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 p articular the total power (within 0.02-2.0 Hz) and the power in the lo w- (0.02-0.2 Hz) and high- (0.2-2.0 Hz) frequency ranges, were compute d from the ECG and respiratory signals of 59 premature and full-term i nfants. The objective of the study was to investigate the development and maturation of the autonomic nervous system from the first day of e xtrauterine life to several weeks of postnatal age. The study populati on 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 infant s 7-49 d old with a conceptional age of 34-35 wk. Group C: seven 1-d-o ld full-term infants of 39-41 wk gestation. Group D: six premature inf ants 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 lo wer 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. Th e mean low to high ratio for the combined groups C and D, 17 +/- 1, wa s significantly lower compared with the combined group A and B, i.e. 4 4 +/- 9 (p < 0.01). The respiratory signals showed two types of breath ing patterns: a single peak in the respiratory spectrum centered aroun d 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 spectru m usually showed a dispersed, wide pattern of the power in the high-fr equency range. The progressive decline in mean HR and in low- to high- power ratio indicates a decrease in sympathovagal balance with gestati onal and postnatal age. This maturation might be associated with a gra dual focusing into a single respiratory peak, both in the respiratory and the HR power spectrum.