Thermal entrainment of heart rate in the preterm infant.

Using spectral analysis we have studied changes in the heart rate during periodic thermal stimulation of one foot of infants during quiet sleep. Twenty-two appropriately grown preterm infants were studied in the first 15 d after birth to quantify responses in comparison with previously reported term infants. Babies were stimulated at 0.05, 0.10, and 0.15 Hz. Spectral power was calculated at the stimulus frequency +/-0.01 Hz and +/-0.02 Hz and over the low frequency range 0.03 Hz to 0.17 Hz. The data show that 1) there is an increase in power around the frequency of stimulation for each frequency studied (p < 0.002); and 2) there is an increase in the ratio of local to low frequency power at 0.05 Hz (p = 0.002) and 0.10 Hz (p = 0.001), but not at 0.15 Hz (p = 0.109). These data confirm the concept of entrainment in the appropriately grown preterm infant but demonstrate that it occurs over a wider frequency range than previously reported. The wider range is the same as that of the term infant, although there are differences in the patterns of entrainment between the two groups. Further work is required to map out the maturation of the autonomic nervous system in both the term and the preterm infant with respect to the low frequency components of the heart rate variability power spectrum.

Effect of periodic thermal stimulus on heart rate in term newborn infant.

We studied heart rate changes in 25 term infants aged 1-7 days in quiet sleep during periodic thermal stimulation of one foot to widen the range of frequencies previously studied by others and to develop spectral analysis methods to quantify responses to thermal and other periodic sensory stimuli. The stimulation frequency was 0.10 Hz in all babies and ranged from 0.05 to 0.15 Hz in some. At 0.10 Hz, there was 1) an increase in spectral power at the frequency of stimulation (P < 0.001), 2) a tendency for overall low-frequency power to increase, 3) a reduction in respiratory sinus arrhythmia (P < 0.025), and 4) attenuation in the response between the first and second minute of stimulation (P < 0.01). At other frequencies of stimulation, essentially similar results were obtained. Respiration and other types of periodic sensory stimulation may also entrain the heart rate; we raise the question of whether low-frequency oscillations in heart rate are in fact related to thermoregulation or are a nonspecific feature of integrative processes in the brain stem.