Is an ECG from your smartwatch actually useful?

For atrial fibrillation (AF), a heart rhythm disorder that can cause strokes, smartwatch ECGs have been well studied. A meta-analysis found a sensitivity of 93% and a specificity of 94%. That is impressive for a wristwatch¹.

In practice, performance varies considerably. A study of 201 patients tested five brands, including the Apple Watch 6, Samsung Galaxy Watch 3, and Fitbit Sense. Sensitivity ranged from 58% to 85%, and in 17 to 26% of measurements the algorithm gave no result at all. When a doctor reviewed those borderline cases, 99% were still classified correctly. Without medical follow-up, such a watch is therefore a limited tool².

In older adults after a stroke (mean age 79 years), the Fitbit Charge 5 automatically missed as many as 66% of AF cases. But when it did raise an alarm, that was almost always correct. A green notification can be taken seriously; no notification is not reassurance³.

A major advantage of a smartwatch is continuous monitoring. AF sometimes occurs in brief episodes that a one-off measurement at the GP's office misses. The Apple Heart Study and Fitbit Heart Study show that smartwatches detect such episodes more often⁴. For other heart problems, such as oxygen deficiency in the heart muscle, the watch ECG does not yet work well enough^5,6.

If you want to track your heart rate variability, a measure of recovery and stress, via a Samsung smartwatch, do so at night. During the day, movement causes too many measurement errors. Nighttime measurements do agree well with a standard measurement⁷. And remember: a smartwatch signals, but does not replace a doctor or healthy lifestyle habits⁸.