The purposes of this study were: (1) to compare heart rate variability between bipolar patients in subsyndromal depressive phase and healthy controls (2) to evaluate the relation between the severity of subsyndromal depressive symptoms and reduced heart rate variability.

Inclusion Criteria:

1. Bipolar group:

   • DSM-IV bipolar disorder
   • Montgomery-Åsberg depression rating scale (MADRS) score for last one month : ≤ 10
   • Clinical Global Impression severity (CGI-S) for last one month : ≤ 3

2. Healthy controls group: Subjects had no history of psychiatric disorder and had no psychiatric symptom when interviewed by a board-certified psychiatrist.

Exclusion Criteria:

• Psychiatric disorder except bipolar disorder
• Neurological symptoms or signs
• History of head trauma
• History of cardiovascular diseases
• Any medication except psychiatric medication.

Thirty three (10 males, 23 females) patients in clinically remitted state diagnosed with bipolar disorder based on DSM-IV criteria were enrolled at one outpatient psychiatric clinic and 59 healthy controls (23 males, 36 females) from health promotion center in a university-affiliated general hospital. We defined an individual patient as being in subsyndromal depressive phase when the patient had a Montgomery-Åsberg depression rating scale (MADRS) score of 10 or less and Clinical Global Impression severity (CGI-S) of 3 or less for last one month. Healthy controls had no history of psychiatric disorder and had no psychiatric symptom when interviewed by a board-certified psychiatrist.