Fitbit has registered a patent to its name for estimating arterial stiffness by measuring the pulse wave velocity from the wrist. The technique is also considered to be promising for continuous, cuffless measurements of blood pressure.
The patent filing with the USPTO
The patent was first published by the USPTO on the 2nd of December. The paperwork was originally filed in March this year under the catchy title “Photoplethysmography-based Pulse Wave Analysis Using a Wearable Device”.
The word Photoplethysmography or PPG in short, is an optical technique used to detect volumetric changes in blood in peripheral circulation. Put more simply, this is a heart rate sensor that is built into most smartwatches and fitness trackers.
It turns out the sensor can be used for more than just counting the number of heart beats per minute. Fitbit wants to utilize it to track arterial stiffness using pulse wave analysis. A pulse wave is the rate at which pressure waves move down the vessel.
As we age our arteries become stiffer. That is a natural part of getting older that can be mitified to some extent by leading a healthy lifestyle. Weight loss, exercise, salt reduction, less alcohol consumption can all contribute to a reduction in the rigidity of the arterial wall. Higher arterial stiffness increases your chance of developing hypertension and other diseases, such as chronic kidney disease and stroke. It is an established independent predictor of cardiovascular risk.
Typically you would need to visit a doctor’s office and wear expensive equipment for a few days to get a reading of your arterial stiffness. Something that works from the wrist would provide a low-cost way of doing this. Plus it would allow for data collection over longer periods and during many different activities and conditions.
Here’s an excerpt from the patent.
Disclosed are devices and methods for non-invasively measuring arterial stiffness using pulse wave analysis of photoplethysmogram data. In some implementations, wearable biometric monitoring devices provided herein for measuring arterial stiffness have the ability to automatically and intelligently obtain PPG data under suitable conditions while the user is engaged in activities or exercises.
There are studies which illustrate the correlation between pulse wave and blood pressure such as this one published in PNAS. It could be the next big thing in sensor technology. The technique could even allow for continuous monitoring of blood pressure.
Of course, it is pretty pointless trying to estimate blood pressure during movement or exercise. With a traditional cuff, you are meant to take readings while at rest. A sensor that sits on the wrist would work in a similar way. The patent shows a flow chart which begins by determining wheather the user condition is suitable for collecting pulse waveform data. Only in this case will it try and estimate a reading.
All of this is pretty exciting stuff. Soon the Fitbit on your wrist might be able to track the health of your arteries, along with blood pressure.
Sense might be the first Fitbit with blood pressure tracking
This follows on from the Fitbit study at the start of this year that hinted that blood pressure tracking could come soon to its Sense smartwatch. The company ran a month-long study on the subject.
It seems blood pressure monitoring will be all the rage in 2022. In 2020, we saw the wide implementation of blood oxygen tracking. Blood pressure monitoring from the wrist is next on the list.
A few months ago we wrote about Aktiia. The Swiss-based startup has already released their solution. The bracelet quietly sits on your wrist capturing blood pressure readings multiple times per day via an optical heart rate sensor. But Aktiia is very much an outlier and has little in terms of real competition.
Fitbit wants to change that.
Fitbit blood pressure study
Fitbit ran the above mentioned month long study on something called Pulse Arrival Time (PAT). This measures the time it takes a pulse of blood to reach your wrist after your heart beats. This is no different than pulse wave velocity. The measurement of PWV requires tracking the arrival time of pressure pulses recorded in vivo. This is referred to as pulse arrival time (PAT).
“If high blood pressure was easier to measure, people could manage it earlier, which might help avoid preventable deaths from heart disease and stroke,” said Shelten Yuen, Principal Scientist at Fitbit.
“It’s a hard scientific challenge, and a lot of work remains to be done to understand the best way to do this, but we have a history of advancing technology to make previously inaccessible health metrics available to Fitbit users from their wrist, so it’s a challenge we’re very passionate about solving.”
The work is being conducted by Fitbit Labs. This piggy-backs on previous internal research with showed a correlation between PAT and blood pressure. However, this was a small 3 week study. Having said that, there are other scientific papers with large samples which show there is a correlation.
Blood pressure monitoring is important. Most of us know this. But let’s face it – strapping on a traditional blood pressure cuff is a bit of a hassle. How many of really do it on a regular basis? Having a device on your wrist that can readily take such readings will make this much easier.
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