frequency of a signal array

Hello, first excuse for my bad english.. I try to estimate the frequency of a ECG signal (by frequency I mean FFT of the QRS not the beat rate). For that I used find peak and I isolate the all the QRS into a array.

I would like to make the FFT of this array in order to caracterise the real frequency of the QRS, but I don't know if there is any method to do that on matlab. I think it's possible to make a mean of these sample and then make the FFT but each sample is only caracterized by few point so the FFT won't be really accurate :

Any advice about that?