There's two features: the irregular heart rhythm notification (available on Series 1 and later) which uses the heart rate sensor and is passively monitoring you

There's two features: the irregular heart rhythm notification (available on Series 1 and later) which uses the heart rate sensor and is passively monitoring you, and the ECG app (only available on Series 4) which is an actual ECG (lead 1). >If the PPG-based arrhythmia detection is enabled, each tachogram is classified using a proprietary algorithm to determine if an irregular rhythm may be present. An irregular tachogram initiates a cascade of more frequent tachogram collection (as frequently as possible, subject to a minimum spacing of 15 minutes) and analysis. Tachograms are collected and analyzed only if the user remains still enough to obtain a reading; because of this, the algorithm is not always monitoring the user, but rather is doing so opportunistically when adequate signal is available for collection/analysis. If five out of six sequential tachograms (including the initial one) are classified as irregular within a 48-hour period, the user is notified of the potential arrhythmia. In addition to the notification, the user can access more information related to these irregular tachograms within the Health app (Figure 1). If two tachograms are classified as not irregular before the threshold is reached, the cycle is reset and tachogram collection returns to the baseline rate (every two hours). Apple published a white paper and a mini site for medical professionals about it [1][2]. [1] https://www.apple.com/healthcare/site/docs/Apple_Watch_Arrhythmia_Detection.pdf https://www.apple.com/healthcare/site/docs/Apple_Watch_Arrhy... [2] https://www.apple.com/healthcare/apple-watch/ https://www.apple.com/healthcare/apple-watch/