Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: scientizscht on 19/05/2019 13:56:23
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Hello!
Photoplethysmography is an optical technique that is used for remote blood pressure measurement.
What does it measure exactly which it then converts into blood pressure values?
Thanks
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What does it measure exactly
Light
"The change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a light-emitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode"
From
https://en.wikipedia.org/wiki/Photoplethysmogram
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What does it measure exactly
Light
"The change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a light-emitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode"
From
https://en.wikipedia.org/wiki/Photoplethysmogram
Ok but how does the light at 120mmHg pressure defers from 130mmHg?
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I don't know.
But, the important thing is that it does differ.
So, you can find people with different blood pressures etc and use them to calibrate the system.
However, I think what it's really used for isn't absolute determination of pressure (120 mmHg vs 130 mmHg) but for changes
"It's higher than it was earlier".
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Plethysmography is the measurement of blood flow and volume, not pressure.
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Plethysmography is the measurement of blood flow and volume, not pressure.
"The PPG technology has been used in a wide range of commercially available medical devices for measuring oxygen saturation, blood pressure and cardiac output, assessing autonomic function and also detecting peripheral vascular disease."
Even if it is for blood flow and volume, how is reflection light affected by blood flow and volume?
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The changes in volume and pressure change the shape of things, and that changes the way it reflects light.
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More blood = more absorption. Since blood flow is pulsatile, it is easy to separate out the AC component of the signal from a photodiode illuminated by infrared reflected from the skin.
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But high pressure doesn't mean necessarily more blood.
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Maybe it's magic then. But it seems to work.
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The amplitude of the AC signal depends on the difference between sys and dia pressure, so you can use PP to monitor changes in blood pressure but it won't give you an absolute value.
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The amplitude of the AC signal depends on the difference between sys and dia pressure, so you can use PP to monitor changes in blood pressure but it won't give you an absolute value.
Ok but what generates that signal?
Is it change in colour? Change is shape from physical movement? And how it is proportional to blood pressure?
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I don't know about the device you are looking at, but I heard of one device that used 2 bands at different positions on your arm.
Each band was a simple pulse detector (like a fitness tracker), which detects the reduced reflection when blood is passing under the detector (mine uses a green LED, whose light is strongly absorbed by blood).
This device measures the delay between the pulse appearing at the upper arm and appearing at the lower arm - the pulse propagation speed increases with blood pressure. You would need to calibrate it with a conventional pressure cuff, but the results should be stable as long as the two LED emitters stay in the same place on your arm.
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Not sure if that's photoplethysmography.
If you search online you will find devices that supposedly measure blood pressure and other stuff remotely. Like the device is at the top corner of a room and it measures the blood pressure of the patient at the bed or something
They have even developed photoplethysmography scanner to measure stuff using the camera of mobile phones.
I am not sure what resolution pf the mobile camera is needed to measure blood pressure and how exactly.
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Ok but what generates that signal?
Is it change in colour? Change is shape from physical movement? And how it is proportional to blood pressure?
Change in volume, primarily. More pressure = swollen vessel.
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Ok but what generates that signal?
Is it change in colour? Change is shape from physical movement? And how it is proportional to blood pressure?
Change in volume, primarily. More pressure = swollen vessel.
So the camera can see the tiny changes in vessel volume through your skin from a distance?
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Ok but what generates that signal?
Is it change in colour? Change is shape from physical movement? And how it is proportional to blood pressure?
Change in volume, primarily. More pressure = swollen vessel.
So the camera can see the tiny changes in vessel volume through your skin from a distance?
Yes
You can get an app that does it
"Cardiio is a free heart rate app that works on Apple's iPhone, iPad, and iPod touch devices. Like Accurate Heart Rate Monitor, Cardiio can also measure your heart rate when you place a finger over the camera's lens but it can also read your heart rate by scanning your face with the front-facing, selfie, camera."
from
https://www.lifewire.com/best-heart-rate-apps-4580412
Obviously, I accept no responsibility if that app also posts naughty pictures to your boss at 3 am or whatever.
The point is that it exists; it works- I have seen it in action.
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Ok but what generates that signal?
Is it change in colour? Change is shape from physical movement? And how it is proportional to blood pressure?
Change in volume, primarily. More pressure = swollen vessel.
So the camera can see the tiny changes in vessel volume through your skin from a distance?
Yes
You can get an app that does it
"Cardiio is a free heart rate app that works on Apple's iPhone, iPad, and iPod touch devices. Like Accurate Heart Rate Monitor, Cardiio can also measure your heart rate when you place a finger over the camera's lens but it can also read your heart rate by scanning your face with the front-facing, selfie, camera."
from
https://www.lifewire.com/best-heart-rate-apps-4580412
Obviously, I accept no responsibility if that app also posts naughty pictures to your boss at 3 am or whatever.
The point is that it exists; it works- I have seen it in action.
Can it measure (this or another) your blood pressure?
How can e.g. 15Megapixel of a mobile camera be strong enough to capture such fine detail that does not seem to appear on photos or be visible with human eye which is has 576Megapixel ?
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Because the human eye
1 doesn't have that good a resolution across the whole field of view.
2 eye can't take two pictures one after another, remember them exactly and then measure the changes.
3 has some exceptionally good image processing, but a fourier transform (in time) isn't one of them.
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Because the human eye
1 doesn't have that good a resolution across the whole field of view.
2 eye can't take two pictures one after another, remember them exactly and then measure the changes.
3 has some exceptionally good image processing, but a fourier transform (in time) isn't one of them.
Ok but I don't think photoplethysmography works by comparing photos, because in that case it would be very inaccurate and would need constant calibration.
Also, thr human eye may not have strong resolution but it should be able to see what a mobile phone camera sees. And I cannot see anything on human skin that changes with blood pressure apart from some people getting red on their face.
And even in these cases the redness may not mean high systematic blood pressure.
And what about tanned or black people?
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Ok but I don't think photoplethysmography works by comparing photos, because in that case it would be very inaccurate and would need constant calibration.
Well, what you think isn't as important as what the developers of the app think.
That app can measure heart rate.
It does that by "seeing" your pulse in your face.
Also, thr human eye may not have strong resolution but it should be able to see what a mobile phone camera sees.
Why?
and, even if it can, can it compare one video frame to the next and look for differences, then to a time to frequency transform on that data to get a heart rate.
Essentially, when the pulse of blood from your heart reaches your head, it "inflates" your whole face very slightly.
The camera senses that change in the image.
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Your eye, or your camera, is taking in a lot of extraneous information. All the remote PP machine has to do is to register the change in the amount of a very narrow spectrum of pulse-coded infrared that is reflected from the target, knowing that the cardiac pulse rate will be somewhere between 20 and 200 per minute, so it can ignore everything else and integrate over 15 seconds (the usual "clinical" count period). The mobile phone app can do pretty much the same, knowing that the only source is your finger, so it's just a slow wave filter.
Skin color has negligible effect in the infrared. You can do glucometry and oxygen saturation using visible light reflected off the inner surface of the lower lip, which is pretty much the same color for everyone.
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t would be very inaccurate and would need constant calibration.
Well, what you think isn't as important as what the developers of the app think.
We need to distinguish a few important marketing issues here:
- Does it have approval from the FDA in USA, or similar organisations in other countries?
- If so, it may be useful in hospital wards, doctors surgeries or emergency rooms
- However, FDA approval takes a long time and costs a lot of money (which means the product is very expensive)
- There is another market, for amateur sports and fitness enthusiasts: people who are interested to know what their heart rate is after running around the park. These are the people who can use a low-cost (or "free") app on their phone, and aren't looking for absolute accuracy - if it works, they will use it; if it doesn't work they will delete it.
- But the availability, cost, and portability of these apps means that they are more likely to be available at the site of an accident.
Increasingly, these apps with data transfer over the cellular mobile network are available to local nurses in poor countries, assisted by consultation with a central doctor.
- Sometimes low-cost sensors, attached to the phone via a USB port can provide data collection and initial analysis that previously would have required a pathology lab
- Typically these need a bit more validation and training in each country than a typical "free" app, but not nearly as much as the FDA requires.
Just another way that the declining cost of powerful computers is changing our lives in many ways...
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We are talking about the "running round the park" market.
So what?
Give it a few years...