Heart rate variability in the subsyndromal depressive phase of bipolar disorder

Aims: To compare the heart rate variability of bipolar patients in the subsyndromal depressive phase with healthy controls and to evaluate the relationship between severity of subsyndromal depressive symptoms and heart rate variability. Methods: Thirty‐three bipolar patients in the subsyndromal depressive phase and 59 healthy controls were enrolled. A patient was considered to be in a subsyndromal depressive phase when the Montgomery–Åsberg depression rating scale score was ≤10 and the Clinical Global Impression–Severity scale (CGI‐S) was ≤3 for the previous 1 month. After approximately 10 min of supine rest, all participants underwent resting electrocardiograms for 5 min in the supine position using limb leads. Different parameters of heart rate variability were analyzed in the time and frequency domains. Results: Bipolar patients had significantly lower standard deviation of all RR intervals (SDNN), proportion of adjacent NN intervals that differ by >50 ms (pNN50), log total power (log TP) and very low frequency power (VLF) compared to healthy controls. There were significant negative correlations between CGI‐S score and some heart rate variability parameters, including heart rate variability index, SDNN, root mean square successive difference (RMSSD), pNN50, log TP, VLF, low frequency power (LF) and high frequency power (HF). Conclusion: Patients with bipolar disorder in the subsyndromal depressive state have reduced heart rate variability relative to healthy controls, and reduction of heart rate variability appears to be correlated with severity of symptoms in bipolar patients.