The Mayo Clinic is known around the world for being thorough and not taking any chances, so when when a patient asked if it was okay to drive a hybrid car with an implented cardiac device and the doctors couldn't find a study on it, they tested things themselves. They monitored people with pacemakers and defibrillators in various positions (driving, passenger, outside the car, etc) and in various conditions, and they're now publishing their results, which can basically be summarized as: "patients with implanted cardiac devices such as pacemakers and defibrillators can safely drive or ride in an electric car without risk of electromagnetic interference (EMI)."
The results are not very surprising; after all, it's not like hybrids and electric cars are the only electromagnetic sources around, and people with pacemakers have been living and working in environments that have exposure to large electric motors for a long time. But it's always better to verify with proper scientific methodology to make sure...
Mayo Clinic researchers used implantable devices from the three major manufacturers and a 2012 Toyota Prius hybrid in the study. Electric and magnetic fields were measured in six positions: from the driver’s seat, front passenger seat, the left and right rear seats and in front of and behind the car from the outside.
Each position was evaluated at different speeds: 30 mph, 60 mph and at variable speeds of acceleration and deceleration.
The 30 study participants with implanted devices were continuously monitored while rotating positions in the car and driving the car, with a particular focus on real-time detection of any interruption in the normal functionality of their devices. (source)
Via Mayo Clinic