Respiratory sinus arrhythmia as a within-session biomarker in depression psychotherapy: integrating resting RSA and RSA reactivity.

Objective: Enhancing affective regulation is a key focus in psychotherapy for depression, yet within-session markers that capture the dynamics of regulation remain limited. Respiratory Sinus Arrhythmia (RSA), a biomarker of vagal mediation of the parasympathetic nervous system, is well-known for its role in affective regulation. This nonrandomized observational psychotherapy process study examined (1) whether resting RSA can serve as a biomarker for depression severity, and (2) whether RSA reactivity and its interaction with resting RSA can serve as a within-session therapeutic biomarker linked to session quality. Method: Twenty-eight clients with MDD underwent Supportive-Expressive short-term psychodynamic therapy. Resting RSA and RSA reactivity were measured during five pre-scheduled sessions throughout treatment. Session quality was assessed using the self-report Session Evaluation Scale questionnaire. The data were analyzed using multi-level modeling. Results: Higher resting RSA was associated with lower pre-session depression and was only significant at the within-client level. The interaction between resting RSA and RSA reactivity was linked to session quality, with higher resting RSA predicting a positive association between RSA reactivity and session evaluation. Conclusion: The findings underscore the value of physiological measures such as RSA as a biomarker for assessing depression severity that can capture affective regulation dynamics during therapy sessions.