Hi!
There are 2 possible solutions I think. The first is to count heart beat during i.e.60 seconds. But it isn't good because it takes a long of time and you may to have a special counter.
The second is counting clock pulses between each of heart beat. This method is very fast, usefull for low frequences and simply to do. But the idea is from measuring equipment, not from medical one.

Jacek