Apple Watch Photoplethysmography (PPG)
It’s quite a big word. What it actually refers to is the use of light to detect blood volume changes and determine all sorts of metrics. In fact, nearly all of Apple Watch’s ability to monitor your cardiovascular health comes from this one sensor. It’s a sensor that’s shrouded in secrecy with Apple disclosing few details on the inner workings. Certainly developers have never been able to access the raw data from it, all they can do is read measurements it generates.
Just before we dive in it seems worth mentioning, this article is all based on my own thoughts, research and deductions from developing Apple Watch apps but I am not a medical expert and welcome feedback on anything you think could be clarified in this post.
The PPG sensor hardware has changed over the generations of Apple Watch. With the Series 4 Apple Watch a simplified sensor array was introduced which appeared to offer more accurate heart rate measurements during workouts. With the Series 6 Apple Watch, the sensor changes again most notably to include Red LEDs alongside the other Green and InfraRed LEDs. (Thank you iFixit for the blog image)
We can infer why the sensor has various different LEDs and presumably different photodiodes (light sensors). InfraRed light travels better through the skin due to its wavelength. This can make it a better choice for more accurate heart rate readings. The InfraRed LEDs are also always on for the Apple Watch Wrist Detection feature so extra power consumption is less significant and, as InfraRed is not a visible light, this means you won’t see it if you wear your Apple Watch at night. However, because the readings are being taken from greater skin depth, the InfraRed readings are much more susceptible to movement which reduces their accuracy. This is why sometimes you will see the Green LEDs on the watch switch on. This happens if the Apple Watch accelerometer detects you moving whilst its trying to background measure your heart rate. The Green LEDs are also the default choice by the watch when you start a workout. If you’re interested in further scientific reading on PPG sensors, I encourage you to have a look at this study: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6426305/
Right back at the beginning we mentioned PPG sensors detecting change in blood volume. The Apple Watch shines the InfraRed light and Green light into your skin and looks at the differences in what’s reflected back over time. The blood volume in your wrist changes as your heart pumps, the PPG sensor can use these changes to determine your heart rate in beats per minute. But what about the Red LEDs, why are they in the Series 6 Apple Watch? This is where the Blood Oxygen Saturation measurements come in. Blood Oxygen Saturation or sPO2 is measured by comparing light return from InfraRed light vs Red light. The comparison highlights the difference between red blood cells with and without oxygen bound to their haemoglobin. This difference gives your Blood Oxygen Saturation percentage. One thing that’s worth nothing though, the Apple Watch relies on Reflectance Oximetry rather than Transmittance Oximetry (as is the case when it’s monitoring your heart rate). This means that, whilst the traditional sPO2 sensor that you clip over your finger shines light through your finger and measures from the other side, the Apple Watch from its position on your wrist relies on light reflected back. This is a less accurate and reliable way of taking PPG readings but unless future Apple Watch models embed sensors in the bands, this is the way it will remain for now.
With this lower accuracy in mind, we can probably put to bed a long standing rumour that the original Apple Watch had a non-enabled Blood Oxygen Saturation sensor. Well, yes it does. It has Green and InfraRed LEDs, the ability to compare the light back from both and thus potentially determine the difference in red blood cells with oxygen bound to their haemoglobin. But, it’s likely that testing at the time found that the measurements, especially with any physical motion, were inaccurate to a point where it wasn’t worth shipping the feature. The Series 6 Apple Watch PPG sensor is probably the result of several more years of research where its been determined that comparing Red and InfraRed light with a Transmittance PPG sensor is more accurate for determining sPO2 rather than Green and InfraRed.
Mentioning the Apple Watch Series 6 Blood Oxygen Saturation accuracy seems important here. I would agree with most reviewers that any individual measurement is not something you can pin a lot of weight to. But if you have a trend of consistent measurements that have suddenly dropped, this may be a cause for concern. If you generally always have readings from the sensor of around 97% sPO2 and then one day it over night measures 5 readings near 90%, that seems worth keeping an eye on and you could consider doing some additional manual readings. At last I can mention Heart Analyzer and why it exists! Just as with your heart rate readings, individual measurements on there own are potentially not very significant or accurate. But great information can be gained from analyzing your trends over time. To check out Heart Analyzer, you can download it free for iOS and WatchOS here: https://apps.apple.com/gb/app/heart-analyzer/id1006420410
A really important thing to note is that software upgrades are just as important for the PPG sensor as hardware changes. If we look at iPhones and their cameras, each year Apple launches a new model which has swanky new features such as night mode, time-lapse etc…. But often when you look at the teardown, the actual physical sensor is the same. Generally with the cameras, Apple’s software and image processing development team only focus on the newest model. So when night mode was added to the iPhone 11, they didn’t retrospectively add it to the iPhone XS despite this having a compatible sensor. A similar situation applies to the Apple Watch PPG sensor. Generally, the sensor capabilities of the device you buy are not expanded over time despite the fact this is perfectly possible. There are some exceptions to this though, with WatchOS 4 Apple retrospectively added the ability to record Heart Rate Variability to the Series 1 and newer Apple Watches. To calculate Heart Rate Variability the existing PPG sensor records for a minute rather than until a single BPM value is obtained. The software then looks at the variation in time between the beat to beat measurement intervals and the output is your Heart Rate Variability. I find this particularly interesting because the Apple Watch Series 6 has the potential to have a similar software upgrade which could be applied instead to calculating Respiration Rate. If you were to measure a users Blood Oxygen Saturation for a longer time period, potentially 30 seconds, you could look at how the Blood Oxygen Saturation increases and decreases over time and thus determine their Respiration Rate. Your Respiration Rate is hugely significant in all sorts of conditions fro Sleep Apnea to Cardiovascular Disease. Such a capability with Transmittance Pulse Oximetry has been demonstrated in studies before. For further reading, do have a look at this study published by the BMJ: https://emj.bmj.com/content/20/6/524
Unfortunately, I think it’s more likely given Apple’s slow push towards Apple Watch sleep tracking that this feature will not be retrospectively added but instead touted as a new feature of the Series 7 model or later. But in actual fact, a rumour could be made that the existing Apple Watch Series 6 has a hidden Respiration Rate Sensor!
A similar parallel also exists to the accuracy of the original Heart Rate Sensors and the PPG technology. A lot of the work going on behind the scenes is in the algorithms that can account for the users motion via accelerometer data and feed this, plus the light returned from the LEDs, into data models which return accurate heart rate readings. Whilst new algorithms could be added to any of the previous Apple Watch models, it seems likely that these changes will be limited to new models as Apple continues to encourage user upgrade adoption. But it is perfectly possible that the Series 7 could include a “more accurate Heart Rate Sensor” despite having the exact same hardware.
The other new Health metric that is currently generating interest is measuring your Blood Pressure via PPG sensor. The Samsung Galaxy Watch 3 claims to offer the ability to measure this all via its PPG sensor! In actual fact you have to calibrate it regularly with a cuff reader or similar and it is only available in limited regions. Accuracy is also not really known at this stage. But as far as reading Blood Pressure from PPG sensors goes, watch this space. With exciting developments at this year’s CES, I’m sure we’ll soon see new products coming to market focusing on this. Do I think it will make it’s way to Apple Watch? Not this year…
Finally, at the start I mentioned how nearly all the Apple Watch’s Cardiovascular monitoring capability came from one sensor, which hopefully if you’re with me this far you’ll know is the PPG sensor. But there is one extra bit of hardware I’ve not really covered. The Apple Watch Series 4 added the ability for the user to take an ECG. This uses a completely different way of determining a users heart rate and produces a single lead ECG trace. It’s done by looking at the electrical activity of your heart as opposed to the variation in blood flow in your wrist. I won’t dive into the specifics on how the ECG works now, but I would highlight one thing I’ve noticed for potential Apple Watch buyers to be aware of. All the Apple documentation on the ECG feature at the time of writing seems to refer to the feature being available on the Apple Watch Series 4 and newer. There are two big exceptions here: what region you’re in and also the new Apple Watch SE. The Apple Watch SE does not have the hardware to take an ECG which is something no amount of software changes will fix!