**2. Autonomic biomarkers of psychiatric disorders**

Autonomic measures are interesting because some of the clinical symptoms of depression and anxiety are related to autonomic disturbances. In depression, appetite decreases, and weight loss is frequent. Bowel movement is also unstable. In anxiety, the patients report palpitation and cardiac discomfort without electrocardiographic abnormalities. Perspiration also increases. These somatic symptoms of depression and anxiety can be generated by autonomic dysregulations, supporting the feasibility of autonomic measures as biomarkers for depression and anxiety.

Although autonomic dysregulations are frequent in depression and anxiety, the methods for objective evaluation of autonomic symptoms are limited. Physiological indices of autonomic activity include pupil size, salivary amylase, gut movement, skin conductance, heart rate, and HRV. Among them, HRV has been intensively utilized in the analysis of autonomic function in depression and anxiety [5, 6]. HRV is easily measured by analyzing the electrocardiogram or pulse oximetric plethysmogram. A measurement takes only a few minutes and is less stressful for the patients. The measurement devices are small and portable and can be installed in regular clinical offices or in non-medical places. HRV is useful and convenient to characterize the autonomic dysregulations found in depression and anxiety.
