Reproducibility and circadian rhythm of heart rate variability in healthy subjects

Low heart rate (HR) variability, expressed as the standard deviation of cardiac cycle lengths, has been shown to be associated with increased mortality in patients with coronary artery disease.1,2 HR variability can be easily measured from 24-hour electrocardiographic recordings using recently developed software.1,2 However, reference values for HR variability and their reproducibility in normal persons are not well studied. We estimated the reproducibility and circadian rhythm of HR variability, measured from repeated 24-hour electrocardiographic recordings, in young healthy adults to establish normal values and determine patterns of reproducibility.