Apple has long been positioning the iPhone and the Apple Watch as wellness devices -- hardware that can help you track markers of fitness including exercise, step count, and even sleep and 'mindfulness minutes'.
But now it's taking that a step further by adding the sort of heart-monitoring technology more usually of interest to doctors -- making it available to anyone with a wrist and a few hundred dollars to spare.
Thanks to the new functionality, the Apple Watch will be able to keep an eye on your heart in two ways. First, the Watch's optical heart sensor quietly measures the wearer's heart rhythm in the background. If it detects an irregular heartbeat, the Watch flashes up a message warning that you might have a condition known as atrial fibrillation, and so might like to consider a trip to the doctor. Secondly, electrodes in the new model's crown and back sapphire crystal allow the Watch wearer to take an ECG themselves; again, an alert is delivered if there are signs of atrial fibrillation.
So what's the big deal with atrial fibrillation?
Atrial fibrillation, also known as AF or AFib, is a condition where the electrical signals that control the heart go awry. In a normal heart, the signal that tells the heart to beat comes from an area known as the sinoatrial node, and spreads out around the heart from there. The signal should be steady and regular, but in atrial fibrillation the signal to beat is generated outside of the sinoatrial node, and at random, giving someone with the condition an irregular pulse.
While that might not sound worrying, the irregular heart rhythm means the atrium doesn't pump out blood as it should. The blood hangs around in the upper part of the heart for too long, and clots may form -- clots that may ultimately end up in the brain, causing a stroke.
Apart from the irregular heartbeat, someone with atrial fibrillation may have no other symptoms -- they may only become aware they have the condition after ending up in hospital. A study published last year found that a stroke was the first symptom of atrial fibrillation in one in five people with the condition, and that around one third of people who were at higher risk of AF had the condition, but hadn't been diagnosed.
Atrial fibrillation is a condition that mainly affects the over-65s -- although it can affect younger people too, they're far less likely to experience the condition. One side effect of Apple's new ECG feature may be that -- as Apple Watch users are typically younger - it could lead to a wave of younger people visiting their doctor after a bout of atrial fibrillation. As atrial fibrillation largely hasn't largely been picked up in a younger population before, because they're not screened for it in the same way as over 65s, there's been very little research done on how best to treat them.
"Atrial fibrillation is an important health issue that affects a lot of people, so there's a lot of sense in trying to recognise people with atrial fibrillation. But what I think is the challenge is that we don't know enough about the early stage of the disease to really guide people on what treatment to have," says cardiologist and researcher Rohin Francis.
"The majority of all our information on atrial fibrillation has been in people who have been diagnosed opportunistically or who have had a stroke or mini-stroke, and often it's something that we'll find after they've had another problem, so we don't have any real evidence on well patients -- particularly younger patients -- who might have a bit of atrial fibrillation from time to time.
"If we detect a short run of atrial fibrillation in someone that is otherwise fit and well and doesn't have major risk factors for stroke otherwise, we don't know whether they would benefit from the same treatment as someone who's, say, elderly and has other risk factors, or who has already had a stroke," Francis says.
Nonetheless, by bringing this sub-population of patients to medical attention, Apple is likely to inspire significant research into how to treat early stage atrial fibrillation in younger patients, particularly as the sheer number of Apple Watch users and the data they bring is likely to be many times what a conventional medical trial could generate.
There are other potential benefits from the Apple's ECG feature too. As well as finding previously-undiagnosed people with atrial fibrillation, it could help track those already on treatment for the condition. By checking in with the ECG feature, patients with AF could keep their doctors up to date on whether starting a new treatment or swapping to a different medication has decreased the frequency of their atrial fibrillation.
There are already some caveats to the Watch's ECG feature, however: it is not to be used by people under 22, for example. The FDA also stipulates that the ECG Watch shouldn't be used as a replacement for normal medical workup. "The ECG data displayed by the ECG app is intended for informational use only. The user is not intended to interpret or take clinical action based on the device output without consultation of a qualified healthcare professional.. [it] not intended to replace traditional methods of diagnosis or treatment," it said.
Also, in around 10 percent of cases, the Stanford research found the Watch was unable to read people's rhythm at all -- meaning some Watch wearers who have atrial fibrillation may still go undiagnosed.
What's next for the Watch's ECG?
Apple's Watch is a one-lead ECG, making it a very blunt instrument -- it's only set up to measure the heart's activity at one point. It can detect atrial fibrillation, but detecting atrial fibrillation isn't actually that hard: any doctor (and even most medical students) will be able to detect it just by taking a patient's pulse.
With just one lead, the Watch's ECG means it won't become a substitute for a full 12-lead workup that doctors use to diagnose cardiac problems. However, there are other conditions that it could potentially be used to diagnose in future, such as other types of supraventricular tachycardia. Thanks to Apple's ECG functionality, we could be witnessing the beginning of an era of home diagnostics: a time when a diagnosis is handed down by a device on the wrist, not a doctor in a clinic.