Hi Saku,

I don't know what kind of detection(sensor) you are using. Assuming you have a clear trigger pulse for every heartbeat, you connect the trigger (ofcourse, -ve going) to say interrupt0.
Use one of the timers as 1msec timebase.
Count the duration between 1 to 5th beat and average and convert it to a minute (bpm = beats per minute). Don't try to do beat to beat measurement and it is clinically insignificant.