HRV vs. VO2 max vs. ECG: Which wearable metric ACTUALLY matters? | Prof. Malcolm Findlay

0:00Welcome to Zoe Science and Nutrition, where world-leading scientists explain how their research can improve your health.

0:13The use of wearable devices that promise to transform our health is booming. But do they work? These watches and rings provide measurements and feedback to help you understand your health and take steps to improve it. Measurements include sleep quality, step count, and others. But the metric getting a lot of attention right now is heart rate variability, or HRV. And it's no wonder. HRV is said to be a good indicator of your stress levels, of your heart health, and your overall well-being. But is it really all that useful to track HRV? Is there anything else that we'd be better off tracking instead?

0:54And how accurate are these wearable devices anyway?

1:03Joining us with answers is Professor Malcolm Finley, a board-certified cardiologist and electrophysiologist and an honorary professor of cardiovascular medicine and devices. A leader in the field of measuring heart health, he's well-positioned to give us the truth. You'll leave today's episode understanding whether a wearable device could help you. How to get the most out of it if you have one? And what are the best things to track to improve your health?

1:33Malcolm, thank you so much for joining me today. Thank you so much. Thanks for hearing me. It's a pleasure. Now, we have a tradition here at Zoe where we always start with a quick fire round of questions from our listeners. And you can answer with a yes or a no, or if you have to, a one-sentence answer. Okay. Can HRV tell you something about the health of your heart? Yes. Can wearable health tech help you look after your heart? I think so. Could focusing on the wrong numbers put your heart at risk? I think if you focus on the wrong numbers, you can do the wrong things and get the wrong idea. So I guess yes.

2:10Can two people with the same heart rate have completely different HRVs? Oh yeah, absolutely. If you want to live longer, is HRV the number to focus on? If you want to live longer, there are lots of things to focus on, and HIV is just one of the many things that you might use to guide you. But actually, I think there are more sensible things you could use to guide you on focusing on HIV. And what's the most common myth that you hear about heart health? Okay, so the most common myth is that one thing, for example, concentrating on doing exercise, can negate concentrating on some really important medical issues such as high cholesterol.

2:52And so just looking after yourself in a general way doesn't negate the need for some proper medical intervention. Well, I'm excited to get into all of that, Malcolm. And actually for today's show, I woke up this morning, I thought I'm going to actually wear two different types of watch, both of which tell my heart rate. So it'll be quite fun to do that as we go through this. And they also tell me my heart rate variability, this HRV thing that you mentioned, which I know is hugely popular at the moment. What is HRV? Heart rate variability is something you can measure from the heart rate. Like the technical side of things is it measures the subtle changes in the beat-to-beat time differences of each heartbeat.

3:34When we talk about heart rate variability with watches and so on, it's not really the big changes that the heart rate will change when doing exercise or when you get excited about something or going to sleep at night. It's more like the subtle wobbles in the heart rate from beat to beat and looking at the course of that over several minutes or even over the whole day. So the measures, quite a mathematically complex measure that was first brought out in the 1970s, I think is a way of guiding medical treatment and risk prediction. I think in hardcore medicine, it didn't fulfill its promise in that people are trying to look for heart rate variability as a good predictor for future disease or future problems.

4:14But actually, it's kind of a weak one compared to some other very serious medical testing that we have, for example, CT scans or MRI scans. But as a wearable device, I think wearables have picked up on this measure because you can take that and it gives another insight about your overall physical general health and general health metrics. Am I healthiest if there is no variability in my heart rate and it's completely steady? It's the other way around. Absolutely the other way around. So the general idea of being healthy would be someone who have both control inputs into their heart, both the sort of fight and flight response, the sympathetic, that's the adrenaline sort of drive, but also the relaxation drive, the parasympathetic that works against that.

4:59And having both inputs from the nervous system to the heart subtly changes how that beat to beat variability happens in the heart. and that is the HRV. And so the HRV being higher, as measured by a watch, really indicates you've got those inputs really strongly affecting the heart. Let's take an extreme example. People with a heart transplant, you measure their HRV, it's completely flat. Why is it completely flat? Does that mean it's zero? It's almost zero. And that's because they've had the nerves to their new heart essentially severed. And over time, there have been cases where people have developed a heart rate variability back in their new transplanted heart, probably because they've regrown some nerve innovation to their heart.

5:43So in an extreme scenario where you remove those nervous inputs, you get almost zero heart rate variability. And so if zero is sort of bad, you would expect variability. Does everyone have basically the same level of this heart rate variability? Is everyone the same? The answer is no. like any human parameter, things vary a great deal. So a sort of average number would be around 50, but it can be anywhere from the low 30s to the high 70s on the typical wearables. And that absolute number doesn't really give you a huge indication on that person's health. And I think that's something we can talk about is how HRV, the exact number, doesn't help us a great deal, But the changes can really monitor whether or not someone's leading a more healthy lifestyle.

6:33The other thing, just to mention, heart rate variability goes down as you get older. Pretty high in the teens. And then it's a downward trajectory ever after that. I'm already fascinated that what's normal is for it not to be steady. Because I think in my mind, you just want your heart beating regularly. The measures you're getting are measured in milliseconds. Like that 50 number is a time variable. So let's not think that the heart is jumping around all over the place. That can be a sign of there being an actual medical problem with the heart rather than it being a measure of health. And I think that in patients with an irregular heartbeat, atrial fibrillation, which is one of the things I see many of my patients with, heart rate variability actually is not a valid measure because in that case, the control systems of the heart have been overrun by abnormal heart rhythms.

7:21It's really a measure that's useful only in people who have the normal heart rate, the sinus rate. I think what you're saying is for a normal healthy person who's measuring their heart rate variability, actually their heart rate is very steady and there's this very subtle change going up and down. Exactly right. And you then said, however, if I'm seeing a bigger change, you mentioned this word atrial fibrillation, I should be worried. Could you talk about that for a minute? Yeah, sure. So just to sort of back up, atrial fibrillation is a specific type of heart rhythm abnormality. It's the most common.

7:52About a fifth of four people will experience it sometime in their life. And actually, it can be pretty serious, particularly if it's there the whole time. Not only is it serious, but it can be treatable. And all the wearables will alert people if they have possible atrial fibrillation or if it's detected atrial fibrillation. And basically, that's been a huge driver for the uptake of the ECG in wearable devices. Now, in that arrhythmia, the top chamber of the heart, rather than being triggered by the sinus node, which is the heart's own pacemaker, there's an electrical chaos going on in the top chamber of the heart, which completely overwhelms any control mechanism for the heart rate from that sinus node.

8:33And so, the ending heart rate is completely irregular, and it's completely driven by these chaotic activities in the top chamber of the heart. And I think until someone like me who puts it right and gets back into the normal rhythm, heart rate variability doesn't really come to play in the way we're talking about it today. It's a disease and it really needs specialist input. If I come back to the HRV, you mentioned that there's these different signals that are driving what's going on. Could you help me to understand a bit more like what's controlling it and what's happening and why? The one we're familiar with is the way the heart rate increases when we do exercise, right?

9:12So that would be the fight and flight response. I mentioned that earlier, the sympathetic activity driving an increase in heart rate. But tugging against that is this idea that you have the nervous system that helps the body relax. So when one sleeps, that really pulls down the heart rate. and it used to be thought that either you were on one either the sympathetic or the parasympathetic system were active and you know if you're doing exercise it would be sympathetic and if you're relaxing parasympathetic but actually in truth both are working at the same time all the time to slightly lower the heart rate or slightly increase the heart rate and the exact ways they're working on the cells in the heart vary very slightly and so the cells will be kind of the timing of those pacemaker cells will wobble from beat to beat.

10:02It won't be a precise clockwork. And that's the wobble that we're measuring in HRV. A bit like putting my phone on the accelerator. An accelerator and a brake. But actually, in a car, you're fairly driving tested, driving the accelerator and brake at the same time. The human body, and actually many complex control systems, have both an accelerator and a brake operating at the same time. I work out regularly because I'm told it's really good for my health. and since I started to have these watches, I actually noticed my heart rate now and sort of see how it changes. And one of the things that I've been most struck by is that my heart rate changes fast.

10:35It goes up fast, but also sort of comes down fast, which I was quite surprised by. I sort of always assumed it was quite a slow and steady change, but it seems like it can change in like a matter of... I mean, if you're going to be chased by a saber-toothed tiger, you don't want to be waiting five minutes for your heart rate to get up before you can run away. So these are deep biological mechanisms which have a huge survival advantage. So if you need to do exercise, your heart rate needs to be able to go up straight away. For decades, we've been led to believe that the brain is completely separate from our body.

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12:20It's a choice you make before you need it. Don't leave your cognitive future to chance. Go to zoe.com slash brain health right now to claim your guide. That's zoe.com slash brain health or click the link in the show notes. And so when you talk about nervous system, like does that mean that there's like sort of signals from my brain going straight through through nerves or? The whole body's connected, right? So the brain, what we think with, is connected to the spinal cord, which also supplies the hormone organs, which will give out hormones for daily function. And I think we're getting more and more used to the idea that every organ is connected to the brain and every organ has an influence on every other organ in the body.

13:05I mean, we're one body. And now the heart, you know, when maybe I was in medical school, the overall thinking was this was a pump and it would just dial up and dial down. But actually, there's this huge number of nerves innervating the heart. These are not directly from your thinking centers, but the thinking centers, the brain, do then pass signals down to influence the nerves actually supplying those heart muscles. And actually, in my PhD several years ago, we were doing some experiments measuring the exact electrical activity in people's hearts whilst we were doing just thinking problems, active relaxation, or even something called anger recall.

13:43So we got people to imagine they were in a very stressful situation and then I'd act the sort of fool to get them very angry. But at the end of that, we were discovering that just having these thinking tasks, psychometric tasks, can directly affect the way the heart cells activate and relax. And the timings of which, the thing I was particularly interested in there was how that could predispose to abnormal heart rhythms. I guess the key learning factor is that the influence of the brain to the heart is extremely deep. So we have these influences which are not just, you know, the heart being a pump, but the whole body connects.

14:21So when you are even just thinking about doing exercise, the heart will start to adapt to get ready to do that exercise. To sort of warming up. Exactly right. Right. So if I understand it right, you're saying there's this sympathetic nervous system, which is the fight or flight, that's going to push my sort of heart rate up. Absolutely. Yeah. And I've got this opposing system, the parasympathetic nervous system, which is sort of trying to get me to relax and is pushing it down. And these are both on all the time. Yeah. And you're saying they're very subtly changing, maybe even by what I'm thinking, that maybe there might be a saber-toothed tiger, or I'm thinking maybe I'm a bit tired or whatever those things are, and that's sort of adjusting my heart rate all the time, are you saying that there's both direct nerves, but also there's these hormones that we hear about?

15:09Exactly right. So the adrenaline hormone can be measured in the blood, but almost more important is the direct nerve innovation to the heart's own pacemaker cells. And so I've got sort of both those things going on? Exactly. So it's not just the nerves, there are hormones as well. So those nerves actually supply the glands that then secrete the hormones. So these things work is a single system. Can I change my heart rate without the hormones? Like, is there also a direct link? Yeah, no, there's a direct link. And this is why it can be so fast? Exactly right. So actually in some medical testing we'll do on people's heart rates, we can stimulate nerves, which will increase and decrease people's heart rate.

15:49So we actually directly stimulate the tiny nerves supplying parts of the heart, which then we see the differences in their heart way. That's fascinating. So you can literally adjust people's nerves to try and get these things back into... Yeah, that's something we do to sort of work out where the best areas are to do treatments. But, you know, there are potentially future treatments that are to come from that as well. So could we talk a bit about in today's world, what is influencing sort of my HRV? Because I'm probably not going to see a saber-toothed tiger. What are the different factors that are likely to be influencing the HRV that, you know, I might end up measured with one of these devices?

16:27Yeah. So, HRV, as we're seeing on our wearables, give a snapshot of how these nervous drives are affecting the heart. This measure can vary quite a great amount between different people. But for ourselves, day to day, it should be fairly similar. In many instances, the trends in that will be an indicator of one's overall sort of health status. So, for example, if you have people who become unwell get an infection you would expect to see the heart rate variability decrease as the body is spending more time fighting the infection the sympathetic system so that's the fight or flight response will be slightly elevated and that heart rate variability tends to decrease likewise in people the same thing's been seen in people who've had a heart attack heart failure or who have quite severe diabetes on the flip side if people are let's say changing their lifestyles, improving what they do on a daily basis, getting a good night's sleep, for example, then we end up finding the heart rate variability will follow those trends and you'd expect to see an increase in heart rate variability.

17:35So that number would generally increase over time. Now you've mentioned a little bit sort of psychological stress, and I know you've actually done some research into the connection between sort of the heart in the brain. Can you tell a bit more about how, as opposed to like maybe an infection, how does stress play into this? Does it increase my HRV? Yeah. So stress, again, would drive that fight or flight response in the way we generally think about mental stress. And so we'd expect things to decrease the HRV. Now, to be honest, the hardcore research into these things is not super well done, but there's a wealth of evidence from patients with, for example, severe psychiatric disorders, clinical depression, as well as patients with severe autism, showing when people are in their worst states, their HRV is decreased.

18:23I've got to say these researchers tend to be from before the wearable era. So these are patients being measured with heart monitors over 24 hours and so on. But I think the data is a good one, that we know that patients who are under severe mental stress will decrease their heart rate variability. It's not really that useful clinically to change treatments. It's really a follow-on. It's like an output measure. So if you've got someone who's needing mental health support, you may find as things get better and things improve, their heart rate variability increases. But it's not great at telling us, oh, this person's heart rate variability is really low.

19:06They need mental health support or they need this doing to them. And what you do to improve things, let's say, tends to come before the heart rate variability becomes useful. Before we move on, I'm conscious that both these watches, in fact, measure a lot of other things to do with the heart beyond HRV. And I think we touched on them briefly. So heart rate is one that I definitely see. One of them says it can do an ECG, which is another thing I'm a bit unclear about what it is, except I've seen it on all those dramas on TV. What are those other things to do with the heart that you can get out of these devices?

19:39Okay, let me just say that ECG is one of my favorite things in the world, right? So this is the direct measurement of the electrical activity of the heart. It's kind of incredible that just this tiny watch can give the real direct indication of those little electrical impulses within the heart, and that can be translated onto the screen on the watch or whichever other wearables are available. these quality of the ECG are really clinical quality. You know, I make diagnoses based on the ECGs that I see from these wearable devices, and they are accurate devices. You would make an actual diagnosis based on the ECG that could come out of my Apple Watch?

20:20Absolutely right. Absolutely right. That is a direct measure of the heart's electrical activity. Every so often we have to be careful, But the majority of the time, I can make a really good diagnosis from the ECG from a wearable technology. And that has really changed the world when we're looking at diagnosing irregular heart rhythms, diagnosing people with super slow heart rhythms, for example, and determining whether people need direct treatments. For those of us like me who still don't really understand what an ECG is, could you explain for a minute what it is? Yeah. So the ECG stands for electrocardiogram.

20:54And this is the direct electrical measure. It's a tracing of the electrical activity of the heart. And typically, in the very first ones performed, it was between the left arm and the right arm in buckets of salt water connected to a mercury galvanometer, which flickered as the electrical balance changed between the two arms, indicating the electrical transmission of the impulses through the heart. And that has gone from being these enormous contraptions, which you can see in the Science Museum of London, if you're interested, to being something that you can get for less than 100 quid to wear on your wrist.

21:34And the neat thing is that tracing is not just an interpreted number. That's the actual recording of those electrical flickers of the heart. And so knowing these and knowing the normal timings between people, it becomes quite straightforward in one sense to then work out exactly what's going on underlying the patient or the person who's having their ECG measured. What's the value of that? How does that tell me something about my health that's important? Yeah, if we just rewind 10 years, 10 years ago, if somebody came to me and said, I feel like my heart goes irregular from time to time, we could say, okay, let's do a tracing of the heart in the office.

22:16We'll do an ECG on the machine. And actually, they're not having arrhythmia at that moment. So they're not having the symptoms. But to get the diagnosis, we really need what's the term is symptom arrhythmia correlation. So we have to do the tracing when people are feeling the symptoms. And if you're wearing it the whole time on your wrist, you can now do that. Anyone with a wearable can do that. And that means that if one develops a abnormal irregular heart rhythm, which for example, atrial fibrillation, as we mentioned earlier, that can give an increased risk of stroke if it's not treated correctly.

22:51So now the watch can detect atrial fibrillation. People go to their GP, oh, I had these funny feelings. My watch says I have atrial fibrillation. GP can give a very simple blood thinner which really removes that risk of stroke not completely but almost completely people have different risk factors so not everyone with atrial fibrillation is at risk of stroke but certainly people over the age of 65 who develop atrial fibrillation well that's really high on the radar people with other health problems that becomes a major risk and so these wearables are not just something which is useful for people on a day-to-day basis to monitor their health and improve their day-to-day health status, but can also prevent serious disease and really catastrophic problems.

23:35Now, it also measures my heart rate, which is different from the heart rate variability, right? Absolutely right. Yeah. So the heart rate is how fast the heart is going. And we expect that to increase with exercise and decrease with rest. And that's something which does have important health status implications, right? So if your heart rate drops low at night, well, that's a good thing. That's implying that you're sleeping very well, that you're having good rest, for example. Likewise, if people are increasing their heart rate significantly during exercise, that's a measure that they're pushing their body, they will be getting benefit from the exercise they're doing.

24:13So these things are important, but they're measuring something different from HRV. One has to be kind of careful that big changes, sudden changes may imply an artifact rather than being a true measure of one's heart health. A great example of that is if people measure an HRV on a day-to-day basis and it's always running about, let's say, just 55, and then one day they're particularly stressed and they measure it down at 20, well, that's far more likely just to be that because you're stressed you haven't attacked your watch properly than a single measure being a measure of a change in your health status.

24:50It's really the consistent, longer-term, subtle changes, which are a more direct implication of one's health status. Within those constraints, would you view these devices as basically quite accurate for these other measures, or are they like a long way behind sort of what you might use when somebody came in to you in the clinic? I think they fill this beautiful gap between guesswork and clinical examination and measuring your pulse in the neck and so on when we're doing exercise. They fill this gap between that and essentially pretty hardcore medical investigations, which are time-consuming, expensive.

25:30Wouldn't it be better if we had a way that people could just have that measurement straight away? And these wearables fill that gap. So I'm a huge fan of the wearables and a huge fan of people using that to take control of their own wellness and their own health. Now, you can spend a lot of money on these devices, and there's a very big range of how much they cost between like sort of the cheapest to the most expensive the key thing is one which measures the ecg i think is important you know the ones which just do a pulse rate well that has its own use the fit the older fit bits and so on but they're not able to go to that next level of actually making that firm diagnosis i actually don't think there's that big a difference between ones which are a thousand pounds and ones which are 40 pounds in fact just an experiment i went onto a website that I didn't recognize and bought the cheapest one that I could find.