Increased Total Heart Rate Variability and Enhanced Cardiac Vagal Autonomic Activity in Healthy Humans with Sinus Bradycardia

Sinus bradycardia can be defined as a sinus rhythm with a resting heart rate of 60 beats per minute or less. While it is assumed that increased autonomic parasympathetic activity is associated with sinus bradycardia, such an association has yet to be proven. The aims of this study were to compute a number of heart rate variability (HRV) parameters in healthy individuals with sinus bradycardia and determine whether there was a significant vagal component to sinus bradycardia. Forty-three healthy adults with normal sinus rhythm and 25 healthy adults with sinus bradycardia had an electrocardiogram recorded for 20 minutes, from which HRV indices were calculated. Results showed significant increases in SDNN (standard deviation of NN intervals) (P < 0.05), RMSDD (square root of the mean squared differences of successive NN intervals) (P < 0.05), and DFA32 (detrended fluctuation analysis) (P < 0.05) in bradycardic subjects compared with subjects with normal sinus rhythm. There were no significant differences in sympathetic frequency domain indices between the two groups. In conclusion, there were significant increases in total heart variability and increased parasympathetic drive in subjects with bradycardia. Clinically, bradycardia is likely to be cardioprotective in aging populations based upon these HRV findings.