Apple Watch
does that Apple Watch detect a heart that misses beats
Apple Watch Series 3
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Your Apple Watch heart monitor function samples heart rate for short periods, every so often (seems to be variable intervals). Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
It would detect single missing beats during a sampling interval as a lower count, so it is unlikely to alert you unless the detected (reduced) rate is below the level configured for alerts. As an example with invented numbers, if the watch samples for 10 seconds and your heart rate is 60 it would count 10 beats. Missing a single beat in that 10 second sample would reduce the observed rate to 54. It would be undesirable to alert for single missed beats, because the overwhelming majority would be false alerts caused by users moving the watch, energetic movement with a loose band, shirt cuffs blocking the sensor, any of the other reasons the watch could fail to see one beat.