“…Next, it was subjected to power spectral analysis using the Sparse Direct Fourier Transform algorithm (Rompelman, Snijders, & van Spronsen, 1982) that was directly applied on the arrival times of the R-waves in the ECG without interpolation of the non-equidistant time series. As absolute power spectral density estimates of HRV are not independent of the mean heart period and the metric of the heart beat time series (i.e., heart period series, instantaneous heart rate series, or R-wave events) (Janssen, Swenne, de Bie, Rompelman, & van Bemmel, 1993;van Dellen, Aasman, Mulder, & Mulder, 1985), spectral estimates were normalized by expressing them as squared coefficients of variation (the squared modulation index). It has been theoretically and empirically demonstrated that normalized spectral estimates of HRV are unrelated to mean heart period and utilized metric (de Boer, Karemaker, & Strackee, 1984;Janssen et al, 1993;van Dellen et al, 1985).…”