Respiratory sinus arrhythmia (RSA) is the heart period variability observed in synchrony with respiration. RSA amplitude is widely used in psychophysiological research to non-invasively index cardiac vagal activity. However, RSA measures are significantly affected by respiratory behavior, even in the absence of changes in cardiac vagal activity. Fifty-to-sixty percent of the variation in RSA can be attributed to respiration rate and tidal volume. This poses a notable challenge for ambulatory RSA measurement where respiratory behavior cannot be experimentally controlled and can show substantial variation. This pre-registered two-day ambulatory study (https://osf.io/57es4) compared four approaches to control for respiratory influences on RSA, to make an empirical recommendation on how to best capture cardiac vagal activity in daily life. We evaluated how well the RSA metric of each approach predicted (1) Minute-to-minute heart period, assumed to be predominantly governed by cardiac vagal activity, and (2) Perceived stress, positive affect, negative affect, and safety—states expected to elicit fluctuations in cardiac vagal activity— at smartphone prompts. The tidal volume-normalized RSA approach was optimal, explaining 1.47 times as much within-individual variance in heart period as that explained by uncontrolled-RSA. The need to use respiratory-controlled RSA was further highlighted by results on safety. Perceived safety was associated with uncontrolled-RSA (p = .033) but not with any of the controlled-RSA metrics. This relationship was driven by higher respiration rate co-occurring with lower safety. We recommend using tidal volume-normalized RSA in ambulatory research to avoid reporting spurious within-individual correlations between psychological states and cardiac vagal activity.

BACKGROUND: Dissociative disorders frequently co-occur with post-traumatic stress disorder (PTSD), yet many individuals lack adequate treatment. Existing interventions often prioritise reducing arousal over promoting safety and self-soothing, tending to neglect the bodily experience.
AIMS: This randomised clinical within-person pilot study examined the effects of the nest position, a physiotherapeutic intervention designed to enhance safety and self-soothing, on patients with dissociative disorders and healthy controls (German Clinical Trials Register No.: DRKS00030669).
METHOD: Eighteen patients with dissociative disorders and 18 healthy controls alternated between the nest position and a neutral supine position across two rounds of a measurement session. The order of the experimental conditions (nest position or supine only) was randomised for each participant. We assessed self-reported distress and comfort (Subjective Units of Distress and Comfort) and autonomic nervous system activity during three baseline phases and imagination of stressful and comforting situations.
RESULTS: Both patients and healthy controls experienced lower distress and greater comfort in the nest position. Heart rate and sympathetic tone decreased, particularly in the healthy controls. There were no significant changes in parasympathetic tone in both groups. Linear mixed models revealed a significant effect of the nest position on distress, comfort and sympathetic tone.
CONCLUSIONS: The nest position is a potentially promising additional intervention for highly dissociative patients. Our findings help to better understand the importance of self-soothing and safety in these individuals and to address the research gap in physiotherapy within in-patient mental health care.