A Comparative Analysis of Signal Quality and Morphological Fidelity in ECG Smartwatches
This study objectively compares the signal quality and morphological fidelity of four leading smartwatches against a Philips reference standard, revealing that the Apple Watch and Withings ScanWatch offer superior signal-to-noise ratios, the Withings device provides the most stable baseline, and the Apple Watch delivers the most balanced ST-segment classification performance.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
The human heart is a relentless pump, but its true language is electrical. Every time it beats, it sends a tiny electrical signal rippling through the body, a pattern that doctors have long relied on to diagnose everything from minor irregularities to life-threatening emergencies. For decades, capturing this signal required a visit to a clinic, where a patient lay still while a machine with multiple wires and sticky pads recorded the heart's rhythm. These recordings, known as electrocardiograms or ECGs, are the gold standard for cardiac care because they offer a clear, detailed picture of the heart's electrical activity. However, this traditional method has a significant limitation: it only captures a few minutes of data in a controlled environment, often missing the very moments when a problem might occur in the middle of a busy day.
In recent years, technology has brought the ability to record this same electrical signal to the wrist. Smartwatches and wearable devices now promise to monitor heart health continuously, offering a window into the body's natural rhythm during sleep, exercise, and daily life. Yet, as these devices have multiplied, a critical question has emerged: do they actually see the heart clearly enough to be trusted for medical decisions? The electrical signal from the heart is faint and easily distorted by movement, muscle tension, or the simple friction of a sensor against the skin. If a device adds too much static or smooths out the delicate curves of the heartbeat, it could hide a warning sign or create a false alarm. The difference between a reliable medical tool and a mere fitness tracker often comes down to how well the device preserves the specific shape and timing of the heart's electrical waves.
A team of researchers set out to answer this question by putting four of the most popular commercial smartwatches through a rigorous, head-to-head test. They wanted to know which device could capture the heart's electrical story with the most accuracy and which features might be lost in the translation. To do this, they did not rely on volunteers walking around a park or running on a treadmill, where movement would introduce too many variables. Instead, they used a controlled laboratory setting with a patient simulator, a machine that generates perfect, known electrical heart signals. They recorded these signals simultaneously using the Apple Watch Series 9, the Samsung Galaxy Watch 6, the Fitbit Sense 2, and the Withings ScanWatch, comparing each one against a hospital-grade Philips TC30 machine that served as the reference standard. By using a machine that produces a known, perfect signal, the researchers could measure exactly how much noise each watch added and how much it altered the shape of the heartbeat.
The results revealed a clear hierarchy in performance, showing that not all smartwatches are created equal when it comes to medical-grade data. The Apple Watch and the Withings ScanWatch emerged as the leaders in signal clarity, producing recordings with the least amount of background noise. In contrast, the Samsung Galaxy Watch 6 showed significantly more noise, making its signal much harder to read. Another crucial factor was the stability of the baseline, which is the flat line the heart signal rests on between beats. If this line wobbles up and down, it can make it difficult to see if the heart is truly struggling. The Withings ScanWatch proved to be the most stable, keeping its line remarkably steady, while the Fitbit Sense 2 showed the most wobble, which could complicate the detection of certain heart conditions.
Beyond just the clarity of the signal, the researchers examined how well each device could identify specific, critical points in the heartbeat that doctors use to diagnose problems. They focused on the J-point, the moment the heart finishes its main contraction and begins to relax, and the ST-segment, a specific part of the wave that indicates whether the heart muscle is getting enough oxygen. The study found a consistent pattern across all devices: they tended to smooth out the signal slightly, causing a small but measurable delay in identifying the J-point. While this delay was tiny—measuring only a few milliseconds—it is significant enough to matter for automated systems that calculate heart health based on precise timing. This smoothing effect suggests that the internal software of these watches is designed to clean up the signal, but in doing so, it sometimes blurs the sharpest edges of the heart's electrical activity.
When it came to detecting dangerous changes in the ST-segment, such as the signs of a heart attack, the Apple Watch demonstrated the most balanced performance. It was highly accurate at identifying both when the heart was under stress and when it was healthy, striking a good balance between catching real problems and avoiding false alarms. The Samsung watch was good at finding real problems but struggled more with false alarms, likely due to its noisier signal. The Fitbit and Withings devices were very good at avoiding false alarms but missed a significant number of real problems, particularly when the heart showed subtle signs of distress. This means that while these devices are excellent at confirming that the heart is normal, they might fail to sound the alarm when it is actually needed.
The study concludes that while these wearable devices have made impressive strides toward clinical utility, they are not interchangeable tools. A researcher or doctor cannot simply pick any smartwatch and expect the same results. If the goal is to monitor long-term stability or detect subtle shifts in the heart's baseline, the Withings ScanWatch offers superior performance. If the priority is a balanced detection of heart stress and oxygen deprivation, the Apple Watch Series 9 appears to be the most reliable choice. The Samsung Galaxy Watch, despite its lower signal quality, still holds value for analyzing the timing of specific heart events, while the Fitbit Sense 2 may be best suited for general rhythm monitoring where extreme precision is not the primary concern. Ultimately, the choice of device depends entirely on the specific question being asked, and understanding these differences is the first step toward using this technology safely and effectively in the real world.
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