time frequency localization using wavelet transform
What information are you trying to extract from your signal?


  I want to know the frequency (heart rate) at every time point


If this is the information you want then any sort of frequency analysis is unlikely to be very useful. It will show you that you have a 1 or 2 hz periodic signal with a particular frequency profile, but you already know all of this information so that won't be especially illuminating.

You are probably better off with peak detection, or local minima/maxima detection to extrapolate beats.

If you can provide a sample of your data, and/or more details about the exact information you are trying to extract then it will help us to find a more precise solution to your problem.