5 ms·

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.

• swayvil 7y ago
  Does it watch heartbeat via the gyroscope? If so then that's a darn sensitive gyroscope.
  • defap 7y ago
    Via an optical sensor that can perform photoplethysmography.
• Symmetry 7y ago
  The Apple watch can look at blood oxygen levels too which might have been what triggered the alert.