Time-frequency analysis of heart rate in determining potential for syncope during Head-up tilt

Short-time Fourier transform (STFT) of RR-interval was used to investigate the autonomic responses of 20 subjects (10) each men and women) to supine control and to 30 minutes of 80° Head-up tilt (HUT). Subjects were either normal volunteers or patients who had experienced prior episodes of syncope. Subjects were classified as either syncopal or non-syncopal by response to HUT. METHODS. Time series of low (0-.15 Hz) and high (.15-.5 Hz) frequency power in RR interval were generated by performing the STFT with a window length of 100 seconds and binning the power in the high and low frequency ranges at each analysis time. RESULTS: Time-frequency (TF) indices were calculated in supine control and at 80° HUT. TF indices included: mean (over time) high (HF) and low (LF) frequency powers, mean (over time) sympathetic (SNS, LF/LF+HF) and parasympathetic (PNS, HF/LF) indices, and frequency content of the HF (FHF) and LF (FLF) time series (i.e. the intermittentcy in the high and low frequency content in time). RESULTS: PNS and HF decreased during tilt for both groups (p<.0001 and p<.003. respectively). SNS increased with tilt for all subjects (p<.03). FHF increased with tilt for all subjects (p=.05). FLF was significantly higher in non-syncopal subjects during control as compared to syncopal subjects. (p<.02). These results indicate: 1) Values of HF, LF, PNS, and SNS failed to predict any difference in autonomic response to tilt for syncopal vs. non-syncopal subjects 2) Parasympathetic activity may become more variable during tilt as evidenced by the increase in FHF. 3) Predisposition to syncope during HUT may be predicted during supine control by examining the intermittentcy (FLF) of low frequency oscillations in RR interval.