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The relationship between heart rate variability and respiration patterns was investigated using spectral analysis techniques in nine full-term infants whose ages ranged from 39-75 h. All the infants were studied during sleep, although no attempt was made to classify rapid eye movement or nonrapid eye movement states prospectively. The data obtained were examined to determine which aspects of neonatal breathing patterns are correlated with heart rate variability. Three spectral regions of heart rate variability could be identified: a very low frequency region below 0.02 Hz; a low frequency region from 0.02-0.20 Hz; and a high frequency region above 0.20 Hz. The dominant heart rate variability activity in these neonates was seen in the very low and low frequency regions, with little activity in the high frequency regions. In contrast to older infants and adults, respiration and heart rate variability were not strongly related through a high frequency region respiratory sinus arrhythmia but rather through a breath amplitude sinus arrhythmia which occurs in the low frequency region of the spectrum. The prominent very low frequency activity and the low frequency activity ascribed to breath amplitude modulation may result from autonomic nervous system mediation of chemoregulation.

The heart rate and respirations of twenty healthy full-term infants between 30 and 60 h postnatal age were studied during quiet sleep with the objective of defining spectral indices which represent normal neonatal heart rate variability (HRV) characteristics. Total HRV power and the distribution of power across different frequency bands varied considerably among infants. Cluster analysis on the measured variables indicated that the population divided into two groups that represented significantly different patterns of HRV behavior. In one group (11 subjects), infants had lower breathing rates and HRV power in a band about the respiration frequency [respiratory sinus arrhythmia (RSA) band] was more than 20% of the total power (TP). Additionally, the ratio of low frequency band power to RSA band power was less than 4. The other group of neonates (nine subjects) had relatively higher breathing rates, RSA power less than 20% of total power, and low frequency to RSA power ratio greater than 4. Regression analysis of low frequency versus TP and RSA versus TP graphs gave strong support to the hypothesis that there were indeed two distinct patterns of HRV behavior. Separation of apparently normal neonates into two groups may be attributed partially to differences in respiratory rates and breathing patterns. However, it is possible that differences in the balance between sympathetic and parasympathetic nervous system control, perhaps related to autonomic maturation, also contribute to group separation. The indices developed from HRV spectral analysis in this investigation may be of value in the study of cardiorespiratory control in neonates.