More and more of the ‘smart’ gadgets like watches and phones that we carry around with us these days come with features that we’d not care to ever need. Since these are devices that we strap onto our wrists and generally carry in close proximity to our bodies, they can use their sensors to make an estimation of whether said body is possibly in the process of expiring. This can be due to a severe kinetic event like a car crash, or something more subtle like the cessation of the beating of one’s heart.

There is a fairly new Loss of Pulse Detection (LoPD) feature in Google’s Pixel Watch 3 that recently got US FDA approval, allowing it to be made available in the US after previously becoming available in over a dozen European countries following its announcement in August of 2024. This opt-in feature regularly polls whether it can detect the user’s pulse. If not found, it cascades down a few steps before calling emergency services.

The pertinent question here is always whether it is truly detecting a crisis event, as nobody wants to regularly apologize for a false alert to the overworked person staffing the 911 or equivalent emergency line. So how do you reliably determine that your smart watch or phone should dial emergencies forthwith?

Budget Medical Devices

One of the amazing things about technological progress is that sensors and processing capabilities that were rather exotic a few decades ago are now being included in just about any smart device you can strap on your wrist. This includes motion sensors, pulse- and oxygen level meters, making these devices in theory capable of acting like ambulatory cardiac monitors and similar medical devices that monitor health parameters and respond to emergencies.

While for a long time the gold standard for heart function monitoring over a longer period outside a hospital setting involved a portable electrocardiogram (ECG) recorder, recently wrist-worn monitoring devices based around photoplethysmography (PPG) have prove themselves to acceptable substitutes. In a 2018 study by Francesco Sartor et al. in BMC Sports Science, Medicine and Rehabilitation the researchers found that the wrist-worn PPG was not as accurate as the ECG-based chest strap monitor, but came close enough to be practical.

Here the difference is such that applications where precision actually matters the chest strap ECG is still the optimal choice, but wrist-worn PPG devices as integrated into many fitness bands and smart watches are an acceptable substitute, such as when monitoring heart rate for signs of atrial fibrillation. A 2022 study by Christopher Ford et al. in JACC: Clinical Electrophysiology examined two smart watches (Apple Watch 4, KardiaBand) for this purpose, finding that their accuracy here was 91% and 87% respectively.

Together with additional sensors like the commonly integrated motion sensor, these devices seem accurate enough to at least determine whether the person wearing them is suffering a cardiac event that requires immediate intervention.

Health Check

The idea of an automatic emergency call isn’t new, with for example the EU making such a system (called eCall) mandatory in new cars since 2018. The idea is that when a serious collision is detected, emergency services are contacted and provided with location and sensor data. Google added its Car Crash Detection feature to the Pixel 3 smartphone in 2019, and Apple added Crash Detection to its Apple Watch and iPhones in 2022. These use sensor data from gyroscopes, GPS, microphones, and accelerometers to determine whether a crash just occurred.

What users of these devices discovered, however, was that activities such as going on a rollercoaster ride could activate this feature, as well as snowmobile rides, skiing, and similar activities. In response, Apple had to adjust its algorithms on these devices to reduce the number of false positives. Despite this, rescue workers in e.g. Canada are still reporting a large number of false positives. One reason cited is that although there’s a time-out before emergency line is called with audible alarm, this can be hard to hear when you’re on a snowmobile.

As it turns out, defining what seems like a pretty clear event to us when you’re limited to just this handful of sensors is much trickier than it seems. After all, what is different between the sensor data from a rollercoaster ride, a car crash, dropping one’s phone or smart watch onto a concrete floor or forgetting said phone on the roof of the car?

In this context, the idea of taking a simple activity like measuring heart rate and pulse, and extrapolating from these that if they cease, an emergency has occurred is fraught with pitfalls as well.

Merging Data

How do you know as a human being that someone has just suffered cardiac arrest? You confirm that they don’t have a noticeable carotid pulse, and the reason why you checked is because they clearly collapsed. This is when you’d pull out your phone and dial emergency services. The LoPD feature that Google has introduced has to do effectively exactly these steps, except that it starts from the loss of pulse (LoP) rather than from seeing someone pass out and collapse to the ground.

Thus the tricky part is establishing whether said collapsing has occurred, not whether the pulse has been lost. After all, the user may have simply taken the watch off. According to Google, to verify their algorithms they hired stunt actors to simulate LoP using a tourniquet, cutting off blood flow, and simulating falls like a person suffering cardiac arrest would suffer.

On the sensor side they use the heart rate monitor (PPG sensor), which initially uses the green light to check for pulse, but can switch to infrared and red lights when a LoP condition is triggered. Simultaneously the motion sensor data is consulted, with a lack of motion taken as a sign that we’re dealing with a LoPD. This starts an auditory alarm and visual countdown on the screen before emergency services are contacted with an automated message plus the user’s location.

Then they needed to calibrate the response to this merged sensor data with clinical data on cardiological events before trialing the result with said stunt actors and volunteers. An article on this research was also published in Nature (paywalled, but here is a gift article), detailing the algorithm and the way they tested its effectiveness. In the paper the authors note one false positive event and subsequent emergency call across 21.67 user-years across two studies, with a sensitivity of 67.23%.

A Matter Of Time

In the case of cardiac arrest, time is of the absolute essence. This is also clearly noted in the Google paper on the LoPD feature, who note that ideally there is a witness on-site who can immediately begin CPR or ideally get a nearby automated external defibrillator (AED). Unfortunately in most cases of cardiac arrest, this event goes initially unnoticed. The LoPD feature on a smart watch thus would be for cases where nobody is around to notice the emergency and respond to it. Although it isn’t explicitly mentioned, it seems that the watch can also detect whether it’s being worn or not, which should prevent false positives there.

With over half a million US citizens each year suffering cardiac arrest, and over half of these occurring outside of a hospital setting, this could potentially save thousands of lives each year. Following cardiac arrest and in the absence of resuscitation the lack of blood and oxygen)being circulated means that within minutes organs begin to suffer the harmful effects, depending on their oxygen requirements. The brain is generally the first to suffer ill effects, which is why the application of CPR is so crucial.

Because of the intense urgency following a major cardiac event like this, the practical use of this LoPD feature will be highly dependent on the location where the emergency occurs. In the case of someone collapsing while alone at home in their city house or apartment, this could conceivably save their life if emergency services can arrive within minutes. Even faster and more useful in less urban settings would probably be having your smart device notify nearby people who can then perform CPR while calling 911 or equivalent.

That said, perhaps the real killer feature that’s missing here is an integrated AED in smartphones since everyone has one of those things on them at all times, or even smart watches that can automatically perform defibrillation while also notifying emergency services.