The Ultimate Cardiologist’s Guide to the Smartwatch ECG

Smartwatch ECG
Dr. Marco Perez
Dr. Marco Perez is a Stanford Associate Professor of Medicine, Stanford Clinical Cardiac Electrophysiologist, Co-PI of the Apple Heart Study, and Co-Founder of Qaly.

Key Takeaways

Dr. Marco Perez is a Stanford Associate Professor of Medicine, Stanford Cardiac Electrophysiologist, Co-PI of the Apple Heart Study, and Co-Founder of Qaly.


Advances in electrocardiogram (ECG) technology are truly remarkable. Not long ago, if you had worrisome symptoms, you had to either visit your doctor to get an ECG, call 911, or drive to an emergency room. Then came the revolutionary Holter monitor in 1962, which allowed you to take a continuous ECG reading in your own home. For a long time, it could only record continuously for 24 hours, and you needed to get lucky to catch any symptoms during that time. But then, newer monitors, including the more convenient ECG patch monitors, stretched that monitoring interval to a few weeks; however, these require a prescription from your doctor.

Now, you can go out and buy a heart monitor that you wear on your wrist, without a prescription from your doctor. It can automatically monitor your pulse multiple times a day, and to take an ECG, all you have to do is put your finger on the watch’s crown. This new heart monitor, known as a smartwatch, would’ve been unfathomable to Dr. Holter 50 years ago. I, too, still remember marveling at the quality of the tracing when I took my first Apple Watch ECG only two years ago.

So what can your smartwatch ECG do? And what can it tell you about your heart? Just what are all the features packed into that heart monitor on your wrist? In this post, I’ll walk you through these questions and help you understand what your smartwatch is designed to do and importantly, what it is not designed to do.


For us cardiologists, the ECG is the most common test that we order for patients. It gives us an electrical picture of how the heart is beating. We use the ECG to tell if electrical signals are flowing smoothly, if there is injury to the heart, if the heart is too big, or if there are other medical conditions.

The ECG you get from a smartwatch gives you a limited view of this picture. Instead of the 12 “leads,” or angles, that you’d get from the ECG at your doctor’s office, you get one lead: “lead I.” The good news is that you can still learn a lot about your heart with this single lead. When professional photographers are asked, “What is the best camera you can get?,” they often respond, “The one you have on you when you need it.” Similarly, taking a single-lead ECG anytime you wish is invaluable.

Note that the ECG application on the Apple Watch and similar devices were designed for one task: to detect Atrial Fibrillation (AFib). Why? AFib is the most common arrhythmia, affecting over 1% of the US population, and it is associated with an increased risk of stroke, along with other poor outcomes. So, it makes sense to focus a lot of the features on this abnormal heartbeat. The FDA has specifically cleared the Apple Watch ECG for this purpose. In fact, when you take your ECG with this device, the only medical condition that it can tell you about is AFib.

The ability of smartwatches to identify AFib correctly from the ECG is not perfect, but it is very good. There are, however, some limitations to keep in mind. If the heart rate is too fast, which often happens during AFib, the app is unable to classify it as AFib. If there is too much electrical noise, which can happen if you are moving or contracting your arm muscles, then the app cannot interpret the ECG. If the electrical signals are too small, it can also be challenging. Some people, for example, have an irregular heartbeat due to extra heart beats or just normal variation, and if the ECG signal is too small, the watch may get confused. But despite these limitations, the smartwatch ECG usually does this one job of AFib detection very well.

Not only does the ECG app aim to detect if you have AFib, but it also stores the ECG reading, which you can then convert to PDF to send to your health care provider or review yourself. As a cardiologist, I believe the quality of the Apple Watch ECG tracing is usually high enough for me to read and make an interpretation. I’ve had several patients identify AFib with their smartwatch, and I’ve been able to validate by reviewing their PDF tracings. And although you only get a single lead with the smartwatch ECG, there are many insights beyond AFib that a trained professional can glean from reading that one lead.

Most importantly, with a clean recording, we can learn what the heart rhythm actually is. The heart rhythm describes the way that the heart beats electrically. “Normal sinus rhythm” is a description of the normal way that the heart beats – with organized contraction at the top chambers of the heart (atria), followed by organized contraction at the bottom chambers (ventricles). AFib is just one of many types of abnormal heartbeats that can cause palpitations, fatigue, shortness of breath, or similar symptoms. There are also extra heartbeats or premature beats, fast heartbeats such as supraventricular tachycardia and ventricular tachycardia, slow heartbeats such as heart block, and other abnormal electrical problems. The tricky thing is that many of these abnormalities can come and go quickly. To get a diagnosis, you need to catch them at the moment of your symptoms. If you have an abnormal electrical heartbeat other than AFib, the ECG app won’t tell you what it is, but you can send a PDF of your ECG to your health care provider, or to Qaly (on Android or iPhone) for a cardiographic technician’s interpretation.

In a few cases, heart attacks, which are problems with blockages in the blood vessels around the heart, can be picked up on the single lead ECG; however, this is not a reliable way to assess whether you might be having this serious condition. Also, a single lead ECG usually does not provide enough information to rule out structural abnormalities of the heart. For these, a full, multi-lead ECG is needed, along with additional heart testing. Likewise, an ECG is not used to tell if you are having a stroke, lung problems, or other serious cardiovascular conditions. This is why we recommend talking to your doctor if you are experiencing any new symptoms, and not relying solely on the smartwatch ECG.

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Irregular Pulse Notification

One major limitation of the ECG is that you must actively check it by starting the app on your Apple Watch, and then physically placing your finger on the crown of the watch for 30 seconds. However, believe it or not, there are people who have conditions like AFib and don’t feel anything. Or, they might feel a bit off, but not know what is going on. In these cases, it may not even occur to them to take an ECG.

Interestingly, there is another way that your Apple Watch can monitor for abnormalities in the background, without you having to take any action. The Apple Watch has an LED light and a light sensor in the back. It shines a light, and using the light reflected from your wrist, it can measure your pulse. If your pulse is very irregular, and the irregularity lasts long enough through multiple checks, then your Apple Watch will notify you of an irregular pulse.

All you have to do is turn this feature on, set it, and forget it. If you’re notified of an irregular pulse, it doesn’t automatically mean that you have AFib, since there are other things that can cause an irregular pulse. However, AFib is the most likely case. In fact, in a large study we conducted using the Apple Watch, we found that 84% of the time, the irregular pulse was consistent with AFib [NEJM study]. Most of the other times, the irregularity was caused by frequent extra heartbeats.

If you ever do get this alert, and if you have access to an ECG, I recommend you take an ECG recording as soon as possible. AFib can go away, and it can be difficult to catch otherwise. If you talk to your doctor after an irregular pulse notification, they may suggest additional testing such as using a clinical heart monitor to see if you can catch an episode – but again, the episodes can be so intermittent that you can miss it altogether. In fact, in the study mentioned above, we were able to catch an episode of AFib only 34% of the time when participants used a clinical heart monitor two weeks after the notification. If you ever do get this notification, and if you cannot catch it on a monitor, you may want to consider getting a wearable device that you can then use to make an ECG recording the next time it happens. An irregular pulse algorithm is present on all Apple Watches, Series 1 and later. Fitbit is currently studying a slightly different version of an irregular pulse algorithm.

These algorithms will not catch every episode. They are not designed to. We know that longer episodes of AFib place you at higher risk of strokes, and that very short episodes represent a low risk. The Irregular Pulse Notification algorithm takes at least an hour to detect an irregular heartbeat that could be due to AFib. This means that an episode of AFib that lasts just a few minutes will not trigger an alert, which is why having an ECG device is helpful. The irregular pulse algorithm is most useful for those who don’t feel their AFib episodes, or those who might have symptoms like fatigue but wouldn’t think about checking an ECG or learning how to read ECG results.

High Heart Rate Notification

There are many reasons why your heart might start beating quickly. If you are exercising or moving around, then your heart could start picking up its beat rate, which is normal. However, if you are sitting still and your heart starts beating fast, then there may be something going on. The Apple Watch’s High Heart Rate notification uses the LED and light sensor to measure your heart rate periodically. It’s smart enough to tell if you are moving, so it won’t alert you unless you have been still for 10 minutes.

A high heart rate while sitting still can happen if you are anxious, or emotionally excited. This is a normal physiologic response. However, if you are not moving and you are not emotionally excited, and your heart starts beating rapidly, then a fast heartbeat could be abnormal. It’s possible that your heart has a rapid and unusual electrical abnormality. It’s also possible that your heart starts beating quickly for reasons that have nothing to do with your heart, such as an infection or other medical problem. That being said, some people can have fast heartbeats for benign reasons, due to normal variation in their response to body position or hydration levels.

If you get an alert like this, and if you don’t feel well in any way, it may be time to call your doctor. If you feel OK, and if you have a device that can check your ECG, take an ECG. Remember that you have the option to adjust the cutoff for the High Heart Rate notification, from 100 to 150 beats per minute. Most people can leave this at the default 120 beats per minute, but if you are regularly triggering this alarm, you may want to make adjustments or turn it off altogether.

Slow Heart Rate Notification

Similarly, there are many reasons for a slow heart rate that can be completely normal. Athletes often have resting heart rates that dip into the 40s or even 30s. Some endurance cyclists get their heart rates as low as 28 beats per minute! Medications such as beta blockers can also slow down your heart, which may be desirable in some people with certain heart conditions.

However, if your heart rate is normally higher, and you suddenly start having a slow heart rate, then there may be something going on. You could be developing an abnormality in your electrical system. There are a few other reasons for new and profound slowing of the heart rate, such as changes in your electrolytes or infections such as Lyme, but most people who experience a sudden and abnormal drop in heart rate have a problem with their electrical system.

If you get this alert, and don’t feel well for any reason, you know the drill: call your healthcare provider. If you do feel OK, take an ECG, since it can tell us where the slowdown is happening, which in turn tells us a lot about the treatments you might need. As with the high heart rate notification, you have the option of adjusting the cutoff from 40 to 50 beats per minute. Most of us can leave this at the default setting of 40 beats per minute. Again, if you know that your heart rate regularly dips below this value and you know this is normal for you (perhaps you are a marathon athlete), then you may want to turn this feature off.


We are just scratching the surface in a new era of wearable technology. From glucose monitors and oxygen saturation today, to exciting new technologies like electrolyte tracking tomorrow, devices are quickly improving. Consequently, we’re increasingly gaining the ability to take control of our own health. We no longer need a prescription to take an ECG. We can check our heart status as often as we like. So, let’s educate ourselves. Let’s learn what our health devices can and cannot do. Let’s understand that these are not replacements for routine medical care, but that they can augment the care we receive in new and unique ways. When in doubt, seek the advice of your health care team. But hey, maybe one day, you’ll even learn how to read your own smartwatch ECG! Stay heart healthy, everyone!

- Dr. Perez

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Heart feels off? On the Qaly app, human experts will interpret your ECGs for arrhythmias and irregular heartbeats within minutes. Get started for free today.

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