What are uses of radioisotopes in pacemakers?

For energy.: Many years ago, there were some pacemakers that had "nuclear batteries" - they were powered from the plutonium-238 radioisotope. These batteries lasted many times longer than even the modern batteries do. However, lithium-powered batteries are now the norm - I am not aware of any manufacturers selling a nuclear battery pacemaker in many years. http://www.nejm.org/doi/full/10.1056/nejmc072143.

None anymore: In the 70s they had a plutonium powered pacemaker than ran off the heat from radioactive decay. Those are no longer made (i think it's ~3000 that were ever deployed). There are no plans that i'm aware of to resurrect any similar technology.