DEVELOPMENT AND MATURATION OF THE AUTONOMIC NERVOUS-SYSTEM IN PREMATURE AND FULL-TERM INFANTS USING SPECTRAL-ANALYSIS OF HEART-RATE FLUCTUATIONS

The changes in the power spectra of heart rate (HR) fluctuations, in particular the total power (within 0.02-2.0 Hz) and the power in the low- (0.02-0.2 Hz) and high- (0.2-2.0 Hz) frequency ranges, were computed from the ECG and respiratory signals of 59 premature and full-term infants. The objective of the study was to investigate the development and maturation of the autonomic nervous system from the first day of extrauterine life to several weeks of postnatal age. The study population was divided into four age groups. Group A: seven 1-d-old premature infants with gestational age of 34-35 wk. Group B: 28 premature infants 7-49 d old with a conceptional age of 34-35 wk. Group C: seven 1-d-old full-term infants of 39-41 wk gestation. Group D: six premature infants 35-97 d old with a conceptional age of 39-40 wk. Mean HR (+/- SEM) of groups C and D combined, i.e. 135 +/- 2 bpm, was significantly lower compared with groups A and B, i.e. 152 +/- 2 (p < 0.01). The mean (+/- SEM) of the low- to high-frequency power ratio obtained from the HR power spectrum decreased progressively from 71 +/- 31 in group A to 34 +/- 8 in group B, 16 +/- 3 in group C, and 17 +/- 2 in group D. The mean low to high ratio for the combined groups C and D, 17 +/- 1, was significantly lower compared with the combined group A and B, i.e. 44 +/- 9 (p < 0.01). The respiratory signals showed two types of breathing patterns: a single peak in the respiratory spectrum centered around the respiratory frequency (mature type, typically found in adults); and a second type showing two separate peaks, one centered around the respiratory frequency and the other at the much lower breath amplitude modulation frequency. In both respiratory modes, the HR power spectrum usually showed a dispersed, wide pattern of the power in the high-frequency range. The progressive decline in mean HR and in low- to high-power ratio indicates a decrease in sympathovagal balance with gestational and postnatal age. This maturation might be associated with a gradual focusing into a single respiratory peak, both in the respiratory and the HR power spectrum.