Distinguishing bipolar II depression from unipolar major depressive disorder: Differences in heart rate variability

Abstract

Objectives. Bipolar II (BPII) depression is commonly misdiagnosed as unipolar depression (UD); however, an objective and reliable tool to differentiate between these disorders is lacking. Whether cardiac autonomic function can be used as a biomarker to distinguish BPII from UD is unknown. Methods. We recruited 116 and 591 physically healthy patients with BPII depression and UD, respectively, and 421 healthy volunteers aged 20–65 years. Interviewer and self-reported measures of depression/anxiety severity were obtained. Cardiac autonomic function was evaluated by heart rate variability (HRV) and frequency-domain indices of HRV. Results. Patients with BPII depression exhibited significantly lower mean R–R intervals, variance (total HRV), low frequency (LF)-HRV, and high frequency (HF)-HRV but higher LF/HF ratio compared to those with UD. The significant differences remained after adjusting for age. Compared to the controls, the patients with BPII depression showed cardiac sympathetic excitation with reciprocal vagal impairment, whereas the UD patients showed only vagal impairment. Depression severity independently contributed to decreased HRV and vagal tone in both the patients with BPII depression and UD, but increased sympathetic tone only in those with BPII depression. Conclusions. HRV may aid in the differential diagnosis of BPII depression and UD as an adjunct to diagnostic interviews.

Key words::

• bipolar II depression
• unipolar depression
• differentiation
• heart rate variability
• cardiac autonomic function

Acknowledgments

This study was supported in part by the grants from the Ministry of Science and Technology of Taiwanese Government (MOST-103-2314-B-016-021), the Tri-Service General Hospital (TSGH-C103-135) and the National Defense Medical Research (MAB-99-I-16).

Statement of Interest

None to declare.