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It watches the heartbeat, probably arrhythmias specifically. You can't monitor blood pressure with just a watch. You need a blood pressure cuff for that.

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The Apple watch can look at blood oxygen levels too which might have been what triggered the alert.

Does it watch heartbeat via the gyroscope? If so then that's a darn sensitive gyroscope.

Via an optical sensor that can perform photoplethysmography.

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