Circadian rhythm of heart rate and heart rate variability

Background-Measurements of heart rate variability (HRV) are increasingly us ed as markers of cardiac autonomic activity. Aim-To examine circadian variation in heart rate and HRV in children. Subjects-A total of 57 healthy infants and children, aged 2 months to 15 ye ars, underwent ambulatory 24 hour Holter recording. Monitoring was also per formed on five teenagers with diabetes mellitus and subclinical vagal neuro pathy in order to identify the origin of the circadian variation in HRV. Methods-The following variables were determined hourly: mean RR interval, f our time domain (SDNN, SDNNi, rMSSD, and pNN50) and four frequency domain i ndices (very low, low and high frequency indices, low to high frequency rat io). A chronobiological analysis was made by cosinor method for each variab le. Results-A significant circadian variation in heart rate and HRV was present from late infancy or early childhood, characterised by a rise during sleep , except for the low to high frequency ratio that increased during daytime. The appearance of these circadian rhythms was associated with sleep matura tion. Time of peak variability did not depend on age. Circadian variation w as normal in patients with diabetes mellitus. Conclusion-We have identified a circadian rhythm of heart rate and HRV in i nfants and children. Our data confirm a progressive maturation of the auton omic nervous system and support the hypothesis that the organisation of sle ep, associated with sympathetic withdrawal, is responsible for these rhythm s.