Heart-rate variability in children. Spectral analysis of developmental changes between 5 and 24 years.

The variation in instantaneous heart rate is most prominent in infants and younger subjects. In a preliminary study of the effects of maturation on heart rate, we compared the heart rate variations of 29 children and young adults in three groups between 5 and 24 years of age. We used spectral analysis to determine the intensity of the variations in each of the two main frequency bands in which variations occur: HF, 0.15-0.45 Hz, and LF, 0.03-0.15 Hz. Three-minute segments of continuous instantaneous heart rate were recorded for each subject in standing and supine positions. The group mean LF and HF amplitudes and the L/H ratio decreased between 5 and 10 years of age in both positions, significantly for LF and L/H in the supine position (p less than 0.05). Half of the youngest group of children had adult LF amplitude values by 5 years of age; the others had much higher levels, indicating increased low frequency variation at this age. Thus the high variation in heart rate in very young subjects is most prominent in the LF range. These preliminary results, considered with previous pharmacological studies, suggest that many children have a significant decrease in sympathetic activity between 5 and 10 years of age and possibly a slight decrease in parasympathetic activity. Spectral analysis of heart rate appears a promising technique for investigating the development of neural control of the heart.