
How Pulse Oximeters Work (2015) - zealotsm
https://www.howequipmentworks.com/pulse_oximeter/
======
maxfan8
Here is the WBM archive of it:
[https://web.archive.org/web/20200708064647/https://www.howeq...](https://web.archive.org/web/20200708064647/https://www.howequipmentworks.com/pulse_oximeter/)

(for those who can't access the original site due to it being hugged to death)

------
danieltrembath
The Royal Melbourne hospital in Australia is using pulse oximeters to manage
COVID positive patients in their own homes. The program was setup in March
this year to manage a large number of patients remotely (cost/safety).

The patients take their own measurements with a pulse oximeter and digital
thermometer (both off the shelf consumer items). The person is prompted via
SMS and submit their vitals via website. A software system orchestrates all of
this and alerts patients and clinicians to anyone with worrying numbers.

This keeps beds free at the hospital, but still gets the small percentage of
patients back to hospital that get really sick.

It also does all the other boring monitoring and administrative work needed
when you're checking up on lots of real people.

Full disclosure. I worked on this project. A version has been open sourced and
if you're a hospital or other medical service you're welcome to use our work.
We're publishing improvements as we go.
[https://github.com/rmhcovid/txtmon](https://github.com/rmhcovid/txtmon)

[https://www.thermh.org.au/news/royal-melbourne-hospital-
impl...](https://www.thermh.org.au/news/royal-melbourne-hospital-implements-
virtual-monitoring)

Using simple standalone devices and a low-coding platform already in use by
hospitals (REDCap) the whole project was crash-built in a couple of weeks and
is saving lives. That platform has many shortcomings (messier even than
Excel), but there's various medical/privacy rules that make more traditional
development unattractive for quickly prototyping. It's been a rewarding
project to work on despite many frustrations.

~~~
elif
Thank you for sharing this. In the program guide it says a temperature reading
of > 42 degrees or an oxygen saturation of < 90 are the thresholds for a MET
call. Is there a basis for these specific values or are they general
approximations?

~~~
Aeolun
Temperature higher than 42 (e.g. 43) is described on Wikipedia[1] as:

> Normally death, or there may be serious brain damage, continuous convulsions
> and shock. Cardio-respiratory collapse will likely occur.

It seems that that's a little bit late to consider calling the hospital?

[1]:
[https://en.wikipedia.org/wiki/Human_body_temperature#Fever](https://en.wikipedia.org/wiki/Human_body_temperature#Fever)

~~~
distant_hat
42C is 107F, it is way too high it seems.

------
sohkamyung
Off-topic, but an inventor of the Pulse Oximeter recently passed away [1]:

"Takuo Aoyagi, a Japanese engineer whose pioneering work in the 1970s led to
the modern pulse oximeter, a lifesaving device that clips on a finger and
shows the level of oxygen in the blood and that has become a critical tool in
the fight against the novel coronavirus, died on April 18 in Tokyo. He was
84."

[1] [https://www.nytimes.com/2020/05/01/science/takuo-aoyagi-
an-i...](https://www.nytimes.com/2020/05/01/science/takuo-aoyagi-an-inventor-
of-the-pulse-oximeter-dies-at-84.html)

------
Fabricio20
Technology Connections [0] also has a great video about these awesome devices,
although not that much in-depth in comparison to this website.

[0]:
[https://www.youtube.com/watch?v=4pZZ5AEEmek](https://www.youtube.com/watch?v=4pZZ5AEEmek)

~~~
ravedave5
Such a great channel and good explanation.

------
ornel
I measured myself at home in Mexico City and got 93%. I got kind of worried,
knowing people sick with COVID-19 can have very low saturation levels without
realising it. Then I remembered I was standing at about 2200 m above sea
level. Lots of unexpected weirdness happens at high altutude

~~~
Engineering-MD
I’ll be honest, a lot if the portable oxygen saturation monitors seem to
report slightly out of range readings. It often picks up after a few minutes,
I suspect they are calibrated poorly tbh. 94-98% is the desired range.

~~~
peripitea
This is only true for people who live close to sea level. At higher altitudes,
saturation even down into the high 80s can be normal:
[https://www.thelancet.com/journals/langlo/article/PIIS2214-1...](https://www.thelancet.com/journals/langlo/article/PIIS2214-109X\(19\)30543-1/fulltext)

------
wlesieutre
Speculated to be included in the Apple Watch S6 this fall.

[https://9to5mac.com/2020/03/08/apple-watch-blood-oxygen-
satu...](https://9to5mac.com/2020/03/08/apple-watch-blood-oxygen-saturation/)

At the time they reported that there were mentions of it in an iOS 14 leak,
but it wasn't clear if it would be a software update to existing watches
(somehow able to detect it with existing heartrate hardware) or in a watch
hardware revision. Since it wasn't mentioned in the watchOS news at WWDC,
looks like it'll be new hardware.

Either that or the code they found in iOS 14 was related to the health app and
support for 3rd party oximiters, but that's not how the 9to5 post frames it so
best I can do is take their word for it.

~~~
deadmutex
IIRC, Fitbit has products on the market that has them. Works both on iOS and
Android.

[https://www.wearable-technologies.com/2020/01/fitbit-adds-
bl...](https://www.wearable-technologies.com/2020/01/fitbit-adds-blood-oxygen-
monitoring-spo2-in-its-versa-charge-and-ionic-device/)

~~~
VectorLock
Wow they finally did it. After the hardware being there for _years_ they
managed to get around to enabling the software. I already jettisoned them for
Garmin after waiting so long.

~~~
Darkphibre
Same frustrations here! Couldn't believe when I bought their smart device
based on advertising that said they had the hardware to do it.... but it
wasn't enabled.

------
dghughes
The problem with pulse oximeters is the people who need them often can't use
them. Yes I know it's not meant as a medical device.

My dad has COPD and IPF but he can't use the SPO2 meter on the Samsung phone I
gave him (and Samsung disabled the SPO2 function for Canada). A Garmin
VivoSmart 4 can't give a decent reading on him or me and it takes forever.
Part of the problem is blood flow dad's fingers are ice cold due to his
condition.

My dad uses a cheap oximeter from Walmart it works better than anything else.
It's the type that clips over a finger it's fast and seems to be accurate. I'd
like to get a wristband or watch that has a good SPO2 meter but nothing seems
to exist.

------
bdcravens
The most painful thing I've experienced medically was an arterial blood draw
to measure my o2 sat levels. (They numb your inner wrist with novocaine then
go in with a giant needle to the artery deep in there. If it's that painful
numb, I can't imagine what it'd be like otherwise.

And each time it was no more accurate than the pulse oximeter.

~~~
joshgel
Most of the time we don’t numb patients to be honest. I’ve had it done to me,
we practiced on each other in Med school. It is painful, but I suspect the
Novocain you got didn’t work because it’s the artery that’s painful not the
skin.

But it does give you different information than a pulse ox. For example,
people can have respiratory failure from too low oxygen (captured on a pulse
ox), but also from top high CO2 (not measured on pulse ox). It also gives a
measure of acid-base status, which can be thrown off for a variety of reasons
(usually renal or respiratory). It is used somewhat too much in my opinion but
can be life saving in some situations.

------
dvt
Went to a party last weekend where the host used a pulse oximeter (and
thermometer) on everyone. Blood O2 level is apparently a leading indicator of
COVID-19. Pretty cool stuff. The inner workings of it are actually quite
simple as well, pretty amazing that it works as well as it does.

~~~
popotamonga
I discovered i have a problem with one. Every one else who tried it had 98-100
all the time. I had < 94 no mater what

~~~
asveikau
I thought I recall reading that 94 is not dangerous or abnormal, and 90 is
when you should get checked out.

~~~
alkonaut
This is something that is still not quite understood by doctors.

I take my kid (Who has a respiratory condition) to hospital at 90.

In my experience if you are in hospital with oxygen because of a respiratory
illness, the oxygen would probably be given once you dip below 92-93 assuming
they know you have a respiratory condition. Not because that’s the danger
level but because they want to be able to tweak the O2 if you fall even lower.
Doctors then usually instruct nurses to adjust the flow so saturation stays in
the 90-95 range.

People at high altitude or in airplane cabins can be at 70% for hours without
even noticing. Meanwhile someone having a dip from 90 to 80 with pneumonia
might be gasping for air.

~~~
vikramkr
IIRC gasping for air has more to do with blood CO2 which is why
hyperventilation is so dangerous - you can run out of oxygen without knowing
it.

------
leetrout
It's always interesting to me how much of a hivemind the internet has and how
many coincidences there are. Just last night I dug out my cheapo pulse
oximeter and checked everyone in my house to get a baseline.

I then googled how they work and read pretty much the same thing.

~~~
jacquesm
This is more a function of how many people read HN than anything else. On a
population that large in a crowd of technically inclined people there's bound
to be _somebody_ who in the space of 24 hours researched how oximeters work. I
read the same article a few months ago.

~~~
leetrout
True

------
DangerousPie
Does anybody have any suggestions for how to check whether a pulse oximeter
actually works properly?

I bought one online from a not particularly trustworthy shop. I have never
seen it show any value other than 99%. How can I figure out whether it
actually does anything and doesn't just show 99% no matter what?

~~~
mturmon
I purchased a relatively cheap one from Amazon a couple of months ago. Less
than $50, I believe.

I tried some simple stuff when it arrived. I believe I was able to get it to
swing within the 94-99% range by holding my breath and by deliberate, deep
breathing, over the span of just a minute or two. It definitely was not pegged
at 99%. The changes were surprisingly fast. My wife was able to get the same
response.

If it matters, I'm at about 500 feet above sea level (not high-altitude at
all).

~~~
DangerousPie
How long did you hold your breath for?

~~~
mturmon
Perhaps 30 seconds? It was uncomfortable but not a big strain. I see someone
nearby saying 2-3 minutes. For me, it was not that long!

~~~
mancerayder
I bought one a few days ago (weird coincidence), from Amazon for around $35 at
the advice of a friend. Skeptical also, my 97% level upped to 98-99 with deep
breathing, and then I held my breath as long as I could possibly do it and it
dropped to about 93-94.

That's obviously not a confirmation that it's calibrated or trustworthy, but
at least it shows a relative measure that seems real in both directions of
oxygen saturdation. Surely if I have a drop due to illness I should notice,
even if the number isn't exact.

------
acidburnNSA
SO is an internal medicine doctor. When covid was ramping up at first and we
thought there would be beds in school gyms and stuff the first thing she did
was get a pulse oximeter off the internet.

------
dreamcompiler
FWIW some Samsung phones (and probably others) have this functionality built
in. I've tested mine and I consistently get readings within 1 point of my
gold-standard Nonin pulse oximeter.

------
progre
Since the site is hugged to death, why not check out Technology Connections
video instead: [https://youtu.be/4pZZ5AEEmek](https://youtu.be/4pZZ5AEEmek)

~~~
herbstein
Technology Connections is such a good channel that mixes humor and in-depth
looks at things people only kind know a little bit about.

~~~
el_oni
It has a weird 80s-90s aesthetic that I can watch for hours. He got me back
into electronics when he was talking about the NA electrical system, and
bashing ring circuits

~~~
Tokkemon
I mean, who wouldn't bash ring mains?

------
amiga_500
Does anyone know the degradation in blood oxygen levels under moderate
exercise for a person who is fairly health (not athlete but slim, does regular
exercise)?

I had covid, so bought one of these, and I'm now building my way back to being
able to jog, after months of shortness of breath. I would like to be able to
understand what level my blood oxygen %, for a normal, healthy person, would
drop to when say jogging.

~~~
pps43
If I really push during my 5k run, pulse oxymeter shows around 85%. Not sure
how much to trust it, as soon as I stop it climbs back up. Unclear if it's due
to vibrations, or my lungs catching up.

~~~
amiga_500
Mine starts beeping at 93% (if I hold my breath for as long as I can it drops
to around 92%). My level does rebound again after a few seconds. 85%, sounds a
bit low, not totally incredulous though.

------
rcar
cached:
[https://webcache.googleusercontent.com/search?q=cache:Gs1gDy...](https://webcache.googleusercontent.com/search?q=cache:Gs1gDy7I1WkJ:https://www.howequipmentworks.com/pulse_oximeter/+&cd=1&hl=en&ct=clnk&gl=us)

------
nerdbaggy
It’s weird, when I have a pulse oximeter on my finger and a blood pressure
cuff on and it’s rather inflated I get electrical feeling shocks up my arm.
The both have to be on the same arm. No idea what it is, probably a pinched
nerve.

~~~
bookofjoe
Likely ulnar nerve compression just above the elbow by the inflated blood
pressure cuff. (retired M.D. here)

~~~
nerdbaggy
That makes sense thanks!

I see that you were an anesthesiologist, now I can finally asked this question
that I’ve always wanted too! When under general anesthesia why do they put you
on opioids? I read that if not people would still react to the pain? Like
would the scream and cry just not know they were doing that?

~~~
bookofjoe
Opioids are a component of so-called "balanced" general anesthesia (GA).

Over my 38 years in practice (1977-2015) and some 25,000-30,000 cases,
probably 95% were GA and the remainder regional/I.V. sedation.

Of the GA's, I estimate I used IV opioids in 95%. My drug of choice was
fentanyl: fast and relatively short acting, easily titrated to response.

Along with fentanyl, I routinely used diazepam or Versed (I started in 1977
and Versed didn't become available until 1985), and skeletal muscle relaxants
for endotracheal intubation and skeletal immobility.

Induction of GA was with sodium thiopental (1977-1989), thereafter Propofol.

Having said that, know that by the time I retired, I was in a small minority
of anesthesiologists (most older like myself) who trained when balanced
anesthesia was standard.

Today an overwhelming majority of anesthesiologists use only inhalation agents
for GA and do not give opioids intraop. Inhalation GA often does not require
skeletal muscle relaxants (depending on the surgical site), and if a patient
is "light," they will react by moving and show increases in heart rate and
blood pressure.

Diff'rent strokes.

FWIW, I am one of very few anesthesiologists with extended time in practice
who have never been sued.

>Like would they scream and cry, just not know they were doing that?

Awareness under anesthesia is a whole different subject of considerable
importance.

~~~
nerdbaggy
Thanks for that, really appreciated!

------
sirius87
Wow! that was a fantastic, insightful read. did not know the pleth was of
note.

------
dmitrygr
It is really grating that all the visualizations are wrong. Blue is shorter
wavelength than red. Infrared is longer wavelength than red. Every single
image is wrong. :(

~~~
alister
The author explains his visualization as follows:

 _One is a red light, which has a wavelength of approximately 650 nm. The
other is an infrared light, which has a wavelength of 950 nm. (Throughout our
description, we will show the infrared light in light blue. In reality,
infrared light is invisible to the human eye.)_

~~~
dmitrygr
i mean this, for example:
[https://web.archive.org/web/20191230203925im_/https://www.ho...](https://web.archive.org/web/20191230203925im_/https://www.howequipmentworks.com/wp-
content/uploads/2015/01/wave_show.gif)

~~~
wtallis
The fact that those curves don't even come close to resembling sine waves
seems more wrong to me, and really helps drive home the fact that the diagram
was made in MS Paint with little regard for accuracy.

------
vwcx
Can anyone recommend an inexpensive fingertip oximeter?

~~~
exhilaration
Back in April, I bought this unit and sent it to my wife's family in New
Jersey. I spent some time googling what to get; I found several discussions on
Reddit where doctors and med students agreed that the "Zacurate" units
reliably matched their hospital units.

[https://www.amazon.com/Zacurate-Fingertip-Oximeter-
Saturatio...](https://www.amazon.com/Zacurate-Fingertip-Oximeter-Saturation-
Batteries/dp/B01HSAG8BE)

------
jacquesm
What a fantastic, no nonsense and well researched article this is. I read it a
couple of months ago and was very much impressed.

------
el_don_almighty
HN is the new /.

~~~
d1str0
Feel free to make a new HN that caters perfectly to you.

~~~
ChrisGranger
I think the parent was referring to the Slashdot effect of knocking small
websites offline.

[https://en.wikipedia.org/wiki/Slashdot_effect](https://en.wikipedia.org/wiki/Slashdot_effect)

------
behnamoh
"Error establishing a database connection"

------
oehtXRwMkIs
Fun fact about pulse oximeters: in California EMTs are regulated so strictly
that it is illegal for them to use a pulse oximeter as it is deemed
"invasive". So if anyone is experiencing a medical emergency that could
successfully be delayed following the algorithm involving checking blood O2,
they better hope the EMT is experienced/lucky enough to suspect low O2 or that
there's a paramedic or nurse on board. It's definitely one of those laws that
aren't really enforced but it's weird that in California EMTs have to break
the law to do basic screening.

~~~
tomjakubowski
Citation please? Page two of LA County Health Services's "EMT Scope of
Practice" document lists pulse oximetry as part of the basic diagnostic
process for EMTs. This would seem curious if state regulations forbid that.

[http://file.lacounty.gov/SDSInter/dhs/206315_802.pdf](http://file.lacounty.gov/SDSInter/dhs/206315_802.pdf)

~~~
oehtXRwMkIs
That's LA, here's California: [https://emsa.ca.gov/wp-
content/uploads/sites/71/2017/10/Scop...](https://emsa.ca.gov/wp-
content/uploads/sites/71/2017/10/Scope-of-Practice-Documents-
Current.12.11.18.pdf)

EMTs in California are indeed allowed to use it, I must have learned something
that was county specific, not state wide. My apologies.

