Why smartwatch-measured blood pressure still isn’t ‘ready for primetime’ (2021)

I'm not sure why you'd even want this. As someone who had hypertension, so many factors affect your BP. I could drop mine by 10 points by just taking a warm shower. That's why I had to take it at the same time everyday, and it was a process to do it too. I'd have to sit in my chair in a prone position for 10 minutes or so, slow my breathing, relax, clear my head of thoughts, etc. Once I felt significantly relaxed, I took 3-4 measurements and averaged them all.

Having a heart condition is a very easy gateway to health anxiety. I had to stop wearing my apple watch, my wife keeps the pulse oximeter under lock and key, etc. because I became way too focused on the numbers while ignoring the context. It became a very bad feedback loop.

I don't want to monitor my BP 24/7.

A bit over a year ago I took the time for little experiment and made practically simultaneous heart rate measurements using a fairly recent FitBit model, another smart watch (forgot which one), the heart rate reading from a medical arm cuff blood pressure monitor and manual counting over a minute. In a rested state, repeated five times.

The arm cuff and manual measurements showed very limited variablity over different measurements and were in very good agreement with each other (like +/- 4 beats per minute). The smart watches were all over the place, showing deviations of up to twenty beats per minute from the former two methods / each other, varying by over 20 beats per minute over consecutive measurements, and not even showing signs of convergence over repeated measurements.

My conclusion (at least for my skin type, which does affect accuracy for watch-based measurements): smartwatch-based heart rate measurements are probably best used as slightly biased random number generators. If tracking blood pressure is much harder than the heart rate, as they say in the article, I don't even want to know.

I'm the CTO at a relatively well established medical device company (EEG/ECG). Before I arrived at my current company I spent a year of my own time tinkering in the BP space. The article, while relatively, light on details gives a pretty good sense of the problem with trying to replicate the traditional BP measurements using the optical methods used by smartwatches.

Smartwatches and similar devices use photoplethysmography (PPG) to measure blood oxygen saturation. The heme units in blood have different reflectance/absorbance spectra depending on their level of oxygen saturation. The only difference between the fingertip pulse oxygen sensors used in clinical settings and those used in smartphones is that the clinical ones use transmission, whereas the watches use reflection. While the methods are different, they arrive at very similar numbers (+/- skin color & racial biases).

Unlike pulse oximeters of yore, smart watches can take many thousands of readings per second. And if you offset your photodiodes by a known distance, you can calculate pulse transit time, and a variety of other measures beyond just your spo2.

The challenge has been in translating these measures back into systolic and diastolic blood pressure. To be accepted by the FDA you need to be able to show that your device is +/- 3mmHg. That's just not doable right now for the reasons mentioned in the article.

That said, the measures that you can get from high-speed PPG are probably not useless. But these measures aren't backed up ~200 years of medical practice. It is a very long road to introduce new measures into the medical system. Not only do you need to get FDA clearance to sell your device, but you need to navigate the reimbursement system to find someone who will pay for your device. Doing this in a 5 year time span would be unheard of. While cardiologists would like a better system than cuff based BP measurement, the upside of a better system is limited by the current medical system. And there's a significant cost and downside risk to anyone pushing for something different.

One company that I spoke to and is still working on the problem comes with an impressive biomedical pedigree, and they're starting in the non-diagnostic "wellness" space.

Another company is using mm-wave radar to measure pulse waves in a different way, but it appears that they've pivoted from being directly challenging the BP status quo into now selling their hardware sensors to more well established medical device companies.

Long story short, I'd be impressed to see blood pressure cuffs retired in my lifetime.

I have hypertension which has been treated for years with medication. My measured pressure is now within healthy limits. I have a cuff-based device bought from a UK chemist and I check it most days, at a similar time and after at least ten minutes sitting down (as per jawngee elsewhere on this thread). Even so my BP and pulse vary by a few percent. When I go into a hospital / surgery I get my BP checked properly and because the result is usually in the same ball park as the home device, I tend to trust that if the home device showed a sudden massive variation, then I should consider consulting a doctor. If smartphones are doing a variation check, as the article implies, then they could be useful. But there is no substitute for a proper check using a calibrated device if you are really worried.

One reason that I check my BP is that I have underlying issues with my heart's electrical systems: right bundle branch block [0]. This is visible on a professional ECG. I've had quiet a few of those following a ventricular fibrillation episode which involved an air ambulance ride to a cardiac ICU where I had a defibrillator implanted. These underlying conditions are not typically detected on the few (n == 1) smart devices I've tried that offer an ECG-lite check.

[0] https://en.wikipedia.org/wiki/Right_bundle_branch_block

Another promising tech for this is using ultrasonic waves to measure the diameter of the artery:

https://www.nature.com/articles/d41586-020-01807-6

A lot of these smartwatch sensors have questionable accuracy. No doubt Samsung will rush out something that hardly works.

I don't get how the traditional cuff method actually works. It's not clear to me how having the thing on your arm tells you what the pressure is inside your blood vessels. Isn't your arm made of things that can stress, therefore the pressure is not the same on either side of it? Same as how how do you measure the pressure inside a tyre or other pressure vessel? The point of the vessel is to make the pressure not the same as outside.

There is aktiia.com. They force you to calibrate every 4 weeks. And the values seem to be reasonable most of the time.

There's a much better location to do continuous 24/7 blood pressure monitoring with a much smaller sensor that could eventually be made unobtrusive with development: the earlobe

Must be some kind of stigma for American market or unwilling investment into "not inventing anything new" attitude.

"It needs calibration"