Development of linear and non-linear properties of heart rate control during quiet and active sleep in healthy infants.

In this study it is analyzed by linear and non-linear methods, how the control of instantaneous heart rate (IHR) develops in infants in quiet and active sleep. Ten healthy and term newborns were studied during their first half year of life. Low (LF) and high frequency power (HF) of the spectrum, and the largest Lyapunov exponent (LLE) were calculated from IHR. By means of the surrogate-data-test, significance of LLE with respect to linear correlated gaussian noise was verified. Significant differences were determined between the original and the surrogate data sets (p < 0.001) assuming non-linear properties. LF, HF, LLE have irregular developmental patterns during the first month, whereby LLE has a distinct low value on the 21st day. HF significantly increases between the second and sixth month in quiet sleep, whereas for LF and LLE no significant correlations were found, neither for quiet nor active sleep. Using linear and non-linear methods, the study shows that the system controlling heart rate pass through mainly two different developmental periods: a period of adaptation in the first four weeks, where parameters distinctly vary, followed by a period with clear developmental courses, lasting till the sixth month.