Rhythmic organization of neonatal heart rate and its relation to atypical fetal growth

The rhythmic organization underlying long-term heart rate variability was examined in 36 newborn infants. Heart rate was registered every 30 s for 2 continuous hr while infants rested in a temperature-controlled isolette. Spectrum analysis of the time-series of the 240 observations detected rhythmically organized changes in the heart rates of 33 of the 36 infants. Thirty of the 33 infants showed a basic rhythm at 1.5 +/- .5 cycles per hr (one cycle every 30 to 60 min). While 9 infants showed this single cycle in behavioral activity, 24 infants showed additional cycles at a wide range of faster frequencies. Infants with signs of atypical fetal growth less often showed evidence of these multiple cycles, had reliably fewer cycles in heart rate, and had a marginally lower power in their basic cycle than infants with typical patterns of fetal growth. Infants with multiple cycles in the power spectra, independent of fetal growth group, were more often observed in Alert and Active Alert behavioral states and less often in Active Sleep than comparison infants. Results indicate that 1) heart rates of newborn infants show evidence of the 30- to 60-min cycle characteristic of the Basic Rest-Activity Cycle found in other behaviors, and 2) the complexity of behavioral rhythms may be affected by prenatal malnutrition. Viewed within a dynamical systems approach to development, results suggest that the complexity of rhythms in behavior may reflect the complexity of behavioral organization.