Increased heart rate in depressed subjects in spite of unchanged autonomic balance?

A clinical study was conducted to examine the effects of depression on cardiac autonomic control. Cardiac autonomic control was measured in 26 nonmedicated patients (19 females) suffering from Major Depression, melancholic type, and in 26 age- and sex-matched normal controls. We measured heart rate and high frequency heart rate variability (respiratory sinus arrhythmia), pulsewave velocity and blood pressure, during 10 min of supine rest under controlled conditions. Using a log transformed time domain measure of respiratory sinus arrhythmia (logRSA), we found an inverse linear dependence between cardiac vagal tone and age in the healthy subjects as well as the depressed patients. logRSA was 0.22+/-0.25 in the patients and 0.25+/-0.16 in the control group. While this difference was not significant (P > 0.1), the deviations from the regression line were significantly (P < 0.0005) greater in the patients (0.21+/-0.12) than in the control group (0.09+/-0.07), indicating a more heterogeneous vagal tone in the depressed patients. Heart rate was also significantly (P < 0.03) greater in the depressed patients (76.6+/-12.4) than in the control group (69.5+/-6.9). No between-group differences were found in pulsewave velocity or systolic blood pressure, but diastolic blood pressure was lower in depressed patients (73.5+/-8.7 vs. 80.8+/-9.1). We discuss the possibility that the increased heart rate seen in the absence of vagal tone changes may not be due to altered vagal or sympathetic tone, as measured in this study. Other factors, including altered autonomous heart rate, may be responsible for the higher heart rate in the depressed group.