I have an old Heart Rate Monitor with Finger Clip
made by REPCO (bought it in a bin of junk $5.00 the lot)
It uses a 3mm incandescent lamp (grain of wheat Bulb)
mounted on a steep angle which shines light into the
finger, the pickup close by is an LDR.
The OP amps inside must look for a peak in the light
levels returned (blood pumping modulates the light).
I had thought about a PICAXE version some time back
but the sampling needs to be constant, better suited
to OP Amps etc. So I didn't bother much after that.
Not saying that it can't be done with a Picaxe, but
there may be better ways to go about it.
Remember that Microchip and Rev-Ed don't endorse
their products to be used for any medical purposes at all.
(you need a special note from your Mother for that type of thing )