Electronics > Projects, Designs, and Technical Stuff
pulse oximeter for health monitoring?
JBeale:
I was curious about these devices, given the current events.
I got a cheap one yesterday: CONTEC Pulse Oximeter Model CMS50M. (from another vendor, but Amazon has them)
https://www.amazon.com/CONTEC-CMS50M-Oximeter-Waveform-Carrying/dp/B07D8R4G5H
This uses 2x AAA batteries, and clips over the end of one finger. It apparently uses two LEDs: 660 and 880 nm both shining through your finger, and the ratio of total absorbance at those two wavelengths goes into a lookup table to compute SpO2 or "peripheral capillary oxygen saturation, an estimate of the amount of oxygen in the blood." It displays pulse rate (BPM) and SpO2 on a LED display, along with a real-time bargraph showing each detected pulse. This one is live-readout-only, some fancy ones have bluetooth so you can log the data to an app.
Normally on my finger it indicates 99% SpO2. Just now I tried holding my breath for 40 seconds, which is about as long as I wanted to. At T=40s when I started breathing again, the meter had only gone down to 97%. Interestingly, although I then continued breathing normally the meter continued slowly down for awhile, reaching 91% at T=59s before quickly rising back up again to 99% at T=70 s. There must be some delay in O2 transfer and blood circulation from lung to fingertip, but I don't know how much of the total readout delay is some real physiological thing, vs. some internal (possibly asymmetric) lowpass filter in the meter.
Of possible interest, yesterday I felt pretty fatigued with slight but noticeable shortness of breath feeling which is very unusual for me. I had no fever, but occasional cough and headache. I did this experiment yesterday with similar results on the SpO2 numbers, starting out 98-99 slowing going down to 93% then bouncing back up, but the difference was that yesterday I could only hold my breath for about 10 seconds. However I am feeling more energetic today and maybe I just have more practice in breath-holding now. FWIW
penfold:
The body is full of feedback loops, so under a step change, there will be a lot going on which ensures the most critical functions are being looked after first, your finger probably being the first to get overlooked and the last to get restored. As a steady state measurement after a period of rest etc, its probably representative of the more critical blood gas saturations within the body, but dynamically there is a lot going on that will influence that measured at the finger.
I couldn't cite any specific sources right now but.. the same kinds of hormone responses that trigger "white-coat hypertension" could have an effect on the pulse oximeter readings through blood vessel contraction/dilation, it is unfortunately immensely difficult to get any valid readings of pretty much anything in the body since even that "being curious" feeling will trigger some very slight physiological changes towards nervousness/anxiety and even being slightly excited about having a new gadget to play with will do similar..
taydin:
My father has advanced stage chronic heart failure, and even with him the oximeter indicates above 97% saturation. Before he received his ICD implant, right before he was going to undergo the operation, the oximeter was swinging wildly between 80% and 100%.
So the oximeter will only show low oxygen concentration when there is a life threatening drop, which would have already so many other symptoms that you would be compelled to go to the ER without even considering the use of an oxymeter.
hans:
My Samsung S7 also has a SpO2 sensor.
I tried it first when laying flat on the couch. I think it read something like 95%. I sat up, and it jumped back up to 97%.
Later that day I went out for a walk, and it fluctuated between 97% and 100%.
I'm not sure how reliable these measurements are. Under stable and repeatable conditions probably pretty reliable, but under changing circumstances there is probably quite some tolerance.
I had similar symptoms as you mentioned. I put them down to stress & anxiety usually, even at strange times like these. I would only start to worry if the readings would drop further, and I actually feel sick. However I'm now medical doctor, so take my findings for even less than 2 cents. :-//
Kleinstein:
The phone has to measure in refection and this can be less accurate than the transmission configuration used in the classical pulse-oxymeters.
Usually the saturation is quite high and one would notice a drop quite a lot. So the saturation is more like an alert signal in intensive care, maybe telling the doctors when they need a respirator or extra oxygen.
For private use the pulse reading may be more useful, especially if used under well defined conditions, e.g. in the morning. It may help the doctors to know the individuals pulse rate at rest when healthy. However usually no need for an extra instrument for this. One can usually do it the old way. For some heart problems the changes in the PBMs may give some help to an expert.
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