The Apple Watch Series 4 has two new features that could help warn users of life-threatening conditions like atrial fibrillation. Here's what happened when I tested Apple's ECG against a medical-grade EKG at the doctor's office.

When I first started testing out the new electrocardiogram (EKG or ECG) feature on the Apple Watch Series 4 -- which launched in December with the update to Watch OS 5.1.2 -- the last thing I expected was to find something abnormal about my heart rhythm. But that's exactly what happened when I was cross-referencing the Watch's readings with medical-grade EKG equipment at the doctor's office.

"We see on your Apple Watch the same early heartbeat that we see on the EKG," said Dr. Gregory Marcus, professor of medicine and a cardiac electrophysiologist at UCSF Medical Center, as I sat on the hospital bed with cables attached to my body and an Apple Watch Series 4 on my wrist.

"These early beats are very common … but they can lead to problems in the long term. So we should talk a little bit more about that," he added.

Heart-rate tracking has always been a big part of the Apple Watch and fitness trackers in general. But until now, it's mainly been used for activity tracking and calorie counting.

With the update to Watch OS 5.1.2, heart rate will play a more important role on the Apple Watch as we get access to the two new FDA-cleared features that Apple announced at its September keynote. There's an abnormal heart rhythm alert for all Apple Watches, except for the first-generation model, and an electrocardiogram (EKG or ECG) exclusive to the Series 4. Both of these could help warn of potentially life-threatening heart conditions.  

Heart-rate tracking isn't new to wearables. Smartwatches and fitness trackers have for years used LEDs and optical sensors on the back to measure the changes in blood flow under the surface of the skin, aka your pulse. When the heart beats, more blood gets pumped into the blood vessels, absorbing more light. Between beats when there's less blood, more light gets reflected back into the receivers of the watch.

Set up the Irregular heart rate notification from the Health app on the Apple Watch Series 1 and beyond.

In 2017, Apple became proactive about how its Watch uses heart rate information by adding the high heart rate notifications, which let users know when their heart spiked above a certain level, and then later adding  low heart rate notifications. These notifications have already been helping users detect serious conditions. 

But the heart rate only measures beats per minute, or the frequency of the heartbeat over time, and not the patterns between each beat known as heart rhythm.

"You can have a regular rhythm that is very fast or too slow … And similarly, one can have an irregular rhythm that is of a normal rate, that is too fast or too slow," Marcus said.

With the new irregular rhythm notification, the Apple Watch uses the optical sensor to measure heart rhythm and alerts users when it detects an irregular pattern that may be atrial fibrillation (AFib), a type of arrhythmia that can increase your risk of stroke and other serious heart complications. This feature will only work for adults over the age of 22 and won't help if you've already been diagnosed with AFib.  

"Sometimes those beats are so early that the heart hasn't had adequate time to fill, even though electrically there may be an early beat that's happening," Marcus said. "We would want to have an electrical confirmation of a diagnosis of atrial fibrillation before we decide on acting on that and not base it, in general, on the pulse recording alone." 

That's where the EKG comes in. An EKG uses electrodes to measure the electrical activity of the heart. A hospital-grade EKG generally consists of 10 electrodes placed on different parts of your body. The Apple Watch Series 4 has two: one electrode on the back crystal and one electrode on the digital crown.

"This 12-lead EKG shows what's happening electrically in the heart from 12 different perspectives, or 12 different directions, whereas the Apple Watch gives you that same electrical activity, but really in just one direction," Marcus explained.

As he was looking at his monitor, I opened the new ECG app (Apple uses the abbreviation ECG, whereas doctors generally say EKG) on the Apple Watch Series 4 to take my first EKG. I put my finger on the digital crown and waited while the screen counted down 30 seconds. The Apple Watch classifies your heart rhythm as either AFib, sinus rhythm or inconclusive. My result: inconclusive.

The notification on the Apple Watch also said I should contact my doctor if I didn't feel well or if I continued to get the same result. Users can share these results as a PDF with their doctors, but luckily my doctor happened to be standing next to me.  

The EKG on the Apple Watch directly coincided with the results of the printed-out hospital EKG. There were intermittent early beats coming from the lower chamber of my heart.

"This would be really useful [as a way] to screen for this or to have the first understanding that you have these early heart beats," Marcus said. "What's missing in the single lead Apple Watch is the information that tells us more specifically where exactly this is coming from."

Dr. Marcus says I probably won't die from the irregularity he discovered in my EKG that day, but he did ask me to schedule a follow-up to discuss my early heartbeat, something I likely wouldn't have caught without this kind of test. And for people with more serious heart conditions, this could help doctors make a diagnosis faster and allow them to treat the problem sooner.

"Some people feel it when they have atrial fibrillation, but a lot of people don't. So there's this hope that we might detect those people who otherwise didn't know they had atrial fibrillation," Marcus said. However, "The flip side is that we recognize there's a risk of false positive results which could lead to undue anxiety," he said. 

The Apple Watch is the only direct-to-consumer device with a built-in EKG. But there are other devices like AliveCor's FDA-cleared KardiaMobile and KardiaBand for the Apple Watch that give users access to an EKG outside of the doctor's office. Fitness wearable competitors like Garmin and Fitbit are also working to improve their heart rate monitoring as more tech companies focus on healthcare as a way to breathe new life into wearables.  

Irregular heart rhythm notification is already available on all Apple Watches starting with the Series 1 and can be set up in the Heart section of the Watch app. The ECG app is only available on the Apple Watch Series 4 and is currently available in the US, Hong Kong and 19 European countries including Spain, United Kingdom, France, Germany and Italy. Apple says it expects to get regulatory approval for this feature in even more countries later on. 

Originally published Dec. 6, 2018. Update, April 5: Adds information about the availability in Europe and Hong Kong of the ECG app and irregular heart rhythm notifications.

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