Wearables get serious about heart rate

At 39 years old, Eric Isakson felt he was in the best shape of his life, but his heart told a different story. It was 11 a.m. when he looked down at his fitness tracker and noticed he had already burned 9,000 calories after sitting in meetings all morning. His heart rate: 155 beats per minute, well above the normal resting rate.

"I didn't feel a thing, that was the craziest part of this whole ordeal," he says. "I thought it was a problem with my Fitbit ."

After resetting it and getting the same results, he decided to see his doctor just in case. Three hours later he was in the emergency room getting treated for an atrial fibrillation (AFib) episode brought on by a leaky heart valve, a condition that if left untreated could've been fatal.

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Heart rate can tell a lot about your overall health; all you have to do is listen to the signs. Or in Isakson's case, look down at your wrist. "It's one of the easiest ways to gauge how well your body is working," says Dr. Anthony Luke, director of primary care sports medicine at the University of California, San Francisco.

Smartwatches and fitness trackers with heart rate sensors have made it easy to keep tabs on your ticker without seeing your doctor. But they're starting to do a lot more than just track your data. The Apple Watch already lets you know when it detects a spike in heart rate, and the company's newest Series 4 Watch will be able to take an electrocardiogram (EKG or ECG) to help screen for serious medical conditions like AFib that increase the risk of stroke. And other wearable makers like Fitbit and Garmin may not be too far behind. Both are developing similar screening features for AFib and sleep apnea.

Potential limitations of tracking technology still stand in the way, but the end goal of these companies is to elevate heart rate trackers from workout buddy to medical device.

Using your heart

There are different ways to measure heart rate. Medical professionals use an EKG machine, which records the electrical signals generated by the contraction of the heart muscle through a series of electrodes placed on the chest and limbs of the patient. A physician then uses this information to detect abnormalities in the rhythm and structure of the heart. Changes in the electrical pattern can also be used to diagnose a heart attack.

Some athletes use the more portable option of heart-monitoring chest straps. Like an EKG, they measure electrical activity in the heart using a single sensor that needs to be strapped as close to the heart as possible. Chest straps are generally worn during exercise and the information is generally used to achieve training goals rather than to warn of something wrong.

Wrist-worn trackers use a different type of sensor called a photoplethysmogram (PPG) that uses light to measure how much blood the heart is pumping under the surface of the skin instead of electrical activity. As the heart beats, blood flow increases and absorbs more light. Between beats, when there's less blood, more light is reflected back to the sensor. This resulting measurement is your pulse.

Dr. Mintu Turakhia, executive director of Stanford University's Center for Digital Health, says this measure is usually accurate for most normal conditions and normal rhythms, but it can lose accuracy under certain conditions.

Movement is one of the variables that can interfere with a good read. While resting heart rate tends to be spot-on, the sensor may have a harder time getting an exact read during high-impact activities when a wearable is bouncing up and down the wrist. Tattoos can also be a problem as they may block the light from penetrating the skin.

Becoming medical devices

For average consumers looking to increase their general fitness level, the drawbacks of PPG technology may be acceptable. But improving accuracy is critical if fitness trackers using the sensors are ever to be relied on to screen for bigger problems.

To bridge the gap, companies like Apple , Fitbit and Garmin have conducted studies on how accurately their devices screen for conditions like AFib by taking heart rate data and feeding it into machine learning algorithms. The goal is to help translate the data into meaningful results for the user.

Shelten Yuen, vice president of research at Fitbit, says hardware and software are equally important in measuring heart rate. "You don't have one without the other," he says.

These device makers are also adding new sensors to work alongside the PPG sensor. With the Apple Watch Series 4, introduced in September, Apple has brought EKG technology to the wrist. The watch uses electrodes in its digital crown and back crystal to measure the electrical activity of the heart. After placing a finger on the digital crown, you receive a notification whether your heart is beating normally or if the beat is irregular. The information is stored in the Apple Health app and can be shared with your doctors. The EKG app has been cleared by the US Food and Drug Administration (FDA). 

"It doesn't give you as much information as a full 12-lead EKG, but it's richer data than a simple pulse recording," says Dr. Gregory Marcus, a cardiologist and director of clinical research at UCSF. "Theoretically, beyond AFib, a trained physician can glean a substantial amount of info and make multiple diagnoses based on a single read."

It's not the first wearable EKG monitor, but it is the first direct-to-consumer device for your wrist. Before the Series 4, Apple Watch users could purchase a separate watchband accessory called the KardiaBand with a similar EKG feature that used the watch's heart rate sensor to detect abnormal heart rate.

Fitbit and Garmin rely on PPG technology for heart rate information but have also added pulse oximetry sensors to their newer devices that can measure the amount of oxygen in the blood. Paired with heart rate data, this information can be key to detecting sleep-related issues like sleep apnea, asthma and allergies.

Heart alerts from your wrist

Beyond making their devices more accurate, manufacturers like Apple have also started to become proactive about how they use heart rate information. Even before its EKG-equipped Watch, Apple added a feature called Elevated Heart Rate notifications with WatchOS 4, released in September 2017. Users who opt in receive a notification when the watch detects a spike in heart rate during a period of inactivity of about 10 minutes. So if, say, you've been lounging on the couch and your heart rate is through the roof, the watch will let you know.

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That's exactly what happened to Heather Hendershot. The 26-year-old mom of two was watching TV with her husband when she received the alert on her wrist. "I thought that the watch had to be wrong because I couldn't feel my heart racing," she says.

After monitoring her heart rate overnight, she decided to take a precautionary visit to the ER the next day, where they confirmed what the watch had been telling her.

Doctors diagnosed her with hyperthyroidism, a condition where the thyroid gland produces excess thyroxine hormone. If left untreated, it could lead to life-threatening complications.

"I am not someone who checks their heart rate randomly," Hendershot says. "So I'm very confident that I wouldn't have been able to detect it without the Apple Watch."

Highs and lows of tracking

But Marcus says an elevated heart rate (or low heart rate for that matter) doesn't always mean there's a problem, as it can also be triggered by other factors like stress. "As physicians, we still take this kind of information with a grain of salt," he says.

Take the 2018 NFC divisional playoffs, for example. The game between the Minnesota Vikings and the New Orleans Saints had fans at the edge of their seats and Apple Watches on high alert. Users on Twitter began posting pictures of their Elevated Heart Rate notifications while they were sitting on the couch watching the game.

Marcus says the benefit of the Apple Watch Series 4 is that it can provide valuable EKG information for the doctor to consider alongside the notification. "Ultimately the physician is the only one that can make an accurate diagnosis using the primary image from that EKG," he says.

He also says that sometimes tracking can cause more stress. "In general I suspect this information is helpful," he says. "But there are some rare cases where I've told patients to take their fitness trackers off because it's making things worse."

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From tracking to screening

FDA clearance is necessary before a company can sell and market a wearable as a medical device in the US. (FDA approval is a separate and more rigorous process.) Prior to the Apple Watch Series 4, the KardiaBand watch strap was the only Apple Watch accessory cleared by the FDA to detect abnormal heart rhythm and AFib.

Vic Gundotra, CEO of AliveCor, the KardiaBand's manufacturer, says it took the company four years and $40 million to get FDA clearance. The EKG app on the Apple Watch has already cleared that hurdle, and others may soon follow. Apple, Fitbit, Samsung and Verily (like Google, a subsidiary of Alphabet) were among the nine companies selected for the FDA's precertification pilot program, designed for companies that want to speed up the clearance process for software for medical devices.

With all this activity in the field, Stanford's Turakhia is optimistic that wearables could play an important role in heart health, particularly for people like Isakson and Hendershot who are otherwise unaware they have a problem. "We estimate that there are almost 700,000 undiagnosed cases of AFib in the US, and most of them would benefit from treatment such as anticoagulation to prevent stroke," he says.

For Turakhia, as for Marcus, the biggest question isn't how to listen to your heart, it's what to do with the information. "Is there benefit to having the diagnostic conveniently as part of your regular life?" he asks. "I think that's the central question that all of us are excited about." 

• This story appears in the Winter 2018 edition of CNET Magazine. Read more CNET Magazine stories here.