Heart rate variability in subthreshold depression and major depressive disorder

Background: The relationship between heart rate variability (HRV) and major depressive disorder (MDD) has been well explored. However, current researches lack an observation of HRV in subthreshold depression (SubD), which increases the risk of MDD and presents significant societal challenges.

Methods: This study compared resting state HRV among 128 MDD patients, 131 SubD individuals and 222 healthy controls (HC) recruited from the hospital, physical examination center, and colleges. Depression, anxiety and sleep quality were assessed by the Patient Health Questionnaire-9 (PHQ-9), Generalized Anxiety Disorder-7 (GAD-7) and Pittsburgh Sleep Quality Index (PSQI) scale. Statistical analyses including Kruskal-Wallis test, spearman correlation and binary logistic regression were conducted to investigate relationships between HRV and emotional issues.

Results: The results revealed increasing trends in HRV across three groups, and the top three indices correlated with self-reported symptoms were standard deviation of the Poincaré plot perpendicular along the line of identity (SD2), standard deviation of all RR intervals (SDNN) and low frequency (LF). After adjusting for demographics, lower HRV was significantly associated with increased MDD risk compared to HC or SubD. Additionally, LF, SDNN and SD2 exhibited significant associations between SubD and HC.

Conclusions: This study is the first attempt to explore the HRV in SubD and compares it with MDD/HC. Our findings indicate that HRV changes are evident in minor depression, with indices in SubD lower than HC yet higher than MDD. SDNN, LF, and SD2 emerge as potential biomarkers for identifying depression early.