Will wearables ever accurately measure blood pressure?

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very year, 9.4 million people die from complications related to high blood pressure, according to the World Health Organization. But many people with hypertension aren’t even aware they have it.

You only know whether your numbers are on the high side if you get your blood pressure checked regularly. Doctors do this with a traditional arm cuff reader – and similar products are also available for home use – but such devices have limitations.

Research published in the International Journal of Preventive Medicine suggests at-home blood pressure cuffs can lead to errors due to ill-fitting bands and improper arm positioning. In a medical setting, ‘white coat syndrome’ (where a patient’s blood pressure is elevated from the stress of being in a medical setting) is a common problem.

The arm cuff device is also not well designed for larger patients. Many health tech companies are now exploring more convenient ways for people to measure their blood pressure using wearable technology.

In April 2021, Fitbit announced a trial to determine whether its smartwatches can accurately detect hypertension. Preliminary research suggests the device can be used to measure blood pressure from the wrist, but the new study will extend those findings to a larger population.

The company acknowledges that although there is great interest in monitoring blood pressure using wearables, this method has not yet been properly validated. Wrist measurements have so far failed to prove as accurate as those from a cuff. Fitbit users in the US who are at least 20 years old are participating in the study.

Fitbit is not alone in trying to find less intrusive ways for people to measure their blood pressure. For instance, Samsung’s new Galaxy Watch uses optical heart rate sensors to measure blood pressure at the wrist, but users still have to calibrate the feature using a cuff every four weeks.

Dr Steven LeBoeuf is co-founder of Valencell, a company that makes biometric sensors for consumer tech companies. In early 2021, Valencell announced its cuffless blood pressure technology could be used in devices worn on the finger and wrist, such as a smartwatch or pulse oximeter finger clip.

The company says the technology is the first commercially available sensor to enable blood pressure readings throughout the day with the same accuracy as a cuff device. The announcement follows findings in January 2020 that its sensors could identify hypertension with 89% accuracy when used in hearing aids or earbuds.

LeBoeuf reveals it was a slow road to success, however. He first had the idea for wearable blood pressure sensors over a decade ago. “But we weren't able to convince anyone to give us money to do it,” he admits. “It wasn't until 2016 that we got funding to work on this problem.”

There could be significant consumer demand for wearable blood pressure devices. A recent survey from Valencell found 62% of Americans with high blood pressure only measure it a few times a month – far below the twice a day recommendation for this patient group.

However, more than three-quarters of respondents said they would measure their blood pressure “more or much more” often if it was possible to accurately measure it passively in the background.

Companies exploring wearable blood pressure monitoring at the wrist are generally looking at one of two main types of technology. Fitbit’s offering will determine whether pulse arrival time (PAT), also called pulse transit time, can be used to obtain blood pressure readings. While Valencell’s sensors are based on another technology called photoplethysmography (PPG).

PAT is the time it takes a pulse wave to travel between two arterial sites. This speed is directly proportional to blood pressure.

“It’s kind of like if you’re trying to shoot something out of a gun,” explains LeBoeuf. “The more pressure you apply to the trigger, the faster the bullet goes through the gun. There’s a similar relationship between the time it takes a pulse to arrive from your heart to an extremity.”

PAT is the most common way of measuring blood pressure without a cuff. And some devices that use this method are already US Food and Drug Administration (FDA) approved. The main disadvantage with PAT is that blood pressure monitors that use this tech still require frequent calibration.

“We decided to stay the hell away from that technology because everything I’ve seen requires a painful amount of recalibration,” says LeBoeuf.

PPG, on the other hand, uses optical and inertial sensors which detect blood flow patterns. The technology has been explored since the 1930s and measures the amount of light absorbed or reflected by blood vessels which tells you the rate of blood flow.

The relationship between the PPG signal and a blood pressure reading is non-linear, however. So Valencell uses a machine-learning algorithm to convert this blood flow information to a blood pressure measurement.

The algorithm has been fed data from tens of thousands of patient results. And the technology takes note of the person’s age, weight and sex to give a more accurate reading.

PPG sensors are already widely used in many wearable devices. They also come in a range of sizes, which means they could potentially be used on almost any body location. However, blood pressure devices that use PPG have yet to be approved by the FDA.

Valencell’s technology has been validated in a clinical study to demonstrate it meets the relevant ISO standard, says LeBoeuf. The company is now getting the paperwork ready to pursue clearance from the FDA. “We need the industry to adopt it and agree this is extremely valuable for public health and for medical use,” he explains.

Healthcare professionals may need convincing though. A recent report by Forrester Research found physicians are sceptical about the value of wearable devices in assisting clinical decisions.

However, LeBoeuf believes the wearable health market will change rapidly over the next five to ten years with digital diagnostics becoming more popular.

Passive blood pressure monitoring would allow individuals to see how their lifestyle affects their readings, he says. It could help people make more informed choices about their diet and daily activities and ultimately help them reduce the risk of serious illness.

“There’s a mortality rate difference between those who monitor their blood pressure and those who don’t because you can address things before they become deleterious to the body,” says LeBoeuf. “We strongly believe this can be democratised through wearables, which will have a substantial public health outcome.”