Today, voluntary and can call 911, tomorrow used for purchasing goods (like in the netherlands) and required. From Reuters:
The Bluetooth wireless technology that allows people to use a hands-free earpiece while making a mobile telephone call could soon alert the emergency services when someone has a heart attack, Ofcom predicts. ...The communications regulator said that sensors could be implanted into people at risk of heart attack or diabetic collapse that would allow doctors to monitor them remotely. ...If the “in-body network” recorded that the person had suddenly collapsed, it would send an alert, via a nearby base station at their home, to a surgery or hospital. ...However, Ofcom also gave warning in its report, Tomorrow’s Wireless World, that the impact of such technology on personal privacy would require more debate.