Around roughly 1982, I was contracted to design a sense amplifier/filter as a front end for a heart pacemaker.  The specifications were as follows:

Sensitivity: 600uV to 4.2mV — adjustable
S/N: 40dB
Filter: Programmable 5-poles, three zeros, nominal 30Hz center and 45Hz BW.
Supply: 2.0-3.3 V with maximum 4.5uA current

The circuit was designed in 5uM Si-Gate CMOS P-well process.

I am pretty sure this was the first switched-capacitor implementation of a pacemaker sensing channel.  The simplified schematics are shown below.