Blog

Apr 26, 2024

Why blood pressure is so hard for smartwatches to measure

Solving this problem can't happen soon enough Smartwatches and fitness trackers have come a long way since the first Fitbit practically invented the modern wearables market in 2009. Back then, the

Solving this problem can't happen soon enough

Smartwatches and fitness trackers have come a long way since the first Fitbit practically invented the modern wearables market in 2009. Back then, the only thing you could measure was how many steps you took. About five years later, smartwatches burst onto the scene, and today the best smartwatches can give their users a heart rate, blood oxygen saturation, and skin temperature sensor on their wrist for just a couple hundred bucks.

But despite all the modern advances in mobile health technology, getting an accurate, cuffless, smartwatch-based blood pressure monitor into the hands of consumers has so far been beyond the grasp of the major device manufacturers. Samsung’s Health Monitor app has claimed to be able to "measure" blood pressure since 2021 on the Galaxy Watch 3, but it requires monthly recalibration with a traditional cuff-based blood pressure device, and it’s not available in the US due to lack of FDA approval.

So what gives? Why is measuring blood pressure so hard for smartwatches? To understand, let’s start with the basics.

Blood pressure, simply put, is the force your blood exerts on your arterial walls. The peak (or systolic) pressure occurs when your heart contracts, forcing blood through your arteries. Between contractions is the minimum (or diastolic) pressure. These two values — measured in millimeters of mercury (mmHg) — are your blood pressure.

The most accurate blood pressure measurement is achieved by inserting a tube (cannula) into an artery and directly measuring the pressure of the blood. This is obviously highly invasive and is generally not used for routine measurements.

The most common form of blood pressure measurement is the auscultatory method (from the Latin for to listen). This involves placing a blood pressure cuff over your brachial artery (the largest artery on your upper arm) and inflating it until it cuts off the flow of blood. The pressure is released and someone (usually a medical professional) takes note of the pressure when the sound of your blood flow resuming is heard (systolic pressure) and when the sound of your turbulent blood flow can no longer be heard (diastolic pressure).

Electronic blood pressure devices work in a similar manner. As with the auscultatory method, a cuff is inflated until it blocks blood flow. As the pressure is slowly released and blood flow resumes, pressure monitors in the cuff measure the oscillations of the pulse pressure. Based on the peak pressure of the pulse and the pressure of the cuff, the systolic and diastolic pressure can be estimated to high accuracy.

In a word: hypertension.

"High blood pressure, or hypertension, is often the first sign of cardiovascular disease," says Dr. Tammy Brady, Medical Director of the Pediatric Hypertension Program at Johns Hopkins University. "Across the globe, over one billion people carry a diagnosis of hypertension which is the leading cause of morbidity and mortality related to cardiovascular disease." In the United States, cardiovascular disease has been the leading cause of death since 1950, accounting for almost 700,000 fatalities in 2021 alone. Globally, that number rises to nearly 18 million.

Blood pressure category

Systolic pressure

and/or

Diastolic pressure

Normal

<120 mmHg

and

<80 mmHg

Elevated

120–129 mmHg

and

<80 mmHg

Hypertension Stage 1

130–139 mmHg

or

80–89 mmHg

Hypertension Stage 2

>140 mmHg

or

>90 mmHg

Hypertensive Crisis

>180 mmHg

and/or

>120 mmHg

Source: American Heart Association

When blood moves through your circulatory system with too much force and speed, it can damage the interior of your veins and arteries. Cholesterol forms plaques along these damaged regions which constrict the blood flow, further raising the blood pressure leading to more damage and more constriction. If this sounds bad, that’s because it is. High blood pressure can cause your arteries to bulge (aneurysm) or rupture. The constriction and plaque buildup it causes in your circulatory system can cause an arterial blockage (embolism) preventing blood from reaching vital parts of your body. All of these scenarios are potentially lethal and require medical intervention to treat.

Compounding all of this is the fact that there are no outward symptoms for high blood pressure. It doesn’t cause headaches, chest pain, nose bleeds, or dizziness. According to Dr. Brady, "many people do not know they have hypertension which leads to missed opportunities for treatment and prevention of cardiovascular events." Therefore, the only way to know for sure is to regularly check your blood pressure.

Unfortunately, most people only measure their blood pressure when visiting a doctor's office, which costs time and money. On top of that, doctors have to deal with the confounding effects of "white coat syndrome," a rise in blood pressure due to doctor-induced anxiety.

Given these challenges, and the massive public health impact of hypertension and heart disease, smartwatch makers are heavily motivated to make blood pressure monitoring available on their devices.

Most research into cuffless blood pressure monitoring has centered on the idea of using data from the heart rate and blood oxygen saturation monitors that are already common on most smartwatches. These sensors use a technique called photoplethysmography (PPG) to measure the change in volume in your blood vessels over time using an LED and a light sensor. The tricky bit is how to turn the data gathered into something that can be used to compute blood pressure. And while there's no one obvious "right" way to do it, we do have a few options:

Samsung, which offers blood pressure measurements via its Samsung Health Monitor app in dozens of countries (most notably in the European Union), estimates blood pressure with a technique called pulse wave analysis (PWA). The basic idea is that a greater volume of blood passing through your arteries equates to a higher blood pressure. The big drawback with this technique is that these measurements only indicate a relative change in blood pressure — it can’t tell you your absolute blood pressure (as traditionally expressed in millimeters of mercury). In order to even make an accurate guess, it has to be calibrated with a traditional blood pressure device once a month.

Another popular technique uses a metric known as pulse arrival time (PAT) to estimate blood pressure. PAT is the measure of the time between the peak of your electrocardiogram (ECG) and the peak of your PPG. The shorter your PAT, the higher your blood pressure. Like PWA, PAT can only tell you relative changes in blood pressure, so it also requires regular recalibration with a traditional blood pressure device.

PAT-based blood pressure measurements have been around since at least 1993, when Casio released the BP-100 — a wristwatch with a PPG and ECG sensor. Fitbit itself launched a study to analyze the accuracy of blood pressure readings using PAT in 2021, but it has yet to publish any results.

Similar to PAT, pulse transit time (PTT) measures the time it takes for a pulse wave to travel between two parts of your body. There are a couple of ways to do this, but they all rely on the same underlying principles. One method is to use two separate PPG sensors and measure the time it takes for a pulse wave to transit the two points.

Some researchers have proposed the use of seismocardiography (SCG — measuring the vibrations caused by the heart) and ballistocardiography (BCG — measuring the body movements caused by the heart) to obtain the first time point in a PTT measurement. These techniques would use the accelerometers in your smartwatch to detect the SCG or BCG signal of when your heart beats, and compare the timing of that to the PPG signal on your wrist to find the PTT. Researchers have already developed a proof-of-concept smartphone app to demonstrate the feasibility of this technique.

Like PWA and PAT, PTT requires regular calibration with a traditional blood pressure device to maintain accuracy over time. There have been no consumer devices introduced yet which use PTT to estimate blood pressure.

One of the biggest drawbacks to these techniques is their reliance on the PPG signal. Even though the PPG sensors in smartwatches are more accurate than they’ve ever been, they’re still subject to a ton of interference. Movement, sweat, temperature, and even skin color can affect the quality of the PPG signal, in turn having a negative impact on the accuracy of any blood pressure measurements based on that signal.

These methods are not based on any generally accepted physiological principles.

Perhaps even worse, according to the University of Pittsburgh's Dr. Ramakrishna Mukkamala, “these methods are not based on any generally accepted physiological principles.” Dr. Mukkamala is an expert on computational physiology and has written extensively on cuffless blood pressure monitoring. He observes that “the PAT and PWA methods have not been convincingly proven to offer any value in blood pressure measurement accuracy.”

Blood pressure is also a very fickle metric. Measuring it at different locations on the body will yield different results. Once calibrated, you have to measure your blood pressure with your smartwatch/PPG sensor at the same height relative to your heart. A change of just a few inches can mean a change in pressure of 10 mmHg or more, which is the difference between healthy and hypertension.

Given the dire consequences of high blood pressure, it’s no wonder that smartwatch makers want to get it right. Underreporting blood pressure could lead someone to put off lifestyle changes or a visit to the doctor that could save their life. This is why all the big players are waiting for FDA approval before they make their features available in the US. And even if everything's working well, that sort of thing tends to be slow to come, as the FDA is very particular about what it will and won’t allow on the market without regulation and oversight.

According to the FDA’s James McKinney, “what the device claims to do is very important in determining if [it] is actively regulated by the FDA.” Basically, the scope of what the manufacturer tells shoppers its device is intended for determines how it’s regulated. Software that is only for general wellness, such as helping to lose weight, manage stress, or track sleep, typically doesn’t set off the FDA’s radar. That means measuring heart rate to track your exercise is fine, but measuring your heart rate to help diagnose arrhythmia requires the FDA’s blessing.

At first glance, tracking blood pressure would seem to fall under the umbrella of general wellness, but compared to other vital signs, blood pressure is a bit different. “Blood pressure is actionable,” Dr. Mukkamala explains. If your heart rate seems a little high or your SpO₂ seems a little low, there’s not a lot of diagnostic utility in those metrics. On the other hand, if you take your high blood pressure readings to your doctor, she'll very quickly find an appropriate treatment for you. “The FDA chooses to regulate blood pressure measurement with the aim of ensuring that devices lead to proper therapy,” says Dr. Mukkamala.

And the FDA takes blood pressure devices very seriously. To get that coveted FDA approval, smartwatches have to meet accuracy standards laid out by the Association for the Advancement of Medical Instrumentation's Sphygmomanometer Committee (of which Dr. Brady happens to be co-chair). "Long story short," she tells me, "continuous cuffless devices aren’t ready for prime time yet."

Despite these challenges, the future of smartwatch-based, cuffless blood pressure monitoring actually looks reasonably bright. Many OEMs will continue to pursue PWA- or PAT-based methods simply because PPG sensors (which are responsible for determining heart rate and SpO₂) are standard on today’s wrist-worn wearables — even if less than ideal, if they're there, might as well take advantage of them. The accuracy of these techniques may not be up to the FDA’s rigorous standards right now, but as new research continues to be published, it’s not unthinkable that they’ll soon be commonplace.

New techniques are also emerging that may obviate the need for calibration altogether. A 2018 paper (co-authored by Dr. Mukkamala) published in Science Translational Medicine outlines a method for measuring blood pressure using a special combination pressure and PPG sensor which is based on the oscillometric method used by automatic blood pressure machines. What’s more, the team behind the paper built a working prototype in a smartphone form factor. The user presses the side of their finger against the pressure/PPG sensor, compressing a small artery in the finger. An onscreen interface then guides them in applying the correct pressure (analogous to a blood pressure cuff) while the PPG sensor (analogous to a pulse pressure sensor) tracks their pulse wave.

Even though this research is five years old at this point, the technology may soon be on our doorstep. Back in January, Fitbit filed a patent for a smartwatch with this tech built into it. Tellingly, one of the authors of the 2018 article, Keerthana Natarajan, has since left academia to join the Fitbit team in San Francisco.

Omron's HeartGuide is one of the few wearables capable of reasonably accurate blood pressure measurement, thanks to the presence of an inflating cuff embedded within the watch's strap. Available in your choice of two wrist sizes, the HeartGuide can add valuable data to your health tracking efforts.

As of 2023, the only smartwatch available to consumers that has FDA approval to measure blood pressure is the Omron HeartGuide which actually uses a tiny inflatable cuff. What has yet to be achieved, and what public health experts dream of, is a cuffless smartwatch that can provide on-demand or continuous blood pressure monitoring. If such a device became available, "[it] could significantly help with efforts to decrease hypertension and cardiovascular disease," says Dr. Brady. Given the confluence of breakthrough research, cutting edge technology, and machine learning, that dream may be just around the corner.

Daniel writes guides, explainers, and technology news. He especially likes deep diving into niche topics that require more than scratching the surface. He's been writing in newspapers, magazines, and blogs for over 20 years writing hard news, entertainment, and science stories. When not writing he enjoys reading science fiction, playing music, and raising a rambunctious toddler. The devices you'll find him using every day are his Pixel 4a, Acer 311 Chromebook, and Amazon Fire HD 10. As soon as he saves up some money he's getting a 3D printer.