Heart rate variability in infants, children and young adults.

Heart rate variability was studied in normal subjects age 1 month-24 years while awake and in active and quiet sleep using 24 h continuous recordings of the ECG. Variability was quantified by spectral analysis for the two frequency bands: low frequency (LF) 0.03-0.15 Hz, high frequency (HF) 0.15-0.6 Hz. Heart rate variability showed an age dependence, being in general an increase in LF, HF and total power from 0-6 years, followed by a decrease to 24 years. The infant group showed some exceptions to this trend. Developmental changes of parasympathetic and sympathetic mediation of heart rate are postulated as important determinants of age dependence of heart rate variability.