Atrial fibrillation and smartwatches: what the sensor really picks up

This site usually tends to explain what a watch on your wrist cannot do. For blood pressure, that is justified. For heart rhythm, the situation is different – and clearly so.

The same sensor technology that reaches its limits with blood pressure identifies atrial fibrillation with an accuracy that professional societies explicitly acknowledge. This is not a contradiction but a question of what an optical sensor sees directly and what it has to calculate.

What this is about: atrial fibrillation

Atrial fibrillation is the most common persistent heart rhythm disorder. According to the German Heart Foundation (Deutsche Herzstiftung), 1.5 to 2 million people in Germany are affected.

The insidious thing about it is not the episode itself but how inconspicuous it is. Prof. Dr. Thomas Voigtländer, Chairman of the Board of the German Heart Foundation, puts it like this: “Atrial fibrillation is a heart rhythm disorder to be taken seriously, because if it goes unnoticed and untreated it can become a life-threatening danger, up to and including heart failure and stroke.”

That is precisely why a technology that runs in the background of everyday life, without you having to think about it, is interesting.

Two methods on one wrist

Smartwatches use two different approaches for rhythm:

• Photoplethysmography (PPG) – the optical sensor on the underside. Using light absorption, it records how blood volume changes with each heartbeat. The software reads irregularities from this curve. Runs passively in the background.
• Single-lead ECG – the top and underside of the watch act as electrodes. You place a finger of your other hand on the top, closing an electrical circuit through the body. This produces a genuine, if greatly simplified, ECG lead. Must be started actively.

According to the German Heart Foundation, both methods identify atrial fibrillation with a certainty of over 90 per cent. Conversely, this also means: fewer than ten per cent of events are missed – not zero.

Why rhythm is easier than pressure

This is the real point of this article, and it can be said in one sentence: for rhythm, the sensor only needs the time interval between two heartbeats – and it sees that directly.

Every heartbeat produces a peak in the light curve. When this peak occurs is a directly readable quantity. Atrial fibrillation shows up as a typically irregular sequence of these intervals. Here the sensor actually measures what matters.

Blood pressure is different. A pressure in millimetres of mercury is not contained in the light curve. It has to be estimated from the shape of the pulse wave using a calculation model that needs calibration and drifts with vessel stiffness, temperature and the fit of the device. Why this is fundamentally a different task is explained in Measuring blood pressure with a smartwatch: what the watch can really do, and why calibration does not change this in Calibrating a blood pressure wristband: why it is not enough.

Time is an observation. Pressure is a derivation. That is the whole difference – and it explains why the same watch is convincing at one task and not at the other. How pressure, pulse and rhythm are connected as three observations of the same process is summarised on Pulse, pressure, rhythm.

What a large study found

The best-known investigation is the Apple Heart Study, published in the New England Journal of Medicine in 2019. More than 400,000 participants were enrolled within eight months. Three figures from it are worth noting:

• Only 0.52 per cent received a notification of an irregular pulse at all. So the function rarely alerts – it is not a constant alarm.
• When an ECG patch was worn at the same time and the watch issued a notification during that period, the finding was confirmed in 84 per cent of cases.
• Of all those who wore an ECG patch after a notification, 34 per cent showed atrial fibrillation during the observation period.

The third figure is often misunderstood as a false alarm rate. It is not. Atrial fibrillation occurs in episodes – it comes and goes. Someone who has an episode today may have a completely regular pulse for days next week. That a later patch finds nothing is to be expected with an intermittent disorder and does not disprove the original indication.

Incidentally, this is exactly where the strength of a watch lies: it is there when the episode occurs. A device you only receive after your doctor's appointment often no longer is.

Cause number one: high blood pressure

For readers of this site, this is the crucial connection. The German Heart Foundation lists high blood pressure as the number one cause of atrial fibrillation and gives a striking figure: around 60 per cent of patients with long-standing high blood pressure also have atrial fibrillation.

The mechanism is plausible: persistently raised pressure puts strain on the left atrium, the tissue changes, and a remodelled atrium fibrillates more easily.

How palpitations, pulse and blood pressure affect one another in everyday life – and why a high pulse does not automatically mean high pressure – is explained on Palpitations, pulse and blood pressure.

Other risk factors named by the German Heart Foundation: age, genetic predisposition and sex on the one hand; on the other, modifiable factors such as excess weight, high alcohol consumption, smoking, stress, extreme endurance sport and electrolyte disorders – plus coronary heart disease, diabetes and sleep apnoea.

This list overlaps almost completely with the list of risk factors for high blood pressure. If you work on your blood pressure values, you are working on more than just the values. What the numbers mean in detail is explained in our blood pressure values table.

What an alert means – and what it does not

The professional society's assessment is clear and easy to remember. Prof. Dr. Christian Veltmann, cardiologist and electrophysiologist, says: “Wearables can identify atrial fibrillation with high accuracy, but they cannot replace diagnosis by a doctor.” And further: “This is why wearables are particularly well suited to screening for atrial fibrillation.”

The two belong together. Screening means: raising a suspicion, not making a diagnosis. The final medical diagnosis is made by the doctor.

An important limitation belongs right alongside this, and it comes from the authoritative German body on this topic. The S3 guideline on atrial fibrillation (the highest-level German clinical guideline; AWMF register number 019-014, 2025 version) states as expert consensus: “General screening for atrial fibrillation with devices that allow continuous monitoring (e.g. smartwatches, wearables) is not recommended at the present time.” The reason given is that the therapeutic consequences of such screening are still unclear.

This does not contradict the German Heart Foundation's assessment; it is a different question. That a sensor identifies atrial fibrillation accurately is a technical statement – and it holds. Whether the entire population should therefore be advised to look for atrial fibrillation with a watch is a health policy question – and the guideline currently answers it with no. Use makes sense where risk is already increased. Which groups these are is explained further below.

In practice, after an alert this means:

1. Don't panic. The notification is a reason for further investigation, not a finding.
2. Save the recording in the app and take it to your appointment – a documented single-lead ECG from the moment of the episode is valuable for the assessment.
3. Note when it occurred and whether you felt anything: palpitations, restlessness, shortness of breath, a sudden drop in performance.
4. At the same time, document your blood pressure over a clean week – see Measuring blood pressure correctly. Given the connection above, both belong on the table together.

What the watch definitely cannot do

A point the German Heart Foundation explicitly emphasises, because misunderstanding it can be life-threatening: a smartwatch does not detect a heart attack. Nor does it pick up every serious rhythm disorder.

If you have chest pain, don't look at your watch. With chest pain, shortness of breath, cold sweat or a feeling of anxiety: call the emergency services on 112 immediately – regardless of what the display shows or does not show.

Nor does the rhythm function replace a blood pressure measurement. What the watch does in this area and where its limits lie is explained in Apple Watch and blood pressure: what the watch really does.

Who benefits most

The German Heart Foundation considers systematic screening for atrial fibrillation useful mainly in two groups:

• in people over 75
• in people over 65 with additional risk factors for stroke

Here the German Heart Foundation and the guideline agree. The S3 guideline also recommends screening – there by ECG – for people aged 75 and over and for people at increased risk of stroke, and it explicitly includes high blood pressure among these risk factors. So if you have had raised values for years, you do not belong to the general population for whom no screening is recommended, but to the group in which a closer look is taken.

According to the guideline, the diagnosis itself should be made by a doctor's assessment of an ECG of sufficient quality. That is another good reason to take the watch's recording to your appointment rather than reading it as a result yourself.

In these groups, a wearable can run alongside as support. For younger people without risk factors, the benefit is smaller – for the same reason that applies to any screening: the rarer a disorder is in a group, the greater the proportion of notifications that are not confirmed.

When you should seek medical advice

• After every alert about an irregular pulse – with the saved recording
• With palpitations, a racing heart, unexplained shortness of breath or a sudden drop in performance, even without any watch
• If you have had raised blood pressure values for years: actively raise the topic of heart rhythm – the connection above is reason enough
• Never change existing medication based on what a wearable shows

In case of sudden severe headache, chest pain, speech or vision problems or one-sided paralysis: call the emergency services on 112 immediately (the European emergency number).