Age related alterations in the complexity of respiratory patterns.

The objective of this study is to investigate the relative contributions of maturation to the dynamic behavior of respiration during ontogeny in the neonate. The phrenic neurogram, an output of the respiratory network, was recorded during eupnea at three postnatal ages (3-7, 12-19, and 26-31 days) in decerebrate piglets. The inter-breath interval (IBI) time series were reconstructed from phrenic neurograms for each piglet and analyzed using the approximate entropy (ApEn) method. The mean values of the approximate entropy were high during the first seven days of the postnatal age [1.02 +/- 0.02 (standard error)] and decreased for the 12-19 days (0.67 +/- 0.008) and increased during subsequent maturation [the 26-31 days age group (0.92 +/- 0.015)]. The mean approximate entropy values for the 3-7 days age group was significantly different from those of the 12-19 days age groups (p < 0.05) using the nonparametric statistical test. The complexity values for the 3-7 days age group were slightly higher than those of the 26-31 days age group, but these differences were not statistically significant (p > 0.05). Therefore, these findings suggest that a decrease in the complexity values is unique to the 12-19 days age groups. This could be due to a reduction in the number of dendritic terminals per cell for the 12-19 days age groups. The results of these preliminary experiments also indicate that the behavior of the respiratory pattern generator in the neonate fluctuates during the early maturation period.