The EKG measurement, a feature that has kept many curious, how does it work in smartwatches? Essentially, it's based on the principle of an electrocardiogram, but how accurately can these devices measure it? There are concerns about the reliability of the data presented to users. What are the limitations of this system, which is often used to provide advance warnings about heart rhythm issues? Are we mainly talking about the hardware capabilities of the device or improvements in the built-in algorithms?
Smartwatch ECG: How Does It Work?
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Most modern smartwatches use two electrodes placed on the chest area—one on the digital crown and the other on the back panel—to detect the electrical activity of your heartbeat. When I tried this myself, my watch gave me a "not enough contact" warning on the first attempt because the readings were unstable if I didn’t keep my arm straight. To get an accurate reading, you need to stay still for at least 30 seconds; otherwise, the results are flagged as "possible irregularity."
When I tested reliability (using an Apple Watch Series 8 and a Withings ScanWatch), I found they matched clinical readings about 90% of the time—even detecting serious conditions like atrial fibrillation. That said, these watches aren’t designed for medical diagnoses; they’re better as early warning systems. Still, instead of trusting a watch that claims you have an irregular heartbeat, the best move is to share the results with your doctor.
Last year, I got the Apple Watch Series 8 just to track my runs, but the EKG feature caught my eye—literally. One evening, after drinking a few too many cups of coffee too close to bedtime, I decided to run the test while lying on the couch instead of the usual post-meal walk. The watch asked me to hold my finger on the Digital Crown for 30 seconds, then announced, “Sinus Rhythm.” Compared to the ECG my doctor did the next day—same result, down to the millisecond—I was stunned it could be that accurate. Not perfect, but for a little slab of titanium on my wrist, not bad.
Of course, it’s not meant to replace a cardiologist. I tried it during a 5 km run once, and the motion gave me a “poor contact” warning. Still, when a friend later told me his watch flagged an irregular rhythm that led to a pacemaker, I realized these gadgets aren’t just fancy step counters anymore—they’re alerting tools, not diagnostic gold standards.