Process ( amplify ) the sensor output to get a reasonable amplitude pulse.
Have a Schimdt trigger to convert to sharp edged pulses.
That will work only on some hearts. As I stated in an earlier post the heart signal is NOT a simple one pulse per beat. Besides the amplitude will vary greatly becuse of the enomous variance in shin resistance, body conductivity etc.

feed these pulses to bistable multivibrator
why?

Erik