What question did this study set out to answer?

The aim is to explore how respiratory waveform, depth, and rate relate to parasympathetic nervous system activity during spontaneous breathing.

February 28, 2026Open Access

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

Q: What does this research mean for the field?

Instantaneous respiratory depth, as measured by Hilbert-derived amplitude, is more strongly associated with parasympathetic nervous system activity than respiratory rate during spontaneous breathing. Novelty: ClaimNovelty.NOVEL_FINDING. Consensus alignment: ConsensusAlignment.NEUTRAL.

Key Result

Instantaneous respiratory depth (Hilbert-derived amplitude) correlates significantly stronger with parasympathetic activity than respiratory rate during spontaneous breathing.

Key Points

The aim is to explore how respiratory waveform, depth, and rate relate to parasympathetic nervous system activity during spontaneous breathing.
Recorded respiratory waveforms and electrocardiograms from 37 healthy adults at rest.
Defined respiratory depth as instantaneous amplitude and respiratory rate as instantaneous frequency using Hilbert transform.
Quantified parasympathetic activity using heart rate variability indices (HF power, RMSSD, CVI).
Analyzed within-participant correlations and used repeated-measures ANOVA to compare correlations among measures.
Significant main effect found; instantaneous amplitude showed stronger correlations with PSNS indices compared to instantaneous frequency.
Depth of breathing indicated by Hilbert-derived amplitude correlates more strongly with parasympathetic activity.

Structured PICO

Which respiratory measure (waveform, depth, or rate) best reflects parasympathetic nervous system activity during spontaneous breathing in healthy adults?

Population

37 healthy adults

Outcome

Correlation between respiratory measures (raw waveform, instantaneous amplitude/depth, instantaneous frequency/rate) and parasympathetic nervous system (PSNS) activity indices (HF power, RMSSD, CVI)surrogate

During spontaneous breathing, respiratory depth (instantaneous amplitude) is more strongly associated with parasympathetic nervous system activity than respiratory rate.

Main Result

Absolute Event Rate: 0% vs 0%

Abstract

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.