If you are among the millions of Americans who wear a wrist device that counts your steps, you probably already know that many of these fitness trackers also can monitor your sleep patterns, nudge you to get up and move when you sit too much and estimate how many calories you burn in a day.

But did you know that some of the newer devices are being credited with saving lives? 

Consider these recent press reports…

• A 73-year-old Connecticut woman with unexplained shortness of breath noticed that her device showed a higher-­than-usual heart rate—and it kept getting higher over a period of a few days. She went to a hospital, and it turned out she had life-threatening blood clots in her lungs.  

• A 34-year-old Utah man noticed his device was showing a much lower-than-normal heart rate. At his wife’s insistence, he went to a hospital, where he learned he had dangerous blockages in two coronary arteries.  

• A 42-year-old man who had a seizure was taken to a New Jersey emergency room, where he was found to have a rapid, irregular heartbeat known as atrial fibrillation (A-fib). The condition can lead to stroke or heart failure. Because he was wearing a fitness device, doctors asked to see his heart rate recordings, which allowed them to time the onset of his abnormal heartbeats and choose the best treatment. 

The people in all of these examples happened to be using Fitbit devices, but the data that alerted them and their doctors to their health risks are available on many devices. 

The key feature in most of the ­devices:­ A sensor, called a photoplethysmogram (PPG), that sits against the wrist and uses light-emitting diodes (LEDs) to continuously detect blood volume changes that can be translated into a heart rate—the number of times your heart beats in a minute.

For a healthy adult who is sitting quietly and not under any unusual stress, the normal “resting” rate is 60 to 100 beats per minute. In general, younger, fitter people have lower resting heart rates. 

While consumer fitness devices are not perfect heart rate monitors, they can give you a good estimate of your resting rate and your rate during exercise. When you work out, you want your heart rate to rise. How much your heart rate should increase depends on your age, fitness level and goals. A rule of thumb is that moderate-to-intense exercise should get you to 50% to 85% of your maximum heart rate, which you can calculate by subtracting your age from 220.

Here’s what you don’t want: A spiking heart rate when you are sitting still or a heart rate that stays sluggish even when you get moving.  

What a fast or slow heart rate could mean: A fast heart rate can be caused by a number of conditions ranging from anxiety or dehydration to infection or an actual heart rhythm problem. A slow beat can signal anything from heart damage to sleep apnea to an underactive thyroid. 

With most popular devices, you can see your current heart rate on your wrist and check your heart rate history—going back hours, days or weeks—on an app or a website. 

At least one device, the Apple Watch, has an additional optional feature—you can set it to send you an alert if a rapid heartbeat develops at a time when the device senses you are not moving much. The watch also can send you alerts for slow heartbeats. The latest version has two more features approved by the FDA—a notification for irregular heartbeats that could signal A-fib and an app that lets you take a brief electrocardiogram (ECG)—a test of your heart’s electrical activity. If results look abnormal, the device will urge you to seek medical care. 

Some wearable devices, including the Apple Watch, also can detect hard falls. If the wearer seems unresponsive, it can even be set to make automatic calls to 911 or other emergency contacts. 

People who may want to consider using a fitness tracker include those who have a personal or family history of cardiac problems…and those who have a medical condition such as high blood pressure or diabetes.

Important: If you have one of these devices, take the time to read through online user guides before deciding whether to turn on the various alerts. 

And don’t forget the cost. The latest Apple Watch will set you back $400 or more, while other smartwatches and trackers with heart rate detection start at under $100 and go up to around $350. In addition to Fitbit, popular brands include Garmin, Polar and Samsung. Other considerations…

• None of these devices or features is a substitute for medical advice. If you see something worrisome or confusing, you should always check with your health-care provider. 

• The risk for false alarms is real. A preliminary report from a study conducted for Apple said that 16% of watch wearers who got a notice about irregular heartbeats were not actually experiencing A-fib. Doctors expect accuracy rates to vary among consumers, with young and healthy users most likely to get false alarms. 

• Risks and benefits remain unclear. There’s no study showing that wearing one of these devices leads to better medical care or outcomes. Even what seems like an obvious benefit—early detection of A-fib—comes with risks. For example, many people diagnosed with the condition are prescribed a blood thinner, which increases bleeding risks. The risks of taking the drug must be balanced against the potential benefit of preventing stroke.

• Your doctor may not be sure how to interpret data from these devices. Because medical systems have not yet developed best practices for incorporating the information produced by wearable devices into the patient’s medical record, doctors typically handle this issue on a case-by-case basis. 

However, there’s reason to think wearable devices that monitor our health will only become more common and useful. Paired with data from other connected devices—such as weight scales, blood pressure monitors and blood glucose monitors—they will produce a torrent of data. Consumers and medical providers will have to work through how best to use it all. 

In the meantime, if you decide to wear a fitness tracker, speak to your doctor about the data you should be collecting and how to best use it to help protect your health.