Just like your smartwatch’s heart-rate monitor can alert you to possible warning signs of atrial fibrillation or sleep apnea, it can also spot warning signs that might signal your body is fighting a flu-like infection — if you know where to look.
App maker Cardiogram pushed an update to its Apple Watch and WearOS apps last week that adds a new stat that shows average beats per minute during sleep. As it turns out, tracking fluctuations in your heart rate during sleep could signal that your body is fighting a viral infection like COVID-19.
Cardiogram co-founder Johnson Hsieh discovered the correlation after tracking his BPM during a bout with the seasonal flu in January. He noticed that his normal sleeping heart rate was about 10 beats per minute higher while his body was fighting the virus and returned to normal as his sickness subsided. The higher BPM was also evident during other parts of the day, but sleep is where it’s easier to spot.
It’s due to vasodilation, which is a fancy medical term for the expansion of the blood vessels during inflammation. As blood vessels expand, signals are sent to your brain to increase your heart rate and provide additional blood supply to inflamed regions.
“A pretty clear signal in your heart rate when you have symptoms that would otherwise be measured exclusively by a thermometer,” said Harish Kilaru, head of product at Cardiogram. “When your body is fighting an infection, both your sleeping BPM and your resting BPM are higher.”
As Kilaru explained, other factors (such as consuming alcohol) can also lead to an increase in resting BPM, so it’s not an exact science just yet. While Cardiogram still recommends taking your temperature and consulting your physician with any concerns, heart-rate monitoring is one more way to stay on top of possible coronavirus symptoms.
“I think there’s a lot of potential here because we have seen a connection between these symptoms and your heart rate,” Kilaru said.
A recent Fitbit study reached a similar conclusion. Researchers discovered a distinct correlation between elevated resting heart rate and influenza-like illness rates and found the data significantly improved flu predictions. Additionally, weekly changes in resting heart rate closely mirrored changes in influenza-like illness rates.
While the Cardiogram app isn’t available for Fitbit OS, the Fitbit watch will natively track your sleeping heart rate each night. And if you have a Charge 3 or Versa 2, you’ll get an even better handle on your COVID-19 exposure due to their relative SpO2 sensors that can track oxygen levels in your blood.
Like Cardiogram’s method, Fitbit’s SpO2 sensor will record your blood-oxygen levels as part of its Sleep Score. While you won’t get a number, each morning you’ll find an Estimated Oxygen Variations chart that will show your levels throughout the night, which can be invaluable for tracking COVID-19 symptoms. A typical blood-oxygen saturation level is above 90 percent, but COVID-19 sufferers can develop severe hypoxia, which lowers the oxygen levels in your blood due to damage to the walls of the air sacs in their lungs.
Monitoring resting heart rate and SpO2 tracking can be excellent indicators of the flu, but they’re still not as good as an actual body temperature reading. While there are very few wearables that are able to accurately show body temperature right now, that’s probably going to change in a post-COVID-19 world.
But even without the ability to track body temperature, your smartwatch can still be useful in monitoring your own coronavirus symptoms. “Just because your wearable devices aren’t taking your temperature regularly,” Kilaru said, “it still provides you a clear and continuous data point that you can use on a day-to-day basis.”
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