Hello,

If you want ECG you need electrodes on legs and arms or on breast, a cumbersome method.
A small analog system produces a 1 mV signal at very low frequency, between 0.5 to 2 Hz. A 8 bit ADC is OK.

If only heart beat frequency is needed, a small
device with a LED and phototransistor measures
skin reflexivity to light, wich changes with
blood pressure. In the past, Hugues Inc.
produced an interface CMOS chip wich processed
the reflectivity signal.

Best regards,

J. M.