The Apple Watch Series 4 is the one of the most impressive smartwatches you can buy today, but when we reviewed it back in September 2018, its headline feature wasn't yet available.

Due to waiting for certification from the US Food and Drug Administration (FDA), Apple had to put the Watch Series 4 on sale without its new electrocardiogram feature working. FDA certification arrived at the end of 2018, enabling the feature on watches bought in the US, and now certification has also been granted in Europe.

Read More:

• Apple Watch Series 4 review
• Six wearables that keep an eye on your heart

Apple Watch Series 4 (GPS, 44mm) - Space Gray Aluminium Case with Black Sport Band

List Price: $429.00

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As of late-March 2019, the Watch Series 4 can perform an ECG on wearers in 23 countries and territories. But do bear in mind, this location restriction is based on where the watch was bought, not where it is being used or where the owner's Apple ID is located. If you bought a Watch Series 4 from a country not on the list below, then it will not work until that country certifies the ECG feature.

Before we get started on explaining how the ECG works (and what it is and isn't capable of) here's a list of the countries and territories which have certified the feature:

• Austria
• Belgium
• Denmark
• Finland
• France
• Germany
• Greece
• Guam
• Hong Kong
• Hungary
• Ireland
• Italy
• Luxembourg
• Netherlands
• Portugal
• Puerto Rico
• Romania
• Spain
• Sweden
• Switzerland
• United Kingdom
• United States
• US Virgin Islands

The majority of these countries were added with the latest watchOS software update, version number 5.2, so if you're not in the US you'll need to update your watch using the Watch app on your iPhone before the ECG app appears.

How to set up and use the ECG feature of the Apple Watch Series 4

The Health app on your iPhone walks you through the ECG setup processGearBrain

Once you have updated your watch software to version 5.2 or newer, the ECG app will appear on the wearable's home screen.

Tap this and, for the first time only, you'll be asked to complete setup on the iPhone's Health app. So grab your phone, open Health, and you'll be greeted by a message introducing the ECG app, and encouraging you to set it up.

Apple Watch 4 Available at Best Buy

Tap 'Set Up ECG App' and you'll be asked to enter your date of birth. This is because Apple recommends that Watch owners under the age of 22 do not use the ECG app.

Next, the app walks your through how the ECG function works. Apple is clear to explain that it cannot detect a heart attack and is not to be used if the wearer thinks they may have had one. An electrocardiogram, as the app explains, represents the electrical pulses that make your heart beat. The app "checks these pulses to get your heart rate and see if the upper and lower chambers of your heart are in rhythm. If they're out of rhythm, that's atrial fibrillation."

Screenshots from the Apple Watch of the ECG app in actionGearBrain

The app then runs you through four possible results Watch wearers may see from the ECG. These are:

• Sinus Rhythm - The heart is beating in a uniform pattern
• Atrial Fibrillation (AF) - The heart is beating in an irregular pattern. AF is the most common form of serious arrhythmia
• Low or high heart rate - Under 50 beats per minute or over 120 affects the app's ability to check for AF, and the recording is considered inconclusive
• Inconclusive - As well as being caused by a low or high heart rate, an inconclusive reading can happen "for many reasons", Apple says, such as not resting your arms on a table while taking the ECG, or if your watch is too loose.

What the Apple Watch ECG function cannot do:

• It cannot detect a heart attack
• It cannot detect blood clots or stroke
• It cannot detect other heart-related conditions, including high blood pressure, congestive heart failure, high cholesterol, or other forms of arrhythmia

How to take an ECG with the Apple Watch Series 4

Setting up the ECG feature of the Apple Watch Series 4GearBrain

Once you have tapped through the app's information pages on your iPhone, it's time to take your first ECG. The app tells you that it typically takes 30 seconds, and for the duration you must keep the watch on your wrist, with the index finger of your opposite hand pressed against the Digital Crown. Rest both arms on a table until the process has finished.

Apple Watch Series 4 (GPS, 44mm) - Space Gray Aluminium Case with Black Sport Band

List Price: $429.00

New From: $414.00in Stock

After 30 seconds, the Watch gives you a result. For me, this was sinus rhythm, followed by: "This ECG does not show signs of atrial fibrillation."

The iPhone app now tells you setup is complete, along with a graph to show your heart's performance during the test, and your average heart rate over that 30-second period. A PDF of your recordings can be made in the iPhone's Health app, to be given to your doctor. Finally the results of your ECG are added to the home page of the Health app, along with all the other readings your watch takes, like daily steps taken and calories burned.

ECG readings are stored in the iOS Health appGearBrain

Irregular Rhythm Notifications

The next time you open the Health app, it will present a notification about another new feature now available on the Watch Series 4 - Irregular Rhythm Notifications.

Tap 'set up notifications' and the app explains what this new system is, and how it works. Basically, the Watch will now take occasional tests of your heart rate through the day, then notify you if an irregular rhythm is detected.

Apple Watch 4 Available at Best Buy

The app states: "Apple Watch will occasionally look at your heartbeat to check for an irregular rhythm that may be AF. This usually happens when you're still to ensure a more accurate reading."

The number of readings taken each day will depend on how active you are, and the time between the readings will vary.

As before, this also requires your date of birth and you must say if you have or haven't been diagnosed with AF. Then, again, you are reminded that the Apple Watch cannot detect a heart attack, and that if you are feeling unwell you should consult your doctor, even if the Watch does not notify you of anything being wrong.

Apple Watch Series 4 (GPS + Cellular, 44mm) - Space Gray Aluminium Case with Black Sport Band

List Price: $529.00

New From: $514.99in Stock