Cardiac vagal control in the severity and course of depression: The importance of symptomatic heterogeneity

Background: Impaired cardiac vagal control (CVC), as indexed by respiratory sinus arrhythmia, has been investigated as a risk factor for major depressive disorder (MDD), but prior findings are mixed with respect to whether impaired CVC predicts greater global depression severity and/or a more severe course of disorder. One possible explanation for mixed findings is that CVC abnormalities in MDD are related more closely to specific depression symptoms than to the syndrome as a whole. Methods: Depression severity (both global and symptom-specific indices) and electrocardiogram measures of resting CVC were obtained from 151 diagnosed MDD participants at intake, before randomization to a novel treatment for depression (acupuncture), and again after 8 and 16 weeks. Results: Resting CVC did not predict global indices of depression in cross-sectional or longitudinal analyses. In symptom-specific analyses, resting CVC was positively related to sad mood and crying and inversely related to middle and late insomnia. Improvement in late insomnia was related to increases in CVC over time. Limitations: Relationships between CVC and MDD were studied only within the clinical range of severity. Symptom analyses were exploratory and hence did not correct for Type I error. Conclusions: Resting CVC did not exhibit concurrent or prospective relations with overall depression severity but a few specific symptoms did. Symptomatic heterogeneity across samples may account for mixed findings within the CVC-depression literature.