Smart watches or fitness trackers are increasingly common, with about 1 in 5 Americans regularly wearing one. They can provide a wealth of data about your activity levels, heart rate, and more - but how much of that is actually useful? Can it help you understand how healthy your heart is?

The smart watch and your heart rate

Smart watches and fitness trackers, first and foremost, report your heart rate, or how many times your heart beats per minute. They use technology that can detect pulsatile changes in blood volume under the skin. This technology is generally capable of recognizing heart rates between 30 and 210 beats per minute. When combined with accelerometer data (your movement), it allows the watch to determine resting vs walking heart rates.

You can see your average resting heart rate as well as heart rate variability (HRV), both of which have implications for your heart health.

• Resting heart rate - Technically, a resting heart rate between 60 and 100 bpm is considered normal. That said, many of my young, fit patients have much lower heart rates (such as 40-60 bpm), and in the absence of symptoms like lightheadedness, this is very normal. In fact, a higher resting heart rate has been linked to lower physical fitness, higher blood pressure, high cholesterol, and increased body weight. It has also been linked to an increased risk for premature death, independent of physical fitness and other risk factors. In one study, a resting heart rate between 81 and 90 bpm doubled the chance of death, while a resting heart rate higher than 90 bpm tripled it.

• Heart rate variability (HRV) - HRV measures the fluctuation in time between each heartbeat. It’s a measure of the impact the autonomic nervous system has on the heart. It’s normal to have a heart rate that varies, reflecting the interplay between the parasympathetic (“rest and digest”) and sympathetic (“fight or flight”) tone. There is also a 24 hour circadian rhythm, with relatively more sympathetic tone during the day and more parasympathetic activity at night. Overall increased sympathetic tone (“fight or flight”) decreases HRV, which can occur for many reasons including stress, poor sleep, inflammation, and infection. Reduced HRV has been associated with increased mortality and a greater risk of cardiac events in several population studies. That said, there is a considerable range of normal values even in people of the same age and gender, with normal ranging anywhere from below 20 to over 200 milliseconds. As a result, it’s more useful to monitor changes in your own personal HRV rather than comparing to others.

The smart watch electrocardiogram (EKG)

Many watches can create a single lead electrocardiogram (or EKG). This analyzes the electrical activity of your heart.

In contrast to the 12 lead EKG we get in the clinic, it's much more limited. Since it’s only one lead (designed to mimic Lead 1), it is not very good at detecting a heart attack, for instance. It can, however, be useful to detect some rhythm issues like atrial fibrillation, for which it has been FDA cleared. While the reported positive predictive value in clinical trials is reasonable (~84%), there have been concerns of high real world false positivity rate (meaning, alerts for which no medical problem was identified). It also may not alarm at high heart rates, falsely reassuring the wearer that nothing is amiss. Of note, I've had many patients present with "irregular heart rhythm" on their tracker that wasn’t atrial fibrillation (or an arrhythmia at all), so just know that it's not perfect and never a substitute for consultation with your doctor.

One of its best utilities is for patients who have very infrequent palpitations, typically allowing us to distinguish sinus tachycardia (your normal heart beat, just fast) vs arrhythmia (an abnormal heart rhythm). This can sometimes avoid the need for an implantable loop recorder.

The smart watch and blood oxygen level

The latest versions also measure your blood oxygen level. This is a newer function, so to my knowledge, we don't know much about the accuracy yet (and the technology it uses, reflectance oximetry, is typically much less reliable than the standard pulse oximetry that goes on your finger). That said, if it proves to be reliable, it could be quite useful during the current pandemic, as well as alerting the wearer to possible underlying lung disease or sleep conditions like obstructive sleep apnea. Of note, these functions have not been evaluated by the FDA.

Can your smart watch detect if you have COVID-19?

A recent study found evidence of pre-symptomatic detection of COVID-19 in patients wearing a smartwatch. Researchers found that

• 81% of individuals positive for COVID had alterations in their heart rate, number of daily steps, or time asleep

• 63% of them had detectable changes in these parameters 4-7 days prior to developing symptoms

Although the original cohort was sizable (over 5000), the number of individuals with COVID was small (32), so results should be viewed with caution. Also, they were not able to distinguish between COVID and a different respiratory infection (ie influenza).

In the recent Warrior Watch study (still in preprint), researchers used heart rate variability to identify patients with COVID-19 prior to developing symptoms. They found that individuals with COVID-19 had decreased HRV, consistent with the increased sympathetic tone and inflammation that would be expected with infection.

The take home

Smart watches and trackers can be useful devices to monitor and trend our resting heart rate, heart rate variability, heart rhythm and a ton of other parameters like number of steps, sleep, stress, and VO2. While some of these may be more reliable than others, they can become a really interesting tool to trend your physical fitness and goals. The technology has advanced such that it may be able to provide the wearer with early warnings about your heart health (like atrial fibrillation), but it may also provide false reassurance. In addition, it’s not for everyone, as some people may experience too much anxiety with all of the “monitoring,” and so as with everything, it’s important to evaluate if it’s a good fit.