Podcast: What Your Smartwatch Tells You About Your Health

Courtney: Welcome to Health Matters – your weekly dose of the latest in health and wellness from NewYork-Presbyterian. I’m Courtney Allison.

From heart rate variability to sleep scores, the newest wearable devices are tracking more than step counts.  And more and more people are using them to keep tabs on their health and fitness. So what’s the best way to use all this information that devices are collecting?

This week, we talked to Dr. Corey Bradley, a cardiologist at NewYork-Presbyterian and Columbia, about the numbers on your smartwatch and which are important to know. We get into what each measurement means, when you should be concerned about alerts, and whether these readings are actually accurate.

Courtney: Dr. Bradley. Thank you so much for being with us here today.

Dr. Corey Bradley: Thanks for having me.

Courtney : So we’re here today to talk about smartwatches. One in three people in the U.S. wear a smartwatch, so a lot of data is coming in. And what we hope to do today is help our listeners better understand how to parse through all that data. So with that in mind, to start, can you explain some of the measurements that smartwatches track?

Dr. Corey Bradley: So most commonly, and it depends a little bit on the brand and what features you have and what model watch you have, but most commonly, your watch can monitor your heart rate. And that will give you some feedback as well about generally heart rate variability throughout the day, which they’ll give you your minimal heart rate and your maximal heart rate over the course of the previous 24 hours.

Watches now can also monitor your sleep, and that’s generally done by measuring, like, a change in your heart rate during traditional sleeping hours and also a lack of movement. Some of the other more novel smartwatches also have, like, a one lead EKG in them, so they can give you an actual ECG analysis of your heart rhythm.

Courtney: In general, how accurate are all these measurements?

Dr. Corey Bradley: So if we talk about heart rate first, which is I think the most common data point that people are using these watches for outside of step counts, the heart rate testing that’s done on these devices is largely done when the person and the device are at rest. And so the data for accuracy of these devices at rest is quite good. It’s usually within about three to five beats per minute of your heart rate. These devices are also relatively good with moderate exercise.

Based off of the way that they work, which is that most of them rely on an optical sensor in the back of the watch that’s essentially providing a light that goes into your skin and then it’s measuring the light, amount of light that’s reflected back and that allows it to see how much oxygen is absorbed essentially in your bloodstream and in your tissues. And that allows it to approximate heart rate. And unfortunately, it does rely on having the light be in a stationary place. And so if you’re doing exercises that require you to use your arms, like for instance, an elliptical or a rowing machine where you’re going to be relying on arm movement, you do generally lose a little bit of the accuracy because the light is just physically shifting on your wrist more.

It’s not like these watches are tested in every form of exercise and so there are always going to be exceptions based off of the person, based off of their wrist shape, based off the exercise they’re doing. And so the higher the heart rate gets, the more variability you’re going to see. But in general, at rest, the accuracy is excellent.

Courtney: Okay, so we’re talking heart rate and this might be a simple question, but could you define what your heart rate is?

Dr. Corey Bradley: Totally. So in the simplest terms, your heart rate is the number of beats your heart takes every minute. And so we, that’s why the unit is beats per minute, BPM. And so how many times does your heart beat per minute? In general, there’s a wide range for what we consider to be normal at rest. Typically between 60 and 100 is what we use for a normal heart rate.

Courtney: So, are there things that people can keep in mind throughout the day when they check their heart rate? For example, if someone has an elevated heart rate, but they’re not exercising, what might that mean? And just personally, I’ve found mine, like sometimes I’m taking a hot shower, and I see my heart rate got high. Or if I’m fighting off a cold, my heart rate’s higher.

Dr. Corey Bradley: Yeah, I think one thing that these devices are making people aware of is how dynamic our heart rates are throughout the day. And that’s good. You want your heart to appropriately respond to times of stress, and you also want it to be as slow as possible within reason, obviously, when you are at rest, because having less stress on your heart when you don’t need there to be stress on your heart is the ideal.

And so people who wear these devices often notice that their heart rate does fluctuate a fair amount, and that can be from a variety of things. Caffeine, you know, that will certainly increase our heart rate. Having any sort of physiologic or psychologic stress can increase our heart rate. Hot water increases our heart rate just because typically hot water causes our blood vessels to dilate.

And so in order to maintain a blood pressure, for instance, in a shower, your heart rate has to increase to be able to compensate for your blood vessels dilating. So that can cause your heart rate to go up. Obviously our heart rates often get up even when we just get off the couch and go on a short walk within our apartment or home. Heart rates can also, as you mentioned, increase sort of for other reasons, or that might be signals of other things going on in your body. Some of the most common things that we see medically that can cause these types of changes are like anemia, omeone develops iron deficiency, or thyroid disease can cause your heart rate to become too fast or too slow. And those can often be things that we identify, even perhaps before other symptoms have developed.

Courtney: Well, so what about stress? Because that’s part of the reason I took a break from my watch, because it just interpreted like going to see a play at night as like, are you okay? Like, your stress has been so high. So is it your heart rate’s changing and that’s indicating that you’re stressed?

Dr. Corey Bradley: So, and I think that’s something that can certainly be like a negative aspect of having these devices be so widely used is that sometimes having all this information delivered to us in real time just isn’t really beneficial to our mental health or to our physical health.

And so, you know, I see a lot of patients who like, let’s say they’re a student and perhaps it’s finals and they notice that their resting heart rate has gone up. And unfortunately, our heart rate is responding to cortisol and our stress hormones and that is normal. I mean, that is a normal response to those types of things. Our body doesn’t know if we’re being chased by a zebra or if we’re just stress out about an email we just got, you know…

Courtney: It’s like the biggest modern problem.

Dr. Corey Bradley: Exactly, and so our heart rate will respond to both of those stressors.

What’s important is not so much what your heart rate does in those moments of stress, because it’s okay if it goes up, we want to know that it’s recovering. And so when you are coming down from those stressors, does your heart rate come down as well? You just want to make sure your heart rate’s coming back down to normal levels afterwards.

Courtney: So, how concerned should users of smartwatches be about low heart rate warnings?

Dr. Corey Bradley: In general, for most people, not concerned. And so, and some of that, too, is the context. So, it is normal, it’s healthy for our heart rates to slow down quite a bit when we’re sleeping. Having a slow resting heart rate is also good. And so as long as you are not symptomatic from it, and by that I mean you’re not dizzy, you’re not lightheaded, feeling like you’re gonna faint.

Having a slow heart rate is not a bad thing. What we also want to look for though is that when you exercise, does your heart rate increase? Because sometimes you can have a slow heart rate, and that’s fine when you’re at rest. But when you get up and you try and do more activity, if your heart rate can’t augment to help you achieve that activity, then you might start to feel lightheaded.

You might feel like you’re too tired easily, and you can’t complete the exercise. So resting heart rate being slow, not a problem, actually a sign of a healthy heart, however, we need to also make sure that your heart rate appropriately increases with exercise, and can help you sort of get through an exercise before it goes back down back to being slow again.

Courtney: So, I’m so excited for this next question because I get to actually ask a doctor versus Googling it because I’m always so curious about heart rate variability. That’s something that always pops up, like when I wake up in the morning. Can you describe what heart rate variability is and is it an important measure of heart health?

Dr. Corey Bradley: Yeah, so heart rate variability in its simplest form just means that your heart rate is changing throughout the day. So we want to see, normal heart rate variability means that your heart rate can slow down when you’re at rest or when you’re sleeping and then it appropriately increases when you’re doing activity.

And so heart rate variability is a sign of cardiovascular health. It’s also a sign of good fitness. And so that definitely can be a marker that things are on track, that there’s nothing sort of else going on. When people develop heart disease, we can see that their heart rate variability can diminish.

So for, so not for everyone is wide heart rate variability necessarily expected. But in general, it is a sign of cardiovascular health and of good fitness. And so a change in that might indicate that you’re a little bit deconditioned, you know, meaning that you’re sort of getting a little bit more out of shape.

Courtney: So in that same vein, let’s talk about sleep quality. I think it’s one of the biggest metrics that people seem to enjoy tracking on their smart watch devices. And you touched on this before, but can we go into a little bit more what’s being measured while we sleep and how accurate are these sleep quality measures?

Dr. Corey Bradley: Yeah. So in general, the sleep quality data is coming from measuring your movement while you sleep and your, and the change in your vital signs, the change in your heart rate. And so they’re not perfect. It might be that with what it is detecting as you waking up or stirring, you know, perhaps you are a very active sleeper, but in the morning you might not actually recall that you were moving that much. And I think that what’s important to know is a lot of these devices are still very new. A lot of this technology is still very new. Whereas technology for measuring heart rate has been around for a very long time and has improved quite a bit. So the sleep data I think is probably the data to interpret with the most caution.

The algorithms that they use are often proprietary, so it’s, you don’t necessarily know the exact technology, the exact inner workings of how they’re generating the sleep score based off of the information that they’re measuring. However, you know, I do think that it can be a helpful place for people to start if you’re, if you have an issue with being overly tired during the day.

I think getting a little bit more information about, you know, am I moving a lot when I sleep? Am I sleeping restlessly? That can be a helpful sort of starting point.

Courtney: And so you mentioned earlier about electrocardiograms or ECGs. Is this how some watches can detect irregular heart rhythms?

Dr. Corey Bradley: Yes. So the smartwatches that do collect ECG data, they are designed to be able to discriminate between what we call normal sinus rhythm, a normal regular rhythm that generated through the sort of traditional electrical pathway in the heart versus the most common irregular rhythm, which is atrial fibrillation.

And so these watches have been marketed and tested to be able to alert someone if they go into atrial fibrillation. Now, what’s important to know is that there’s a lot of different things that can cause temporary irregular rhythms. Some people have early extra beats, but they’re actually not in AFib, they’re otherwise in a normal rhythm. They just have what you call ectopic beats, these early triggered beats. And that can cause your watch to think that you’re in AFib. If you have a series of these, you know, within sort of a small amount of time, your heart rate will appear irregular and the watch might alert you that you’re in AFib.

Similarly, sometimes if there’s artifact, so, for instance, if you’re moving, or let’s say you’re using the elliptical and you’re doing an activity where you’re using your hand. You might end up, it might have noise in the background, meaning that the baseline is a little bit erratic. And so it’s picking up artifact and noise when it’s trying to determine your heart rhythm. And so as a result, that can lead to false alerts for atrial fibrillation.

Courtney: What should smartwatch users do if they get an irregular heart rhythm alert?

Dr. Corey Bradley: So, what I would recommend is seeing if your device does have an option, like in an app, to be able to look at the raw data, to be able to look at the ECG data that generated that alert, then I would advise sharing that with your healthcare provider so that they can see that information. If it doesn’t, or if perhaps, you know, it doesn’t save, or for whatever reason, you don’t have that data, you just have the irregular heart rhythm alert.

I would bring that up with your provider, because it might mean that we either need to look more into the data that you’re getting from your device, or that we might want to do an external heart monitor, where we give people a monitor that they can wear on their chest for a defined period of time, anywhere usually from 24 hours to two weeks, and wear that monitor to be able to detect if we’re finding any evidence of what might have triggered that alert.

Courtney: So we’ve covered a lot about collecting data at home. Is there such a thing as too much information?

Dr. Corey Bradley: I definitely think so! You know, we as humans are very curious. I think we’re always curious about our health. I think this is an exciting time where patients have the ability to collect so much data independently rather than relying that on going to a doctor’s office and collecting this information there.

And so I think it’s important to reflect personally on what you’re collecting this information for, what types of questions you’re trying to answer. If it’s just to kind of optimize your personal fitness, you know, I think that’s great and I think it can be empowering. People see real time results and that can be really rewarding. But I do always like to advise people that these devices are validated for recreational use.

You know, these are not validated to be used in an experimental or in a medical setting. And so to keep that context in mind, they’re not going to be perfect. If you notice a change and you certainly, if you’re having symptoms that make you concerned, again, whether it’s about sleep disorder or fast heart rates, you know, we always want that information to be verified by devices that are approved for medical use.

You know, the heart rate monitors that we’ve mentioned, sleep studies. So I think they’re a great starting point. But certainly some people really begin to obsess over this information. It can create a fair amount of anxiety. And so I would just urge people to sort of be honest with themselves about whether or not they feel like this information has been helping them or perhaps creating a greater distraction and more of a source of stress.

Courtney: Dr. Bradley, thank you so much for being our guest and sharing all this amazing information about heart data.

Dr. Corey Bradley: I’m so happy to have joined, and I can’t wait to be back.

Courtney: Our many thanks toDr. Corey Bradley. I’m Courtney Allison.

Health Matters is a production of NewYork-Presbyterian.  

The views shared on this podcast solely reflect the expertise and experience of our guests.

To learn more about Dr. Bradley’s work with patients, check out the show notes.

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