How could I resample a RR interval signal extracted from ECG?

I have an ECG vector, sample at 250 Hz. Using Pan-Tompkin algorithm I have extracted RR intervals, to study HRV. Now I would plot the power spectrum density, and study what happens in high frequency (0.04-0.15) and in low frequency (0.15-0-4). But obviously my sample frequency is no longer the same of ECG. How can I calculate it? If I use function pwelch(data), my frequency axis is not correct; I may use [pxx,f] = pwelch(data,fs), but I don't know my fs. Any ideas? Thanks