How are batteries chosen to be put in pacemakers?

Manufacture specific: The companies that manufacture pacemakers design their own batteries to optimize the longevity of the device. The batteries are not replaced when depleted; the entire pacemaker is replaced.

They're not: We're not picking duracell vs energizer here. These things were all lithium iodide cells until very recently because of its ideal characteristics (no off-gassing, stable discharge, heat stable, and extraordinary energy density). Icds need a different chemistry with lower internal resistance, and both devices now have hybrid battery techs. Batts aren't replaceable without changing the whole pm.