HEART-RATE-VARIABILITY IN PASSIVE TILT TEST - COMPARATIVE-EVALUATION OF AUTOREGRESSIVE AND FFT SPECTRAL ANALYSES

The dynamic response of the autonomic nervous system during tilting is assessed by changes in the low (LF) and high frequency (HF) component s of the RR series power spectral density (PSD). Although results of m any studies ore consistent, some doubts related to different methodolo gies remain. Specifically, the respective relevance of autoregressive (AR) and fast Fourier transform (FFT) methods is often questioned. Bea t-to-beat RR series rr ere recorded during 90 degrees passive tilt in 18 healthy subjects (29 +/- 5 years, eight females). FFT-based (50% ov erlap, Hanning window) and AR-based (Levinson-Durbin algorithm) PSDs w ere calculated on the same RR intervals. Powers in very low frequency (VLF: < 0.04 Hz), LF (0.04-0.15 Hz), and HF (0.15-0.40 Hz) bands were calculated either by spectrum integration (FFT and AR(IN)), by conside ring the highest AR component in each bond (AR(HP)), or by Summation o f all AR components (AR(AP)). LF and HF raw powers (ms(2)) were normal ized by total power (%P) and by total power after removal of the VLF c omponent (nu). AR and FFT total powers were not different, regardless of body position. In supine condition, when compared to AR(HP) and AR( AP), FFT underestimated VLF and overestimated LF, whereas in tilt posi tion FFT overestimated HF and underestimated LF. However, supine/tilt trends were consistent in all methods shorting a clear reduction of HF and a less marked increase of LF. Both normalization procedures provi ded a significant LF increase and further magnified the HF decrease. R esults obtained with AR(IN) were remarkably close to those obtained wi th FFT. In conclusion, significant differences between AR and FFT spec tral analyses do exist, particularly in supine position. Nevertheless, dynamic trends provided by the two approaches are consistent. Normali zation is necessary to evidence the LF increase during tilt.