How do I deal with varying ECG signal to extract the length of RR interval properly?

Hi,

I've been wrestling with the following problem:

I use peak detection in Matlab, to extract the length of R-R intervals, from the ECG wave. But it does not work properly when the height of the R spikes (and that's generally the highest portion of the ECG wave) varies too much. Does anyone know how I might go about this issue?

Thanks!!