Exploratory Correlation Analysis of Respiratory Waveform, Instantaneous Respiratory Depth and Rate in Relation to Parasympathetic Indices During Spontaneous Breathing.

Respiration is closely related to the parasympathetic nervous system (PSNS). This relation occurs within a single respiratory cycle, with PSNS activity reduced during inspiration and increased during expiration. Over a longer timescale, deep and slow breathing has been reported to enhance PSNS activity, indicating that not only the timing (phase) but also the respiratory depth and rate may influence autonomic nervous system activity. However, under spontaneous breathing, it remains unclear which of the following best reflects PSNS activity: (i) raw respiratory waveform, (ii) respiratory depth, or (iii) respiratory rate. Respiratory depth was defined as instantaneous amplitude and respiratory rate as instantaneous frequency, both derived from the Hilbert transform. Respiratory waveforms and electrocardiograms were recorded at rest in 37 healthy adults. PSNS activity was quantified using heart rate variability indices reflecting parasympathetic modulation, including HF power, RMSSD, and CVI. Within-participant correlations between each respiratory measure and PSNS indices were obtained, and repeated-measures ANOVA with respiratory measure as a factor was used to compare correlation strengths. Results showed a significant main effect, with instantaneous amplitude consistently exhibiting significantly stronger correlations than the instantaneous frequency across all PSNS indices. These findings suggest that Hilbert-derived amplitude serves as a useful indicator of respiratory depth during spontaneous breathing and that depth is more strongly associated with PSNS activity.