Hi there!
In this video, we’re going to be talking about a medical device.
Now, I am not a healthcare professional of any sort and have absolutely
no qualifications to be giving medical advice whatsoever.
And so, I won’t.
This video, like nearly all of mine, is borne from simple curiosity about how something works.
My goal is to show you an example of
how everyday technologies can be used in clever ways which improve our lives,
not to make suggestions on how to do so.
To that end, I’ll be explaining what the device does and how it works, but not how
- or even whether -
it should be used in any given situation.
And with that, let’s continue.
If you’ve ever made an unplanned visit to a hospital, you might have had this happen to you;
someone appears with a clip-like device,
puts it around your finger, and it lights up with a couple of numbers.
That device was a pulse oximeter,
or oxy-meter if you want to say it that way,
and it measures your blood’s oxygen saturation as a percentage.
This serves as a quick-but-limited snapshot of how well your respiratory system is functioning.
I’ve got one right here, and simply by pressing this button and then sticking my finger into it,
it tells me my heartrate and my oxygen saturation in moments, and completely noninvasively.
Now, if you know a thing or two about blood, you’ll probably know that in general
it should be inside your body, and not outside of it.
Usually to analyze something about your blood
you’ll need to get a sample of it through means of various unpleasantness,
but this device manages to do without that.
How?
Well, with a sensor.
And a very, very ordinary one at that.
So first, what is a sensor?
(or, as they say in Star Trek the Motion Picture, a Sensore)
That’s a surprisingly complicated question.
Our bodies are filled with sensors, like the light sensors in our eyes,
the temperature and pressure sensors in our skin,
or the pressure sensors in our ears.
"Wait," you ask, "pressure sensors? Don’t you mean sound sensors?"
No, and that’s precisely the point.
A sensor is on a very low level something that reacts to a change in its environment or stimulation.
But it’s how we interpret that change that makes a useful sensor.
For example, our ears detect rapid changes in air pressure,
but that alone isn’t sound.
It's the way that our brains interpret that change that produces
the sensation we call sound.
Similarly, the photodetectors in our eyes simply send stimulation to the brain.
It’s up to our visual cortex to synthesize that stimulation into a useful image.
Moving into the artificial realm, our human-made sensors work in essentially the same fashion.
For example, take a thermistor. Its electrical resistance changes depending on its temperature.
That by itself isn’t useful to us, but if we study how that change occurs,
and learn the correlation between a given resistance and a given temperature,
we can measure its resistance in order to determine its temperature.
In that way, we can design a temperature sensor using a thermistor.
A pulse oximeter is one of countless examples where we use a simple electrical device
as a means to determine… something.
In this case, we use light.
If you take a look inside where you put your finger you’ll find a little blinking LED.
As your finger approaches it it lights up solid.
Now look on the other side and you’ll see a photodiode opposite the LED.
Your finger goes between the LED and that photodiode,
and the oximeter shines light through your finger.
If you were ever a child
(which, you were)
you’ll have discovered that your skin is
actually quite translucent, and you can shine light through your thinner parts like fingers
or earlobes.
It comes out red on the other side because -
BLOOD! [said all spookily]
And that of course means
you’re shining light through your blood, and it’s absorbing some of that light.
And because scientists can’t help themselves from asking questions and performing experiments
(to which we owe them our thanks),
we learned that the absorption spectrum of hemoglobin
(that’s the protein in our blood that carries oxygen around the place)
is quite different between its oxygenated and non-oxygenated states.
In layman’s terms this means it changes colors,
but it’s not nearly as drastic as the pop-culture myth that deoxygenated blood is blue.
‘Cause it’s not.
But it is enough of a difference to be measurable.
And you can make the difference quite obvious if you measure it with two different wavelengths of light.
Here comes a twist - there are actually two LEDs down there!
There’s a red one, and an infrared red one.
And they’re actually pulsing, not steady.
The light flashes red, then infrared, and then off over and over again.
And by analyzing what the photodiode on the other side sees after the light passes thorugh your finger,
we can actually determine how oxygenated your blood is with fairly good accuracy.
Look at this graph!
This is the absorption spectra for oxygenated and deoxygenated hemoglobin.
The red LED’s output is right about here.
Oxygenated hemoglobin absorbs hardly any of this wavelength,
but deoxygenated hemoglobin absorbs a fair bit.
As we move into the infrared range, this actually flips.
The infrared LED’s output is right about here, where oxygenated
hemoglobin actually absorbs a little more light than deoxygenated hemoglobin.
But there’s more in your finger than just blood.
There’s skin, there’s bone, there’s a finger nail, and other goodies,
so how can light alone tell us anything?
Well, because your blood isn’t just sitting there in your finger!
It’s pulsing thanks to that heart thing in your chest cavity.
With a little signal analysis, the microprocessor inside the pulse oximeter
can isolate that pulsing component of the signals it's receiving
and ignore all the not-blood.
That allows it to tell you your heart rate,
but more importantly it allows for the next most important bit; (more important? Or next-most? This was a bad line)
determining what percentage of the hemoglobin is oxygenated.
Once the microprocessor isolates the pulsing of your blood, it simply has to make a comparison
between how much infrared light passes through, and how much red light passes through your blood
during that pulse.
That ratio itself will change over time as your cells absorb the oxygen
and each new pulse refreshes the blood.
A simple look-up table can then convert the peak ratio observed to the percentage of hemoglobin which is oxygenated,
and then that value is displayed as SpO2,
or peripheral oxygen saturation.
This method has good-enough accuracy to make
this an acceptable tool for a quick snapshot of someone’s respiratory function.
There are many applications for this technology.
A device like this is a simple indicator meant to give a one-time reading.
This would be useful in an application like patient triage or a simple health checkup,
in fact the display is oriented for it to be read more easily by someone other than you.
But there are also styles where the sensor is placed separately
from the microprocessing unit to allow for things like logging of oxygenation data over time,
which can help with diagnosing things like sleep disorders.
Pilots who fly unpressurized aircraft may use these as a sort-of early warning device.
If your blood oxygenation falls too much you may be near hypoxia and when flying a plane that’s not good.
I mean, it’s never good, but it’s especially not good when you’re piloting
something that can fall from the sky.
So, wearing a monitor like this one or otherwise checking
your pulse ox every once in a while can help you to not get in that situation by alerting
you that you need supplemental oxygen or otherwise need to descend.
But you should know that these are not perfect devices.
There are situations where they can give false readings.
One such situation is with carbon monoxide poisoning.
To the pulse oximeter,
hemoglobin carrying carbon monoxide looks the same as hemoglobin carrying oxygen,
so it may tell you your SpO2 is fine when in fact it’s very not.
There are specialized versions of these devices
which use additional wavelengths of light to look for carbon monoxide poisoning,
but the common pulse oximeter cannot do this.
Some finger nail polishes purportedly interfere with the device’s ability to function.
In theory it should be able to filter out the influence of such a polish like it can your
skin and bones,
but depending on what the polish might absorb that may be impossible.
Of course there are other places where a device like this could work like your earlobes,
but the fingertip is the most common.
And there are other situations where it might not be able to get a good reading.
To help with this, many oximeters like this one will have some
sort of pulsing indicator like this or other means to show you roughly how good of a reading
it’s getting.
But the most important thing these can’t do is tell you anything else about your blood.
Arterial blood gas sampling, where you actually get some of your arterial blood out of you
and analyze it in a lab, is the only way to know oxygen saturation precisely, as well
as learn other things things like carbon dioxide concentration, blood pH,
or even how much hemoglobin is in your blood.
However, the process of getting blood from your arteries is…
unpleasant.
You can’t just draw it from your veins because then that’s not oxygenated blood so…
well you get the picture.
However, as a first-check, these are quite useful.
And since they’re really just two LEDs, a light sensor, some sort of display,
and a basic microprocessor,
they’re essentially commodities at this point and are accordingly pretty cheap.
When demand hasn’t peaked, anyway.
All it took to create this device was someone
studying hemoglobin, discovering that its absorption spectrum is very different between
its oxygenated and deoxygenated states, and then applying that knowledge.
Knowing that red and infrared wavelengths would be affected differently between the two states,
and making the connection with available light emitting diodes,
allowed Takuo Aoyagi and Michio Kishi to develop the modern pulse oximeter in 1972.
I say “all it took” like it was easy, but it goes to show that our world is made
of discoveries built on top of one another.
When we learn things about ourselves, we often find ways to apply that knowledge.
Sometimes it happens long after we’ve made the first discovery, or perhaps
(as was the case with pulse oximetry)
we knew the basics of it since the 1930’s but it took until the modern technological age
for it to become truly practical.
But learning new thigns and applying that knowledge is what makes us human.
When we do it, it can not only blow your mind, but can save your life.
I’d like to end by saying that these are obviously an invaluable tool.
They provide quick access to vital information about our bodies,
and because they’ve been refined through the decades
and the technology that’s inside them is very cheap at this point,
they’re easy to purchase.
But I’m not about to say that you should run out and buy one.
If it interests you, I would encourage you to do your own research on what to look for,
both in terms of the devices themselves and how to interpret the data they provide.
Because again, I am not a healthcare professional. I’m not an authority. And I don’t want to be perceived as one.
The only conclusion I want to lead you to today,
is that these are pretty neat,
and human ingenuity is awesome.
Thanks for watching.
♫ oxygenatedly smooth jazz ♫
[off-camera] OK, this video is something like...
11 - 12 minutes long. If I cannot record this in less than a half-hour I’m gonna be very upset with myself.
Hi there!
Hi there!
Hi there!
Hi there! [this repeats a total of 15 times]
...discovering that its absorption spectrum is very different between its oxygenated and
deoxygenated degenerated
...hemoglobin, discovering that its absorption
spectrum is very different between its oxygenated and deoxygenated
genated
genated
genated
degenated
This videl, like nearly all of mine, is borne from simp… nope, don’t like THAT
As your finger approaches it, it lights up solid.
And it stopped.
...to percentage of hemoglobin which is oxygenated…
eughh there are a lot of big words.
No! [said in a cartoonish voice]
So, wearing a monitor like this or otherwise
checking your pul ba buhhh
...finger in it, it tells me my heartrate
and my oxygen sat, ah be be dah be deuhhh
But this device manages to do that… without!
Eh, ooo.. Nope.
...terpret that change that produces the sensation
we call sound.
Similarllll….
Similarly.. That… don’t write that word.
Boopydoopy Boopydoopy Boopydoopy Boopydoopy
That's what always goes through my head at this part.
be bah da daaaaahhh
daaaaaaaaahhhhhhhh
ba da DAA
ba da DAA DAA
da da daaahhhh
boo
BAAAAAAAAAAAAAAA
boopy doopy
