Can Apple Watch help manage my heart rhythm?

Apple Watch series 4 and above includes an app that allows you to make a 30 second recording of your heartbeat’s electrical activity (also called an “electrocardiogram” or “ECG”). The app itself is called “ECG” and can link to the “Health” app on a paired iPhone. You can then use the iPhone display to review the data you have captured, and even use email to send the recording to someone else, for example your electrophysiologist. There is also a built-in algorithm within the app that Apple say can identify the most common heart rhythm abnormality - atrial fibrillation - automatically.

Lots of my patients have an iPhone and an Apple Watch, and I am often asked whether we can use the data from their Apple Watch to diagnose or manage their heart rhythm problems.

First off,  we should be clear that we are not talking about heart rate data. Separate from the ECG app, Apple Watch can continuously measure your heart rate and feed this into the Health app on your iPhone to give a continuous graph of your heart rate over time, measured in beats per minute (bpm). All Apple Watches way back to series 1 can do this. In certain circumstances the heart rate graph alone can be really useful and in some rare cases it can provide enough information to diagnose an arrhythmia. However, a simple graph of your heart rate does not often provide enough detail on its own to tell if your heart rhythm (as opposed to the rate) is normal.

From series 4 onwards there is an additional app, “ECG”, available on the Apple Watch. This adds the ability to record a 30 sec snapshot of your heart’s electrical activity, similar to the ECG test you will recognise having done in GP’s surgery, A&E, etc, that produces a series of squiggles on a page of paper with red lines on it. Apple essentially records the same squiggles, but without the red paper.

There are plenty of videos on YouTube about how to use the ECG app to record an ECG. Here is a good example.

The real value of the Apple Watch ECG is that you can record it whenever you want. Therefore, you can use it to try to detect heart rhythm abnormalities that might be intermittent, or “paroxysmal” in medical-speak.

For example, if you are troubled by palpitations but these only affect you once every couple of weeks, then using the Apple Watch ECG function could allow you to record an ECG at the time that you feel your symptoms. Before we had access to devices such as Apple Watch, which allow “symptom driven” ECG recording, the only solution we had to try and pick up episodes of intermittent arrhythmia was to monitor patients’ ECGs for longer and longer times and see what we picked up. This approach had severe limitations.

A law of diminishing returns applies to recording longer and longer ECGs in the hope of picking up short episodes of heart rhythm disturbance. In one study that sought to diagnose atrial fibrillation in patients who had a stroke, 15.7% had the arrhythmia detected on a 10 second ECG recording taken at a random time when they were admitted to hospital, and a further 12.7% had arrhythmia diagnosed when the ECG recording was extended to 7 days. For each individual 24hr period of ECG monitoring done, roughly 4.8% of patients had the arrhythmia. You can quickly see that this is very inefficient: you have to do a lot of ECG recording to pick up relatively few additional diagnoses of atrial fibrillation. Read the original scientific report of the study here.

In contrast, Apple Watch and other symptom driven ECG devices can record an episode of arrhythmia whenever it occurs, as long as it lasts long enough for you to activate the ECG function on your device. This probably takes most people around 10 seconds or so, so if your symptoms last less than this, symptom driven ECG recording may not be the solution you need and we may need to look at using prolonged ECG recordings for your problem instead.

However, most  people with symptoms from significant heart rhythm disturbances  will have symptoms that last long enough for them to use Apple Watch to capture an episode. If this is the case for you, the data from Apple Watch can be used in two ways:

First, we can use the device to make a diagnosis. For example, if a patient comes to me complaining of intermittent palpitations, we can use Apple Watch to record an ECG during their symptoms. They can then e-mail the recording to me - or share it with me on screen during an appointment - and in the majority of cases I can reach a diagnosis, or recommend further tests to confirm a suspected diagnosis based on the Apple Watch data.

The second potential use is for what I call “symptom-rhythm correlation”: Many patients with arrhythmia suffer very variable symptoms and sometimes it can be unclear whether all the symptoms are caused by the arrhythmia or not. If we are trying to work out whether to use a particular treatment, for example an ablation, you will probably want the reassurance of knowing that the treatment is going to prevent all - not just some - of your symptoms. In this setting we can use Apple Watch to correlate the symptoms that you feel with the heart’s rhythm at the time, which will inform your decision on whether the treatment is worthwhile or not.

These are the two main uses for Apple Watch ECG in my own medical practice. You may find that other practitioners use the data rather differently. In particular you may find that some doctors rely more heavily on Apple’s built-in “AF detection algorithm”. Apple’s algorithm does have impressive accuracy under certain conditions for detecting or rejecting a diagnosis of atrial fibrillation, but it will only work when you have an excellent quality ECG signal, and when your heart rate is less than 120bpm - in my experience anything above this will simply be labelled “tachycardia” or “indeterminate” by the algorithm. Given that a lot of people get a heart rate far higher than 120bpm during attacks of AF, this is a significant limitation. For many patients therefore, there is simply no substitute for having your ECG data reviewed by a specialist electrophysiologist.

In summary, I find Apple Watch and other symptom driven ECG devices an invaluable addition to our toolbox for managing patients whose arrhythmias come and go over time, but to make the most of the data you capture often requires input from a heart rhythm specialist too.

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You can read more about atrial fibrillation and other cardiac arrhythmias on the patient information pages of my private practice website at https://www.kentlondoncardio.com.