Why Measuring Your Blood Pressure With a Wearable Device Isn't a Great Idea, Yet
Some smartwatches and fitness trackers are starting to read this vital measurement, but the technology isn't fully developed
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If you have high blood pressure, home monitors can be a great way to track your levels and keep them under control. Most of the devices currently available are versions of what your healthcare provider uses, stand-alone devices with cuffs that you apply to your wrist or upper arm.
But smartwatches and fitness trackers are starting to get in on the action, too.
That could be a good thing because uncontrolled hypertension is on the rise, says Jordana Cohen, MD, hypertension specialist and assistant professor of medicine and epidemiology at the Perelman School of Medicine at the University of Pennsylvania in Philadelphia.
āI think people are increasingly interested in their numbers,ā she says. āIf we can get people engaged in what their blood pressures are, then hopefully that can help lead to much better blood pressure control.ā
Indirect Blood Measure Measurement
A traditional monitor measures blood pressure by sensing the pressure exerted by blood on an artery. Thatās what is happening when the cuff tightens and then relaxes around your arm.
Smartwatches and fitness trackers, on the other hand, gather other biological data from your body and use that to approximate your blood pressure.
The Samsung Galaxy Watch Active, as well as the Samsung Note 9, S9, S9+, S10, and S10+ smartphones, for example, can measure something called pulse transit time, which is how long it takes a pressure wave created by a heartbeat to travel between two points along an artery.
Importantly, pulse transit time on its own can only provide you with relative percentage changes in your blood pressure from measurement to measurement. To come up with an absolute blood pressure reading, you first have to establish a baseline by using a standard blood pressure cuff and entering the data into the device. That calibrates the device to your bodyās norms and allows it to provide you with blood pressure readings expressed in the familiar ratio of systolic to diastolic blood pressure measured in millimeters of mercury (for example, 120/80 mmHg).
Moreover, for devices that work in this way, you need to periodically repeat that process, says Tammy Brady, MD, associate professor of pediatrics at Johns Hopkins University School of Medicine in Baltimore and co-chair of the Association for Advancement of Medical Instrumentation Sphygmomanometer Committee. Brady says the accuracy of the readings tends to drift over time, and scientists arenāt yet sure how fast or to what degree that drift occurs.
This method of detecting blood pressure is still in the early stages, though itās already being used by some devices, including Samsungās watches and phones, which detect pulse transit time using optical sensors. Samsung says the blood pressure app available for download to its devicesācalled My BP Labāshould not be used to diagnose high blood pressure but is for research purposes only. In fact, anyone who uses that app must agree to join a study led by researchers at the University of California, San Francisco, who worked with Samsung to develop it.
Users of the app are asked to measure their blood pressure using their phone or watch several times a day and answer questions about their sleep, stress levels, and more, according to the studyās principal investigator, Wendy Berry Mendes, PhD, professor in the department of psychiatry at UCSF. The goal is to better understand how stress affects blood pressureāand to gather data to improve wearable blood pressure measurement in the future.
Other products that use optical sensors to gather biological data used to estimate blood pressure include BioBeat and Heartisans smartwatches, both of which also require calibration with a traditional cuffed monitor, similar to My BP Lab.
But smartwatches and fitness tracker-based blood pressure monitors face another challenge: ensuring accuracy. Scientists have settled on standards for testing the accuracy of traditional digital, cuff-based blood pressure monitors. But those standards donāt apply to available cuffless devices and canāt be used to validate their accuracy, says Ramakrishna Mukkamala, PhD, a professor of bioengineering and anesthesiology at the University of Pittsburgh who has studied cuffless blood pressure technologies. Thatās one reason he urges caution to people considering using them.
Omron's Healthcare Heart Guide
Omron, a leader in traditional home blood pressure monitors, has introduced a model that has an inflatable cuff built into the wristband of a fitness tracker and that syncs up with an app on your phone. CR did test the Omron Healthcare Heart Guide BP8000-M Blood Pressure Monitor, along with other cuffed blood pressure monitors.
While Omron blood pressure monitors typically score high in CRās ratings, the PB8000-M one did not. In fact, it was the lowest of any model we tested. At $499, it was also the most expensive.
CRās testing has found that monitors with a cuff that you wear on your wrist are generally not as accurate as those with a cuff that you put on your upper arm. In our tests, the Heart Guide received a Poor score for accuracy, and our testers found it somewhat hard to use.
āWith wrist monitors, it is difficult to position them correctly for the most accurate reading,ā says Susan Booth, CRās test engineer in charge of blood pressure monitor testing.
Carol Lucarelli, Omron Healthcareās director of marketing and e-commerce, told CR that the device is āheld to the same standards as all other blood pressure monitors in our portfolioā and that many customers report that using the monitor has helped them make taking their blood pressure a regular habit. āAs a wearable, it is essential that the size, fit, and placement be correct for accuracy,ā she said. She also encouraged anyone having challenges with fit to contact Omron customer service.
CR's Advice
For accuracy and reliability, stick with devices that have upper arm cuffs and have performed well in CRās tests. You could also check out a list from the American Medical Association of home monitors that adhere to established standards.
Whichever monitor you try, remember that blood pressure can vary depending on activity, emotions, and other factors. To get the most accurate readings, take these steps, even in the doctorās office:
- Donāt smoke, drink caffeine, or exercise for at least 30 minutes beforehand.
- Use the bathroom and then rest without talking for 5 minutes before the reading.
- Place the cuff on your bare upper arm at heart level.
- Sit on a sturdy chair with a supportive back, with your cuffed arm on an armrest or a table and your feet flat on the floor.
If you buy a standard home blood pressure monitor, the American Heart Association recommends taking it to your healthcare providerās office to check its results against those of equipment in the office.
Recommended Blood Pressure Monitors
Here are three blood pressure monitors that earned high scores in CRās tests. Members can see our full ratings here.