How does my heart rate monitor work?

How are smart devices able to measure heart rate without being invasive?
24 August 2021


Image of heart rate signal



Douglas has been in touch to ask, "I wear a watch that measures my heart rate with a light - how does that work, and will we be able to monitor more things with tech like this in the future?"



Jess Wade from Imperial College London doesn't miss a beat in helping explain the answer to Douglas' question...

Jess - Actually your watch is remarkably sophisticated and it can measure lots of different things. The heart rate monitor is interesting. It uses light to measure changes in pressure in your arteries. So it's called the photoplethysmography sensor, and it shines a light on your arteries and your wrist is a good place to put it because there's a lot of blood nearby and you can detect it quite easily. It shines a light on that and it looks at the light that's reflected. So there's light out and then there's a detector on the watch as well. And when your heart beats, the main chamber of your heart, which I think is your left, according to my GCSE Biology, pushes this pulse of blood around your body. That causes these arteries to temporarily swell. And when they swell up, they change the way that light is reflected back into your watch, your mobile phone or whatever it is that you're using to measure your heart rate.

Jess - It can also hopefully measure changes in pressure, which is useful for people who have hypertension, because you can look at the amplitude of that signal and then a bunch of clever algorithms designed by whoever made your watch will convert those signals that they're getting into an actual heart rate. I think the other things that your watch can already do probably is measure the blood oxygen levels. So to measure the amount of oxygen in your blood, and that's something called the pulse oximeter, which is, again, a technology that's been really useful and important with COVID that looks at how much light is absorbed by your blood. And we can use that to look at how high your blood oxygen levels are. And particularly in people with coronavirus, they got very low, so it was a really good warning sign that they were suffering from the disease. But in the future, I see so many different applications of wearable technology and almost everyone I speak to has a new idea every day. You know, making blood glucose testing less invasive, being able to detect and monitor for diseases early, kind of coming into what Jo was saying about maybe changes in a scent or a biomarker that you could trace.

Jess - People are doing absolutely extraordinary things. You know, there's a fantastic research group in Northwestern University in the States lead by a guy called John Rogers. And he develops tech for monitoring the rehab of stroke patients, develops tech for looking at all different deep tissue blood levels and things like that, which are quite hard to monitor because they're deep inside you, but can tell us a whole bunch about different diseases that you can have. So I see it as this kind of beautiful area of really biology and physicians working with material scientists and physicists to see where technology could be applied and then use it to make doctors' and patients' lives easier. I really love it.

Eva - It's amazing to think how much data we must be creating, everyone wearing their watches and the heart rate being stored somewhere in a cloud, somewhere in a server some way. As we get further and further along, we're going to end up with such an enormous amount of data, we'll need the computer scientists to be developing new ways to store data more effectively too.

Jess - They're already doing that. And I think particularly in kind of rapid analysis of MRI images for the diagnosis of breast cancer, people are making really, really clever algorithms now to be able to identify things earlier and then be able to come in with a proper treatment plan. But I think it's incredibly exciting how much we're generating and really not only from a medical imaging perspective, but from a 'making your doctor's life easier' perspective. You know, you don't have to go physically into the hospital as much if you've got something monitoring your rehabilitation from a stroke, because that data can be sent wirelessly and efficiently to your doctor. That doctor can make a call about what you need to do next and then send you that information. So I think it's not just making our health easier, but it's also making our lives easier and society easier.


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