Investigating Phase Coherence between Respiratory Sinus Arrhythmia and Respiration in Depressed Patients with Obstructive Sleep Apnea across the Sleep Stages

The aim of this study is to investigate if the phase coherence of respiratory sinus arrhythmia (RSA) and respiration exists in obstructive sleep apnea (OSA) patients with and without major depression disorder (MDD), and if it exists, how does it differ across the sleep stages. Overnight electrocardiograms (ECG) and thoracic movements using thoracic pneumography were recorded from control subjects (38); OSA subjects with major depression (40) and OSA subjects without major depression (40). The interbeat intervals (RRI) and respiratory movement were extracted from 5-minute segments of ECG signals with a single apneic event during rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Both RRI and respiration were resampled at 10 Hz and band passed filtered (0.10-0.4 Hz). Hilbert transform was used to extract instantaneous phases of the RSA and respiration. Subsequently, time dependent phase coherence (λ) between RSA and respiration was obtained. Heart rate variables (HRV) were also calculated. During REM and NREM sleep, higher λ and lower low frequency (LF) were found in the OSAMDD+ group compared to the OSAMDD- group (p<0.05). Our findings suggest that depression has lowered the sympathetic activites when it is accompanied with OSA, allowing for stronger synchronization between RSA and respiration.