I found a journal paper quite helpful, which is written by the engineers from St.Jude Medical. The paper was published in 2004, titled “A very low-power CMOS mixed-signal IC for implantable pacemaker applications.” Besides the paper, a book, titled “Bio-Medical CMOS ICs” published by Springer, is also quite informative.
Both the paper and the book give an outline of an implantable pacemaker system. A simplified++ version colored by me is given as follows:
The system seems quite straightforward. The input path includes sensing the heart signal, doing filtering and amplifying, using ADC to convert analog signal to digital one, letting a MCU to do some digital processing. The output path includes pace driver/high voltage pulse generator. Two additional blocks are also important in the pacemaker system: a battery with its power management system and a crystal clock generator.
Though Fig. 1 shows a simple system, many detailed design techniques and challenges haven’t been demonstrated yet. I will try to mention somewhat based on my further reading, especially with a focus on ADC design in pacemaker.