I am no doctor, but i can assure you that a person's cardiac rhythm is not unique... It can vary depending on many factors and two or more people can have the exact same one in any give moment as long as they have similar body stature and are under the same stress level. So this is a funny thing, but as secure as making a lock which opens when you enter a one digit number...
Got a question: Under a life threatening situation (e.g. chased by a bad guy or a bear), the wearer runs some distance to his/her car, with heartbeat rising to 200 and changing cardiac rhythm, will this device still be able to open the car's door and ignite the engine?