Thank you for the answer. I will think about it.
Meanwhile, I found this. Open Hardware – World Famous Electronics llc.. It is basically the same problem. They transformed an unusable signal in a signal which could be interpreted as heartrate by using a threshold. They won't say how they did it, but they said they used op a,ps and filters (preety misterious).
Do you have any idea what amp and filter could I use?
Thank you.