The current heart beat monitor works quite well, but a pulse oximeter would be better to get the pulse rate and the saturation of peripheral oxygen SpO2.
After a lot of research I found this promising reflective pulse oximeter design. The circuit outputs amongst other things directly the peak IR and red light voltages. The simple equation
is used, and S represents the value of StO2 in a calibration table.
I will use a commercially available pulse oximeter to approximate a function of the saturation of peripheral oxygen depending on S.
Redrawn and slightly modified schematic:
Next step will be to build a prototype and see how it works.
PCB layout of the prototype: