Heart rate variability is currently analysed through the power spectral density (PSD) of the time series of RR interval (tachogram), estimated by either Fourier transform based or parametric methods. The hypothesis of stationarity of the tachogram, implicit in these methods, cannot be assumed when the series are derived during autonomic tests.

The PSD estimation based on the Wigner-Ville time frequency distribution (WD) shows the best performance, in terms of time and frequency resolution, among the various techniques that have been devised for computing time dependent PSD estimates. Such a technique has been applied to the analysis of tachograms, derived from multichannel recordings in normal, diabetic and Chagasic individuals stimulated by autonomic tests. The chosen tests were able of eliciting an increase in vagal tone (Valsalva manoeuvre and phenylephrine), or a sympathetic activation and a parasympathetic withdrawal (head-up tilt). The cross terms artefact associated with the WV technique has been effectively reduced by the smoothed WV estimate, trading off artefacts with a slight decrease of time resolution. The spectra of the tachograms obtained by this procedure allowed to achieve an unique characterisation of the evolving tachogram variability, with high time and frequency resolution, in all the autonomic tests.