Nonlinear broad band dynamics are less complex in major depression

Objectives: Cardiac mortality is known to be increased in depressive patients. However, the underlying mechanisms remain elusive to date. Decreased heart rate variability (HRV) has been discussed as contributing to increased cardiac mortality, but studies examining patients suffering from major depressive disorder (MDD) have revealed inconsistent results. This study aimed to investigate long-term and broad band parameters of heart rate regulation in MDD, which have been shown to be more sensitive for the assessment of autonomic dysfunction. Methods: A total of 18 non-medicated patients suffering from MDD and 18 matched control subjects without cardiac disease were recruited and 24-h ambulatory electrocardiograms were recorded. Data were recorded during three distinct time intervals linear and nonlinear parameters as well as autonomic information flow (AIF) were calculated. Results: The power law slope was significantly reduced in the patient group for all intervals investigated and correlated with symptom severity, whereas standard deviation of the 5-min NN intervals (SDANN) and area under the AIF curve (INTNN) showed significant differences between groups in the morning hours only. Analysis of standard HRV parameters in the time and frequency domain revealed no significant differences between groups. Conclusions: The evidence for decreased complexity of cardiac regulation in depressed patients presented here might be useful as an indicator of the increased cardiac mortality known in depression, especially since these parameters are capable of predicting cardiac mortality in other diseases. The importance of these parameters for patients at risk should be evaluated in future prospective studies.