The Ultimate Cardiologist's Guide to the Smartwatch ECG

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Introduction

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.

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Electrocardiogram

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.

You've probably heard or read a story about someone who was first alerted about their irregular heartbeat by their watch — specifically, a smartwatch with ECG capability built in. With an increasing number of people wearing these devices, the accounts of how they helped someone detect a health issue are becoming more and more common.

But these stories can sound almost too good to be true.

It's easy to believe that a smartwatch can improve your overall health — that's why you wear one. But can it really warn you of a heart condition silently developing?

It's just a watch, after all.

According to Dr. Maged Amine, an interventional cardiologist at Houston Methodist, it turns out that smartwatches with ECG capability can play a role in screening for and monitoring at least one heart condition — atrial fibrillation (AFib), one of the most common types of arrhythmia. In fact, through its Medical Device Development Tools program, the FDA recently designated Apple's AFib History Feature as a qualified tool for assessing estimates of AFib burden within clinical studies.

"AFib occurs when the rhythm of the heart becomes irregular, usually too fast," says Dr. Amine. "This leads to reduced function of the heart. It's a common heart condition that is often asymptomatic, meaning it doesn't always manifest with obvious symptoms. Unfortunately, the first presentation of AFib is often having a stroke due to blood clots in the heart."

In fact, AFib increases a person's risk of having a stroke five-fold, making it one of the leading causes of stroke.

"Now, with people wearing devices with built-in ECG capability all day, every day, we're seeing the emergence of a convenient way to broadly screen for AFib," explains Dr. Amine. "And this technology can also help improve how we monitor this condition long-term, too."

What is an ECG and what is its role in detecting AFib?

"Ideally, we catch AFib before it leads to stroke," explains Dr. Amine. "And whether performed in a doctor's office or via a portable device worn at home for a few days, we've used electrocardiogram (ECG) to help detect AFib for decades."

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An ECG measures how fast the heart is beating, providing clues whether a person's heart rhythm is normal or irregular. Traditionally, this test uses electrodes attached to your chest and other parts of your body. The information collected by these electrodes is sent to a computer, then reviewed by your doctor to determine whether your heart rhythm might be irregular.

Your doctor may recommend an ECG if you're noticing heart palpitations, chest pain or shortness of breath as well as the more subtle signs of AFib such as lightheadedness and extreme fatigue. An ECG is also part of the screening practice for those who are at high risk for heart disease or have a family history of heart disease. (Related: 5 Commonly Ordered Heart Tests & What They Show)

"But, given that AFib is fairly common and can present without symptoms, a simpler and more broad way of screening for it would be very beneficial — helping catch AFib in people who are asymptomatic and not eligible for traditional ECG screening," adds Dr. Amine. "The thought is that smartwatches with ECG capability could perhaps help fill this gap."

There is a precedent — not with a smartwatch, but with a device that functions very similarly. A 2017 study assessing whether regular use of an at-home ECG device could more effectively help screen for AFib found that it uncovered more cases than standard screening guidelines.

Are smartwatches with ECG capability really smart enough to detect AFib, though?

The ECG technology in a smartwatch uses LEDs flashed against your skin to detect blood flow and measure your heart rate. Sensors collect this information and algorithms process it to make sense of your heart's rhythm. Your smartwatch takes these measurements throughout the day, and, if an irregular rhythm is detected, it sends you an alert.

Because it's so different from the traditional ECG technology used in the clinic, you might be skeptical about whether what's crammed inside a tiny watch is truly sophisticated enough to detect if you're in AFib. But Dr. Amine gives these devices high marks.

"The ECG technology in smartwatches is very accurate," Dr. Amine explains. "And people have options with these watches now. The Apple brand was the leader in this technology, but other brands have followed, including Fitbit and Samsung."

Their accuracy and utility haven't gone unnoticed. Certain series of smartwatches made by the brands mentioned above are FDA-approved to help users identify signs of AFib.

If you wear one of these devices and get an alert, don't brush it off. Your next step is to check in with your doctor. He or she can order the tests needed to confirm or rule out AFib.

ECG-capable smartwatches can also help better monitor AFib

The implications of having on-demand ECG technology in a personal device that's worn frequently by many people is probably most readily apparent for screening of AFib.

"But these devices can also be very helpful for the long-term monitoring of those already diagnosed with AFib, too," explains Dr. Amine.

That's because AFib, unfortunately, can come and go. This means ECGs aren't just used for the initial detection of AFib, they're also an important part of regular checkups after treatment — used to help catch it in case it returns.

"What's been missing is a convenient way to continue monitoring a person's heart very closely so that we're able to quickly detect AFib if it comes back before their next checkup," says Dr. Amine.

In some cases, an implantable ECG device is used to monitor a person's rhythm for several years after treatment. But implanting this device is a medical procedure, and Dr. Amine points out that this option isn't right for everyone.

"Amazingly, these smartwatches are almost as accurate as implantable ECG devices — with the added benefit of not requiring a medical procedure," Dr. Amine adds. "They're amazing devices."

Additionally, Dr. Amine points out that smartwatches might eventually help ease the reliance on blood thinners after AFib treatment.

"Treating AFib usually includes taking medications that help control the heart's rhythm or undergoing a procedure called ablation therapy," explains Dr. Amine. "It also almost always includes using blood thinners to reduce the risk of stroke. Once we get someone out of AFib and back into a normal rhythm, they often wonder when they can get off blood thinners. It's an important question to consider since there are risks to taking these medications."

But, there's risk in stopping blood thinners too soon since it leaves a person more vulnerable to stroke if AFib does return — especially if it returns without symptoms. But perhaps smartwatches may someday help safely discontinue blood thinners in some people.

"If we can rely on these devices for long-term monitoring of AFib, we can use them to help inform if and when we should re-initiate treatment," Dr. Amine adds.

A smartwatch doesn't substitute the healthy lifestyle behaviors that help manage AFib, though

Smartwatch aside, the best way for people with AFib to manage their condition is to regularly check in with their cardiologist and prioritize the healthy lifestyle behaviors proven to reduce the chances of AFib, such as:

"Your smartwatch can be an added layer of protection, though," says Dr. Armine. "It can provide your doctor with more information that can help guide treatment, as well as help you better monitor your condition and avoid visits to the ER. I hope someday this can be something we prescribe to patients to help monitor AFib. These devices are costly, but they are powerful."

(Related: Can a Fitness Tracker Help Improve Your Heart Health?)

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