> I kinda view Lithium batteries as road flares with good PR. So that inside my body next to my brain seems..sketchy.

From "Trends in Cardiac Pacemaker Batteries" (2004) [0]

  The first implanted cardiac pacemaker used nickel-cadmium rechargeable battery, later on zinc-mercury battery was developed and used which lasted for over 2 years. Lithium iodine battery invented and used by Wilson Greatbatch and his team in 1972 made the real impact to implantable cardiac pacemakers. This battery lasts for about 10 years and even today is the power source for many manufacturers of cardiac pacemakers.