Autonomic imbalance as a predictor of pain expectations and pain experience in chronic back pain: study protocol of a pilot study on the application of heart rate variability biofeedback in a mixed-methods design (BACK.BEAT)

INTRODUCTION: Chronic back pain is a leading cause of disability worldwide and is increasingly understood as a biopsychosocial condition involving not only nociceptive but also autonomic and cognitive-emotional mechanisms. Dysregulation of the autonomic nervous system, reflected in reduced heart rate variability (HRV), has been identified as a potential biomarker of chronic pain vulnerability. Simultaneously, maladaptive pain expectations can amplify perceived pain and contribute to symptom persistence. HRV-based biofeedback, which aims to restore autonomic balance through paced breathing, represents a promising but underexplored approach to modulating both physiological and psychological aspects of chronic pain. This study aims to examine the relationship between daily HRV patterns, pain expectation and pain experience in individuals with chronic back pain, evaluate the feasibility and effectiveness of HRV biofeedback training in improving HRV and reducing pain and explore potential changes in patients' illness understanding following the intervention. METHODS AND ANALYSIS: The study employs a mixed-method randomised controlled design. Participants with chronic back pain will be randomly assigned to an HRV biofeedback intervention group or a control group. Daily HRV will be continuously monitored using the Garmin Vivosmart 5 smartwatch and correlated with daily pain expectations and experiences. HRV-biofeedback under laboratory conditions will be carried out with the NeXus-4 biofeedback system (MindMedia). Feasibility and acceptability of the study design will be analysed through rates of recruitment, retention, adherence and the suitability of study procedures and outcome measures. Secondary to this, HRV will be analysed using both time- (SDNN, RMSSD) and frequency-domain (lnLF, lnHF) parameters. Pain expectation and daily pain experience will be assessed through ecological momentary assessment. Quantitative analyses will include Pearson/Spearman correlations, repeated-measures analyses of variance (ANOVAs) (Group×Time) and exploratory regression models. Qualitative data on illness perception will be analysed thematically. ETHICS AND DISSEMINATION: The study was approved by the Local Psychological Ethics Committee of the Center for Psychosocial Medicine at the University Medical Center Hamburg-Eppendorf (LPEK-0866). All participants will provide written informed consent prior to participation. Findings will be disseminated through peer-reviewed publications, conference presentations and academic theses, and de-identified data and analysis code will be made available in accordance with Findable, Accessible, Interoperable, Reusable (FAIR) data principles where ethically and legally feasible. TRIAL REGISTRATION DETAILS: The study was registered with the ISRCTN registry (ISRCTN19147367).