LINEAR AND NONLINEAR PROPERTIES OF HEART-RATE IN POSTNATAL MATURATION

Objective: An investigation was made of how postnatal maturation of ca rdiac control can be described by linear and non-linear methods of tim e series analysis, Methods: Sixteen healthy and term newborns were stu died during their first 6 months of life. Power spectrum analysis incl uding total power (TP), low frequency power (LF), high frequency power (HF) and LF/HF ratio were performed on the instantaneous heart rate ( IHR) time series and mean heart rate (HR) was derived. The largest Lya punov exponent (LLE) was calculated using a modified Wolf algorithm an d checked by means of the surrogate-data test. Results: There is an ag e dependency for the parameters LLE, HR, TP, HF, LF, LF/HF for active and LLE, HR, TP, HF, LF/HF for quiet sleep. HR is characterised by a s teep increase between the 5th and 7th day, HF demonstrates a distinct development with high values around the first week and 90-180th day an d low values around the 10-60th day. TP, LF and LF/HF show significant ly higher values in active sleep in comparison to quiet sleep. For all ages and sleep stages, positive LLE were found, indicating sensitivit y to initial conditions, a hall-mark of chaos. For the period between the 7 and 90th days of life, the LLE for active sleep took on larger v alues compared with the LLE of quiet sleep. Conclusions: This study sh ows that by linear as well as non-linear analysis one can reveal the c omplexity of the IHR development in humans and may gain insight into t he system controlling the heart during the period considered. The posi tive LLE indicate that there is a non-linear component in the heart ra te control. There is no ''straight line'' development for the paramete rs analysed within the first 6 months. This may result from manifold i nfluences on the autonomic system, due to structural and functional ma turation in this period of life.