On an Apple Watch ECG, atrial fibrillation shows up as an irregular, jagged rhythm without the small, rounded bumps that normally appear before each tall spike. Instead of the neat, repeating pattern you’d see with a normal heartbeat, the tracing looks disorganized, with the tall spikes spaced unevenly apart. The watch itself will display “Atrial Fibrillation” as the result classification after the 30-second recording.
What a Normal ECG Looks Like on Apple Watch
To spot AFib, it helps to know what a healthy reading looks like first. When the Apple Watch shows “Sinus Rhythm,” you’re looking at a repeating pattern with three main features. First, there’s a small, gentle bump called the P-wave, which represents your heart’s upper chambers squeezing. Next comes a tall, sharp spike (the QRS complex), which is your lower chambers contracting to pump blood. Finally, there’s a broad, rounded wave (the T-wave) as the heart resets for its next beat.
The key signature of a normal rhythm is consistency. The distance between each tall spike is roughly the same, and every spike is preceded by that small P-wave bump. The whole pattern repeats like a steady drumbeat, with a heart rate between 50 and 100 beats per minute.
What AFib Looks Like by Comparison
AFib disrupts that orderly pattern in two visible ways. The first and most obvious difference is the spacing between the tall spikes. In a normal rhythm, these spikes march across the screen at even intervals. In AFib, the gaps between them are uneven: some are close together, some are far apart, with no predictable pattern. This reflects the heart’s upper chambers firing chaotically instead of in a coordinated rhythm.
The second difference is harder to see on a wrist-based recording but still important: the P-wave disappears. That small rounded bump before each spike is gone, replaced by a wobbly, irregular baseline. In a clinical 12-lead ECG, this is unmistakable. On the Apple Watch’s single-lead tracing, it can be subtler because the recording is more prone to noise and artifacts. The baseline may look jittery or wavy rather than smooth, but the missing P-waves won’t always be as clear as they would on hospital-grade equipment.
If you’re looking at your own recording in the Health app on your iPhone, focus on the tall spikes. Are they evenly spaced like fence posts, or do they look scattered? Uneven spacing is the single most reliable visual clue for AFib on a wrist ECG.
What the Watch Actually Tells You
You don’t have to read the waveform yourself. After the 30-second recording, the Apple Watch classifies the result into one of several categories:
- Sinus Rhythm: A uniform heartbeat pattern between 50 and 100 BPM. This is the normal result.
- Atrial Fibrillation: The algorithm detected an irregular rhythm consistent with AFib.
- Inconclusive: The watch couldn’t make a determination. This often happens when the heart rate falls outside the app’s detection range.
- Poor Recording: Too much noise in the signal to analyze, usually from movement or poor skin contact.
The ECG app version matters here. Version 1 can check for AFib only when the heart rate is between 50 and 120 BPM. Version 2 extends that upper limit to 150 BPM. If your heart rate is above or below these thresholds, the app returns an inconclusive result even if AFib is present. On version 1, a heart rate between 100 and 120 BPM without AFib also triggers an inconclusive reading.
How Accurate Is the Detection?
A 2025 meta-analysis pooling data from multiple clinical studies found that the Apple Watch ECG detects AFib with 94.8% sensitivity and 95% specificity. In practical terms, that means it correctly identifies AFib about 95 times out of 100, and it correctly identifies normal rhythm about 95 times out of 100. Those are strong numbers for a consumer device, but they also mean roughly 1 in 20 readings could be a false positive or a missed case.
The watch is FDA-cleared as a Class II medical device, but with an important caveat: it’s designed to distinguish AFib from normal sinus rhythm only. It is not intended to detect heart attacks, other types of arrhythmias, or structural heart problems. If you have a known arrhythmia other than AFib, Apple explicitly notes the app is not recommended for you, because the algorithm may misclassify your rhythm.
Why Recordings Sometimes Fail
A “Poor Recording” result usually comes down to physical factors during the 30-second test. The sensor needs clean electrical contact with your skin on both wrists: one through the watch’s back crystal and one through your fingertip on the Digital Crown. Incomplete skin contact, dry skin, and any movement during recording are the most common culprits. The sensor is also more prone to errors with excessive motion or a loosely fitting band.
For the best recording, rest your arms on a table or your lap. Press your fingertip firmly but gently on the crown without squeezing. Stay still and avoid talking for the full 30 seconds. If you get repeated poor recordings, try slightly moistening the skin on your wrist or adjusting the band so the watch sits snugly against the back of your wrist.
What to Do With an AFib Result
An AFib classification on your Apple Watch is a screening result, not a diagnosis. The waveform PDF that the Health app generates is genuinely useful, though. You can export it and share it directly with a doctor, who can review both the tracing and the watch’s classification. Many physicians now accept these recordings as a starting point for further evaluation, which typically involves a standard 12-lead ECG or a longer-term heart monitor to confirm the finding and assess how frequently the irregular rhythm occurs.
If the watch shows AFib once but subsequent recordings show sinus rhythm, that’s still worth mentioning at your next appointment. AFib often comes and goes, especially in its early stages, and a single captured episode can be the clue that leads to a diagnosis that would otherwise be missed during a brief office visit.