  A reliable spectral analysis requires sampling rate at least twice as large
as the frequency bound, otherwise the analysis will be unreliable and plagued
with aliasing distortions. The RR samplings do not satisfy the above
requirements and therefore their spectral analysis might be unreliable.
  In order to demonstrate the feasibility of aliasing in RR spectral analysis,
we have done an experiment which have shown clearly how the aliasing was
developed. In the experiments, one of us (A.G) had kept his high breathing rate
constant with the aid of metronome for more than 5 minutes. The breathing rate
was larger than one-half the heart rate. Very accurate results were obtained
and the resulting aliasing well understood. To our best knowledge this is the
first controlled experiment of this kind coducted on humans.
  We compared the RR spectral analysis with the spectrum of the ECG signals
from which the RR intervals were extracted. In the significant for RR analysis
frequencies (below one-half Hertz) significant differences were observed.
  In conclusion we recommend to study the spectral analysis of the ECG signal
in the free of aliasing frequency range.
