A normal ECG on Apple Watch displays the words “Sinus Rhythm” along with a heart rate between 50 and 100 beats per minute. The screen shows a smooth, repeating waveform with evenly spaced peaks, and the result is stored in the Health app on your iPhone alongside a detailed trace you can zoom in on and share with your doctor.
What “Sinus Rhythm” Means
When the Apple Watch classifies your reading as sinus rhythm, it’s telling you two things: your heart rate falls between 50 and 100 BPM, and the beats are arriving in a uniform, regular pattern. “Sinus” refers to the sinoatrial node, a tiny cluster of cells in the upper right chamber of your heart that acts as a natural pacemaker. In a normal rhythm, this node fires an electrical signal, the upper chambers contract, then the lower chambers follow in sync. That coordinated squeeze is what the watch is looking for.
On the results screen, you’ll see a green heart icon alongside the sinus rhythm label. Below it, Apple displays the explanation: “The heart is beating in a uniform pattern between 50 and 100 BPM.” If your heart rate is normal but falls outside that window (say, 48 BPM or 104 BPM), the app may return an inconclusive result even though nothing is actually wrong.
Reading the Waveform Itself
After the 30-second recording finishes, the Apple Watch shows a scrolling waveform, and a more detailed version appears in the Health app on your iPhone under the ECG Detail screen. A normal trace has a recognizable repeating pattern made up of several small waves and one tall spike per heartbeat.
Each cycle starts with a small, rounded bump called the P wave, which represents the upper chambers of your heart contracting. Next comes the tall, sharp spike known as the QRS complex, the most prominent feature on the trace. This is the lower chambers firing. Finally, there’s a gentler, rounded hump called the T wave, which is the heart resetting its electrical charge before the next beat. On a normal reading, these three landmarks repeat at regular intervals across the entire recording, like a steady pattern on a strip of wallpaper.
The spaces between these waves matter too. The gap from the start of the P wave to the start of the tall spike typically lasts between 0.1 and 0.2 seconds. The spike itself should be narrow, finishing in under 0.12 seconds. And the flat line between the spike and the T wave should sit close to the baseline, neither climbing noticeably above it nor dipping below it. You won’t need to measure any of this yourself. The watch’s algorithm handles classification automatically. But if you’re staring at your trace wondering whether it looks right, those evenly spaced, consistent peaks with a flat baseline between them are exactly what normal looks like.
What a Normal Trace Does Not Look Like
Knowing what’s normal is easier when you can contrast it with what isn’t. Atrial fibrillation, the main condition the Apple Watch is designed to catch, looks distinctly different. Instead of evenly spaced tall spikes, the intervals between beats are irregular, some closer together, some farther apart, with no predictable rhythm. The small P wave before each spike often disappears entirely because the upper chambers are quivering chaotically rather than contracting in an organized way. If the watch detects this pattern, it displays “Atrial Fibrillation” instead of sinus rhythm.
You might also see results labeled “Inconclusive” or “Poor Recording.” These don’t necessarily mean something is wrong with your heart. Major signal interference (from a loose watch band, sweaty skin, or movement during the recording) is one of the strongest predictors of an inconclusive result, increasing the odds by roughly tenfold. Certain heart-related factors also raise the chance of an unreadable trace: having a pacemaker, extra beats (premature contractions), low-voltage signals, or conduction delays all make it harder for the algorithm to classify the rhythm confidently.
What a Single-Lead ECG Can and Cannot Tell You
The Apple Watch records a single electrical perspective of your heart, equivalent to Lead I on a standard clinical ECG. A hospital ECG uses 12 leads, meaning 12 different angles, to build a complete picture. Think of it like photographing a building: the Apple Watch gives you the view from the front door, while a 12-lead gives you every side plus the roof.
That single view is genuinely useful for one thing in particular: spotting atrial fibrillation. In clinical testing, the Apple Watch ECG showed a sensitivity of about 93.5% for detecting AFib and a specificity of 100% for confirming sinus rhythm when it returned a clear, classifiable result. Those are strong numbers. The catch is that a meaningful portion of recordings come back inconclusive. When those unreadable results are factored in, overall accuracy drops considerably, with sensitivity falling to around 66% and specificity to about 73%.
The watch explicitly warns during each recording that it cannot detect heart attacks. Identifying a heart attack typically requires looking at multiple leads to see which region of the heart is affected, something a single lead simply can’t do. The same goes for many structural heart problems, valve disorders, and other arrhythmias beyond AFib. A normal-looking Apple Watch trace is reassuring for rhythm, but it’s not a clean bill of cardiac health.
Getting the Clearest Possible Reading
Your results are only as good as the signal quality. Rest your arm on a table or your lap so it’s still, and place the index finger of your opposite hand gently on the Digital Crown. Don’t press hard, just maintain light, steady contact. Stay still and quiet for the full 30 seconds. Even small movements can introduce artifacts, those jagged, noisy squiggles that make the trace harder to read and increase the chance of an inconclusive result.
Make sure the watch is snug against your wrist, not sliding around, and that your skin isn’t excessively wet or covered in lotion. If you get an inconclusive result, wait a minute, reposition, and try again. Many inconclusive readings clear up on a second attempt simply because the contact improved. The recording and its classification are saved automatically in the Health app, where you can pull up the full waveform chart, compare it to past recordings, and export a PDF to share with your doctor if anything looks different from your usual baseline.