When Apple introduced the fourth iteration of its smartwatch, the big new selling point wasn't a feature we typically associate with a watch or any sort of smart device. Instead, the company added a feature that had only recently arrived in the form of specialized consumer devices: an electrocardiograph (ECG), a device made for monitoring the heart's electrical activity.

But the watch was ready before the software was, meaning an examination of the technology wasn't possible in our comprehensive review of the Apple Watch Series 4. Last week, Apple finally enabled the missing features, and we've spent a few days checking them out.

Basic features

People who haven't used the Apple Watch may not realize just how much it's an extension of an iPhone. This includes the heart-monitoring software, which requires an update to both the Watch and iPhone OSes before it will work. (This caused a small bit of confusion when the software wouldn't launch after we upgraded only the watch's OS.) Once the update is done, the Health app on the iPhone will incorporate any ECG data generated using the watch. On the watch side, the update will install a new app.

From there, doing an ECG is phenomenally easy: launch the app, place your finger on the crown of the watch, and wait. For 30 seconds, the electrical activity of your heart will draw a red trace across the watch's screen. It's incredibly convenient—not much more obtrusive than taking your pulse. And as long as you're wearing your watch on a given day, it's always available to you.

Once complete, the app will let you know whether the trace captured normal heart beats, termed a sinus rhythm. The ECG trace, however, can't be viewed on the watch; the only thing the app allows you to do is record another trace. To look at the results yourself rather than trusting Apple's software, you have to switch to the phone.

That's somewhat annoying. The whole point of the watch appears to be to allow you to interact with it rather than your phone. And there's something more disruptive about pulling out your phone and fiddling with your screen rather than taking some quick glances at your watch.

That annoyance aside, the traces look great. Given how far your fingers are from the heart itself, there's a lot of noise in an ECG signal taken by the watch that software has to filter out. Apple's software does an admirable job. The trace is shown as a thick, red line, making it easy to interpret. On the phone side, the display is equally good, and it's available to share through the usual options. Email or Air Drop will leave you with a PDF that you can keep for your records or send on to your doctor.

The evolution of ECGs

Those who have read my previous coverage know I have a personal stake in this system working well. Genetics have left me susceptible to atrial fibrillation, a heart arrhythmia with severe health implications. I currently experience an erratic mix of occasional atrial fibrillation and harmless premature atrial contractions, both of which I can feel as they happen. Medically, I have to monitor things to make sure that the frequency of atrial fibrillation remains low and doesn't start appearing without symptoms I can feel. This doesn't require the detailed information you can get with doctors'-office-grade hardware—a simple outline of the activity is sufficient (but obviously won't be for everyone).

Over the years, this monitoring has changed. It started with week-long sessions with a Holter monitor, a compact, portable version of ECGs that is worn around the clock for a week or so. The hope is that a week is long enough to accurately sample the behavior your heart shows for the rest of the year—something that's typically not true in my case. Over time, I also developed an allergy to the adhesive used to hold the electrodes on.

Around this time, technology caught up with my needs. A company had created a small, affordable device called the KardiaMobile that would read my heart's activity from my fingers and send the data on to my phone. Assuming that I'd continue to be able to feel whenever my heart is out of rhythm, this would allow me to track what type of arrhythmia I was experiencing and send the information on to my cardiologist. Over time, I got a sense of how to interpret the traces as well.

John Timmer

John Timmer

Better, but not ideal. The Kardia device communicates with phones ultrasonically, making it sensitive to the precise positioning of it and the phone; small shifts mid-recording create strange effects on the trace. While it was very portable, it wasn't something I always had with me, and I didn't always have the space to set it up so it could communicate well with my phone. Finally, it wouldn't be able to answer a critical question: have I started to have periods of arrhythmia that I don't notice because they're asymptomatic?

In many ways, the Apple Watch's heart function seems like it's the next step in the evolution of my monitoring. It's on my wrist enough hours of the day that, should asymptomatic arrhythmias start, there's a good chance it will pick them up. (I have it set up to notify me of high heart rates while resting, and I lied to it about my past atrial fibrillation diagnosis so it would monitor for that, too.) It seems to produce a good enough trace that I can pick out premature contractions. And it's not fussy about how it's set up—I could potentially produce a good trace while continuing a conversation at dinner, for example. Yes, it's more expensive than the dedicated device, but it obviously does a lot more than just monitor my heart.

It will take several months of monitoring and a few more incidents of odd heart behavior before I'll get the same sense of confidence I have in my existing routine. But if the first results are anything to go on, Apple has done a fine job with this software.