Respiratory cycle-related EEG changes: response to CPAP

Abstract

Study objectives: Respiratory cycle-related EEG changes (RCREC) quantify statistically significant synchrony between respiratory cycles and EEG spectral power, vary to some extent with work of breathing, and may help to predict sleepiness in patients with obstructive sleep apnea. This study was designed to assess the acute response of RCREC to relief of upper airway obstruction by positive airway pressure (PAP).

Design: Comparison of RCREC between baseline diagnostic polysomnograms and PAP titration studies.

Setting: Accredited academic sleep disorders center.

Patients: Fifty adults referred for suspected sleep disordered breathing.

Interventions: For each recording, the RCREC in specific physiologic EEG frequency ranges were computed as previously described for the last 3 h of sleep not occupied by apneic events.

Results: The sample included 27 women; mean age was 47 ± 11 (SD) years; and median respiratory disturbance index at baseline was 24 (inter-quartile range 15-43). Decrements in RCREC, from baseline to PAP titration, reached 43%, 24%, 14%, 22%, and 31% for delta (P = 0.0004), theta (P = 0.01), alpha (P = 0.10), sigma (P = 0.08), and beta (P = 0.01) EEG frequency ranges, respectively. Within each specific sleep stage, these reductions from baseline to PAP studies in synchrony between EEG power and respiratory cycles still reached significance (P < 0.05) for one or more EEG frequency ranges and for all frequency ranges during REM sleep.

Conclusions: RCREC tends to diminish acutely with alleviation of upper airway obstruction by PAP. These data in combination with previous observations support the hypothesis that RCREC reflect numerous, subtle, brief, but consequential inspiratory microarousals.

Keywords: Polysomnography; RCREC; computer-assisted; electroencephalography; obstructive; respiration; respiratory cycle-related EEG changes; signal processing; sleep apnea.

Figures

Figure 1

Computation of respiratory cycle-related EEG changes (RCREC) is illustrated. The variation of the EEG signal power is computed for a specifi c frequency band within the time period of each respiratory cycle. The process begins by applying a Savitzky-Golay (polynomial) smoothing fi lter to the recorded thoracic excursion to produce a respiration signal with reduced artifacts (A). The minimums and maximums of this signal, along with temporal midpoints in between, are used to defi ne 4 time segments corresponding to different intervals of the respiratory cycle. A digital bandpass fi lter is applied to the measured EEG signal (B) to produce a time series corresponding to a specifi c frequency band. This signal is squared to produce a time series giving the variation of EEG power with time in the frequency band (C). The mean EEG power in the frequency band (from C) is then computed for each of the 4 respiration cycle time segments defi ned by the fi ltered respiration cycle (A). The mean power for each interval is normalized by the mean frequency-specifi c power over the entire respiration cycle. One is subtracted from each result to get the measures shown for the 4 segments in (D). The segment-specifi c EEG powers are averaged over many respiratory cycles to compute the average measure for each of the 4 respiratory segments (E). The RCREC for this subject, on this sleep study, is computed as the difference between the maximum and minimum mean relative EEG powers for each of the 4 respiratory segments. The RCREC then refl ects the extent to which EEG power varies in synchrony with the average respiratory cycle. *RCREC is maximum difference in normalized segment power after averaging segment power over many respiration cycles.

Figure 2

Changes from baseline to titration studies in respiratory cycle-related EEG changes (RCREC), for delta, theta, alpha, sigma, and beta EEG frequency ranges, are summarized (left panels) and also shown individually for each of 50 subjects (right panels). Perc, percentile; serifs show 90th and 10th percentiles. These graphs show that in general, the RCREC—as a measure of the extent to which EEG activity is gated by or at least synchronized to the respiratory cycle—diminishes from baseline values when positive airway pressure (PAP) is applied.