Instability of parasympathetic nerve function evaluated by instantaneous time–frequency analysis in patients with obstructive sleep apnea

The purpose was to determine whether the instability of parasympathetic nerve (PN) function is associated with fragmentation of sleep and the instability can be improved by CPAP treatment in obstructive sleep apnea (OSA). Fifty-three OSA and 50 non-OSA subjects were examined by full-PSG and pulse rate variability (PRV) was recorded simultaneously using a photoplethysmograph and evaluated by instantaneous time–frequency analysis using the complex demodulation method. PN and sympathetic nerve (SN) activity were assessed by the mean high-frequency (HF) amplitude and ratio of low-frequency (LF) and HF amplitude (LF/HF ratio), respectively. Furthermore, the shift in central frequency (CF) of the main HF peak over time was monitored continuously. The relative times over which the same main HF peak was sustained for at least 20 s and 5 min in total recording time (%HF20s and %HF5min) were considered as markers of PN stability. Twenty-two of 53 patients with OSA also examined under the treatment with continuous positive airway pressure (CPAP). A significant increase in mean LF/HF ratio and decrease in HF amplitude were observed in severe OSA. Furthermore, both %HF20s and %HF5min were significantly decreased not only in mild-to-moderate OSA but also in severe OSA, and %HF20s was the strongest independent determinant for arousal index. Treatment with