I was watching a documentary about a woman who survived without breathing for like a half hour after falling in an icy river, and then was pulled out and survived without any permanent damage. It turns out that if the body temperature is cooled below like 70 degrees Fahrenheit (i.e. not very cold), the blood vessels and brain cells and shit take much longer to die, on the order of an hour instead of just minutes at the normal body temperature.
So my idea is to create an implanted medical device that detects when your heart stops and, instead of shocking your heart to start it again like a pacemaker, it cools your body to the temperature where the blood vessels die much slower, giving you up to an hour to be rescued instead of only a couple minutes.
It has a blood pump, a cooling mechanism (practicality not established), and a cellular communication device. It will alert 911 when it detects that your heart stops, and will not start pumping and cooling the body until it gets confirmation from 911 that the alert was received.
The idea would be to implant this device in anyone with a life threatening medical condition and above a certain age, and it would virtually eliminate death due to not getting to the hospital in time. According to http://www.apa.org/pi/aids/programs/eol/end-of-life-factsheet.aspx, 20% of people don't die in the hospital, and "most people" who make it to the hospital arrive too late to get the most benefit possible from hospital services.