Check this out:
An 81-year-old woman with a mercury-zinc powered permanent pacemaker experienced the sudden pain on her pacemaker pocket followed by an explosion. We are aware of no other report of the spontaneous and symptomatic bursting of a generator battery with fracture of the pulse generator capsule.
Emphasis mine. It’s an old paper (1987) and you can get the full text for free at the journal’s site, here.