Your chances of surviving a cardiac arrest while out on the high street are slim. It's estimated survival rates decrease by ten percent for every minute you don't get medical help. The nearest ambulance may be on its way but could take several minutes to arrive. But what if an off-duty paramedic was just around the corner and could help out? BBC World Hacks looks at a new alert system that informs people with first aid training when they're in the vicinity of a medical emergency. Nick Holland investigates whether it works work and what difference it could make to survival rates?

Image: The app that shows people with first aid training the location of a cardiac arrest / Credit: BBC