difference, same with the color of the LEDs. I'm pretty pale and one of my friends is pretty tan, my signal was much stronger than his. The IR (infrared) ones work better as they are able to penetrate the skin better and there are clearer absorption lines from oxidized blood. I also tested different locations to place the device, I did notice most are fingertip or wrist (fitbits and smart watches), but found I got substantially stronger signals by placing the device near the upper forearm (god did this help in debugging). This was true for everyone I tested (unfortunately only white people) and helped smooth out the data a little, but it was still noticeable. On how these devices work, it is typically two LEDs the produce light that bounces through the skin, some light is absorbed by blood (different absorption by different oxidation and density) then the reflected light is captured by a photodiode. All of this is on one surface, they are very basic. The density changes rapidly and that's how you capture the pulse (easy to get btw) and the oxidation is based on absolute levels (THIS IS THE PROBLEM!!!!). After building one I learned to not take these things too seriously, especially fingertip and wrist mounted ones. I encourage others to build them. You can do most of it in a weekend if you have some soldering experience (best if you can do surface mount soldering), some programming, and are comfortable doing data analysis. And no, I do not expect my device to be on par with the ones in hospitals, but I was quickly able to match readings from a finger based device that I bought from the local drug store.

Edmond 6y ago

The article notes that the devices inaccurately report higher oxygen levels for darker skin tones. Meaning if someone dark skinned reports a result that falls into the 94-100% range, they may in fact be at lower oxygen levels that put them at risk.

PascLeRasc 6y ago
That's true. There are other context clues that we can use, like a patient's medical history, how their breathing looks/sounds, or if their environment or mental status suggests CO poisoning (that also won't show up even on the perfect pulse ox, since it binds to hemoglobin the same way oxygen does). Overall we just need a better way to get information about oxygen levels.
- sbierwagen 6y ago
  At first thought, a hemoglobin protein loaded up with CO molecules should have slightly less mass than an oxygenated hemoglobin complex, which could... theoretically... be measured, maybe with Raman spectroscopy. That effect is going to be tiny, though. Wikipedia is telling me a hemoglobin tetramer masses "about" 64,000 daltons, so a complex loaded up with 4CO will mass 64,111 vs 64,127 amu. 0.024% difference.
treis 6y ago
The bias is smaller than the actual measurement error. The measurement error is around +/- 1. So a 96 reading means actual blood O2 of 95-97. The bias introduced by skin color is around 0.5. So a 96 reading means 95-97 in a white person and 94.5-96.5 in a black person. The bias from skin color isn't significant because the measurement itself is pretty inaccurate.

m463 6y ago

here is one: https://web.archive.org/web/20200708064647/https://www.howequipmentworks.com/pulse_oximeter/ https://web.archive.org/web/20200708064647/https://www.howeq...

rscho 6y ago

Personally, I've never seen a case where the difference was clinically meaningful. So, meh...

RcouF1uZ4gsC 6y ago
Yeah, and if there was a real pressing need to get an more accurate number, you would get an arterial blood gas. Pulse oximetry is for monitoring change. When a critical value is needed for medical intervention, such as a decision to put in a breathing tube, tests like arterial blood gas has are used.

tomphoolery 6y ago

This device seems pretty flawed if it can't read fingers on like 80% of people...

rscho 6y ago

If people were in a fire it's true the value may be flawed, but we usually say that more because of potential carboxy- and methemoglobinemia than because of soot.

adrianmonk 6y ago

> We just look for the 94-100% range and would only take action if we get a reading around 87% or lower I'm not very familiar, so I'm not sure how to interpret this. Does this describe the normal procedure for when you can rely on a pulse oximeter, or does it describe the modified procedure for when you can't?

PascLeRasc 6y ago
That's the normal procedure, as far as I know. Taking a pulse ox reading is done at the same time as we'd get blood pressure, listen to breath sounds and watch chest expansion depth/rate, and talk to the patient about how they're feeling or what's wrong. It's just one factor that could tell us that they're short of breath and we should provide oxygen, but it's important to consider the patient as a whole. They like to say "treat the patient, not the data".
- adrianmonk 6y ago
  Thanks. So in a case where you cannot rely on it, do you completely avoid using the pulse oximeter at all? Or do you use it but place less faith in the readings and interpret the readings differently?
  - PascLeRasc 6y ago
    Since I'm just a student I think it's best if I defer to the textbook: "If your service has a pulse oximeter, you should have a protocol describing when to use it. Generally this will include all patients complaining of respiratory problems or otherwise at risk for hypoxia. When used properly, the device can help you to assess the effectiveness of artificial respirations, oxygen therapy, and bronchodilator (inhaler) therapy."

godelski 6y ago

Just so you have some extra information (I actually build a heart rate monitor and pulse oximiter) skin color and transparency makes a big difference, same with the color of the LEDs. I'm pretty pale and one of my friends is pretty tan, my signal was much stronger than his. The IR (infrared) ones work better as they are able to penetrate the skin better and there are clearer absorption lines from oxidized blood. I also tested different locations to place the device, I did notice most are fingertip or wrist (fitbits and smart watches), but found I got substantially stronger signals by placing the device near the upper forearm (god did this help in debugging). This was true for everyone I tested (unfortunately only white people) and helped smooth out the data a little, but it was still noticeable. On how these devices work, it is typically two LEDs the produce light that bounces through the skin, some light is absorbed by blood (different absorption by different oxidation and density) then the reflected light is captured by a photodiode. All of this is on one surface, they are very basic. The density changes rapidly and that's how you capture the pulse (easy to get btw) and the oxidation is based on absolute levels (THIS IS THE PROBLEM!!!!). After building one I learned to not take these things too seriously, especially fingertip and wrist mounted ones. I encourage others to build them. You can do most of it in a weekend if you have some soldering experience (best if you can do surface mount soldering), some programming, and are comfortable doing data analysis. And no, I do not expect my device to be on par with the ones in hospitals, but I was quickly able to match readings from a finger based device that I bought from the local drug store.