Interaction of Heart Rate and Respiration in Newborn Babies

Variability of heart rate (HRV) and transthoracic electric impedance respirogram (TEZ) were examined by spectral analysis in three groups of neonates: healthy term babies (22), healthy preterm babies (21), and preterm babies with respiratory distress syndrome (RDS) (11). Heart rate, TEZ, Ptco2, and Ptcco2were monitored during quiet sleep on the 1st, 3rd, and 5th day of postnatal life. Autospectra for trend-corrected segments of heart rate and TEZ as well as their cross-spectra and coherence spectra were computed. The peak of HRV spectral density was in < 0.2 Hz [low frequency (LF)] area (< 12 cycles/ min) in all the neonates. Intergroup comparisons of average band-integrated spectra revealed that the LF spectral density of HRV was greater in the term babies than in the preterm babies on day 3. In the babies with RDS, both LF and high-frequency (HF, > 0.2 Hz) were abnormally low throughout the study. In the term infants, the TEZ amplitude spectrum was flat on day 1. On later days, a peak corresponding to the average respiratory rate emerged. In the healthy preterm babies, there was a LF peak in TEZ autospectrum on all days. In the babies with RDS, the peak of ventilator frequency was initially present; finally, the respiratory activity accumulated in the LF area. In the cross-spectra of term babies, there was a LF peak on all days. On day 5, an additional HF peak appeared, representing respiratory sinus arrhythmia. In the healthy preterm babies, only a LF peak was present. In babies with RDS, there were three peaks in the cross-spectra: one in the LF area, another corresponding to the ventilator frequency, and a third one representing the first harmonic of the ventilator frequency. With increasing age, the ventilator-induced peaks disappeared and only LF activity remained. No correlation was found between the spectral patterns of HRV and cross-spectra and the values of PtcO2and Ptcco2. The spectral patterns of HRV and TEZ indicate that in newborn infants most of the activity is generated by LF mechanisms. Respiratory sinus arrhythmia is detected in term infants when regular breathing becomes organized. In RDS, respiratory sinus arrhythmia HRV is induced by ventilator. In both groups of preterm babies, the LF regulation of HRV seems to be more firmly connected with regulation and mechanics of respiration.