I've been reading up on the Apple Watch ECG a bit, but do not own one nor have I used one.
1. To call this an ECG is misleading, but from a product perspective I can understand why they went with that (over)simplicity.
2. To function in "ECG" mode, you need to have the watch on your wrist and finger on the crown. This completes the circuit. ie, it would be very difficult to use during exercise (I also assume that the additional electrical "noise" would stymie it's accuracy)
3. I'm fairly sure that this would be able to "detect" many arrhythmias at rest, however, this is unlikely during exercise (see above). The only arrhythmia that it seems to categorise is atrial fibrillation. This may be valuable to some, or not. Unfortunately, I doubt that it has the ability to detect the "silent killers" that cause "athletes" to "drop dead". (Long QT, WPW, etc)
4. I also doubt it will be able to detect perfusion issues: ST-segment depression from ischemia, "heart-attacks", etc. (However, I'll go on a limb and say that this ability is of more value to the general public than rhythm issues. Double so if possible during exercise).
In my opinion, it is super-cool that they're trying to integrate this technology into wearables. I do, however, feel this is premature as a product.
If anyone is considering this because they fear cardiac issues, please see a cardiologist to discuss your concerns. Resting EKG, Stress Tests and Holter monitors are all of more significant clinical value.
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