last updated 5 Nov 2018

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Health functions

Heart rate:
To fit on the wrist, they use a sensor called a photoplethysmogram (PPG) which uses light to determine how much blood the heart is pumping under the surface of the skin.
Dr. Mintu Turakhia, executive director of Stanford University's Center for Digital Health, says this measure is usually accurate for most normal conditions and normal rhythms, but it can lose accuracy under certain conditions.
Movement is one of the variables that can interfere with a good read. While resting heart rate tends to be spot-on, the sensor may have a harder time getting an exact read during high-impact activities when the watch is bouncing up and down the wrist. Tattoos can also interfere with a reliable read as they may block the light from penetrating the skin.

How EKG works:

Source: Cnet

The Series 4 has electrodes on the digital crown and on the electrical heart rate sensor on the back of the watch. Once in the ECG app, the user just places his finger on the digital crown to get a read. After 30 seconds the watch lets the user know if the results were normal or of they have signs of atrial fibrillation (AFib).

Fall Detection and Emergency SOS
When I was an EMT the most frequent reason for a 911 call was a fall.
Fall detection, where the accelerometer and gyroscope work in tandem to figure out the kind of fall and then decide whether they should get the watch to phone the emergency services on your behalf. This only kicks in if you donŐt move within a minute, and itŐs turned off by default if youŐre under the age of 65.

You can choose to contact emergency services or dismiss the alert by pressing the Digital Crown, tapping Close in the upper-left corner, or tapping "I'm OK" and choosing an option on the screen.
If your watch detects that you have been immobile for about a minute, it will make the call automatically. After the call ends, your watch sends a message to your emergency contacts with your location letting them know that your watch detected a hard fall and dialed emergency services. Your watch gets your emergency contacts from your Medical ID.

Apple Watch Series 4 |
  Apple•Watch Series•4 - Health - Apple
The Apple Watch Series 4 takes heart monitoring to next level | Cnet
New Apple Watch May Be Part of the Future of Wearable Heart Devices |