Associations between indices of 24-hour heart rate variability and inflammation in individuals with major depressive disorder

Biological cardiac risk factors, including reduced heart rate variability (HRV) and inflammation, are already prominent in patients with major depressive disorder (MDD) without existing cardiovascular disease. Although inverse relations between HRV and inflammation have been found across several populations, little work has been done concerning MDD. The present work thus intended to examine whether measures of HRV indices based on 24-h electrocardiograph recordings (24-h, daytime, nighttime) relate to levels of circulating inflammatory markers such as C-reactive protein (CRP), interleukin (IL)-6, and tumor necrosis factor (TNF)-α in eighty antidepressant-free individuals with MDD. A sample of 40 age- and sex-matched non-clinical controls was also involved to verify biological alterations in MDD. Individuals with MDD exhibited reduced total 24-h HRV (i.e., triangular index) and reduced daytime HRV (i.e., triangular index, HF-HRV, LF-HRV, RMSSD), as well as increased levels of all inflammatory markers. Multivariate analyses adjusted for age, sex, body mass index, and smoking revealed robust inverse associations of total 24-h HRV (i.e., triangular index) and daytime HRV (i.e., Triangular index, HF-HRV, LF-HRV, RMSSD) with IL-6. An attenuated daytime HRV may relate to higher circulating levels of IL-6 in the context of MDD. These findings show that biological cardiac risk factors may act in concert in MDD.