When Apple launched the Watch Series 4 back in September, it said the smartwatch would be able to take an ECG (electrocardiogram, also known as an EKG), and would be one of the very first consumer devices to do so.

Today, December 6, Apple has finally released the ECG function along with a Watch application to use it. All you have to do is install the latest software update, which is available in the US now.

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The announcement was tipped off by The Verge, who appeared to have published an embargoed post prematurely, before deleting the article. Apple tends to release software updates from 10am PST, six hours after the post went live.

This app is exclusive to the Series 4, but the software update also brings a new function to all previous models of Watch, apart from the original, known as the Series 0. This new function notifies the wearer if the watch detects an abnormal heart rate, which could be a symptom of atrial fibrillation and other heart issues.

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On opening the ECG app, you are asked to rest your index finger on the Digital Crown and keep it there for about 30 seconds while the Watch gets to work. It performs the equivalent of a single-lead ECG and save the data it gathers to your Health app on your iPhone. The data can be saved as a PDF and sent to your doctor.

The irregular heart rate function is passive, and works by taking a measurement every couple of hours. If the Watch spots five consecutive irregular heart rates (either high or low), it will notify you and suggest you speak to your doctor.

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Apple has gained clearance from the Food and Drug Administration to switch these features on in the US, but is yet to gain clearance in other countries where the Watch is sold. So for now, only US-based Watch users can access the features.

It is also important to note that Apple has gained 'clearance' from the FDA, not 'approval' and it must be stressed that the Watch functions are not to be used instead of a visit to the doctor.

In a September letter sent to Apple confirming the ECG app's clearance, the FDA said the 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...[the ECG app is] not intended to replace traditional methods of diagnosis or treatment. The ECG app is not intended for use by people under 22 years old."

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"Apple Watch has helped so many people around the world and we are humbled that it has become such an important part of our customers' lives," said Jeff Williams, Apple's chief operating officer. "With the release of these heart features, Apple Watch takes the next step in empowering people with more information about their health."

Apple wants to make sure its customers fully understand the limitations of these new applications before they are switched on for the first time. The Health app for iOS walks Watch owners through several pages of information to make sure they understand what the Watch can and cannot do.

For example, Apple explains how the Watch cannot detect heart attacks, and that if you are feeling unwell you should still see your doctor; in other words, the Watch should not be relied upon to give you the all-clear, or notify you when there if something wrong. Apple also states how the Watch cannot detect other heart conditions, like high blood pressure, congestive heart failure, and high cholesterol.

Despite all these limitations, this software update marks an import move forward for Apple and the Watch, as it transitions from a piece of fashion, to a fitness tracker, and now towards a health device.

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