Resting State Heart Rate Variability in Depression: An Introductory Narrative Review of Cross-Sectional and Longitudinal Evidence

Cardiovascular health and depression influence each other bidirectionally and negatively, leading to high comorbidity rates, and favouring higher morbidity and mortality. Heart rate variability (HRV) has received much attention as a "biomarker" for major depressive disorder, with studies suggesting its potential both as a diagnostic and as a predictive biomarker. This narrative review offers a first orientation to the evidence base for researchers entering the field. We present and discuss the state-of-the-art evidence of cross-sectional and longitudinal studies (including observational, pharmacological interventions, and non-pharmacological interventions) linking depression and/or depressive symptoms to HRV by highlighting meta-analyses and key studies in the field. We briefly discuss the physiological context for interpretation of HRV and important confounders to consider, including the influence of genetics, age, sex, antidepressant medication, and lifestyle factors. Finally, with this information at hand, we discuss and provide guidance for factors to consider when using HRV in designing a study. Our literature review indicates that while there is potential for vagally mediated HRV to be of value in predicting future depression, more in-depth and stratified research of HRV is beneficial to the field and the understanding of what HRV can mean for depression research.