Windowed FFT - a time-variant spectral analysis: applicability during the head-up tilt test

Journal of Clinical and Basic Cardiology 1999; 2 (2): 241-244

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Keywords: Herzfrequenzvariabilität,  Kipptisch,  Spektralanalyse,  head-up tilt test,  heart rate variability,  spectral analysis

The spectral assessment of heart rate variability (HRV) and blood pressure variability (BPV) is a well-established method for the identification of rhythmic fluctuations during stationary conditions, but there is no generally accepted method of describing dynamic changes in such spectral patterns. Our goal was to introduce an alternative means of assessing the dynamics of spectral HRV. Continuous ECG and non-invasive BP recordings on 29 subjects during head-up tilt testing were subjected to analysis. The total spectral power and the power over the low (LF: 0.04-0.15 Hz) and the high-frequency (HF: 0.15-0.4 Hz) spectral bands were recalculated in an overlapping series with constant time shifting of the initial data-point. The time course of LFHRV augmentation, with an early peak and subsequent levelling within the first 2 of tilting, was also documented, with parallel changes in LFBPV (LFHRV ms²/Hz: 290 ± 96 supine, 2707 ± 1557 maximum after tilt, p < 0.05 LFBPV mmHg²/Hz: 10 ± 4 supine, 58 ± 26 maximum after tilt, p < 0.05). In a subgroup of 7 patients who exhibited syncope upon tilting, a statistically significant early increase HFHRV was also detected, followed by significant decline by the second minute of tilting (HFHRV ms²/Hz: 143 ± 80 supine, 1054 ± 902 maximum after tilt, 125 ± 84 minimum after tilt). This early HFHRV peak was absent in the group of tilt-negative subjects. Another characteristic feature of the tilt positive group was a second phase of LFBPV and LFHRV elevation preceding the syncopal episode. Windowed fast Fourier transformation is a suitable method for assessment of dynamic HR and BP spectral changes during upright tilt testing. The method is well applicable for the analysis of tilting-induced autonomic responses. J Clin Basic Cardiol 1999; 2: 241-4.