Cardiovascular Fluctuations and Transfer Function Analysis in Stable Preterm Infants

Abstract

To examine the baroreceptor reflex function, a beat-to-beat analysis between systolic blood pressure (SBP) and R-R interval fluctuations was studied in 10 stable appropriate-for-gestational age preterm infants (range, 27.2–33.7 wk) in the first postnatal week during quiet sleep. Spectral power analysis, using fast Fourier transform, and transfer functions (gain and phase difference) between SBP and R-R fluctuations were estimated in a low-frequency band (LF, 0.03–0.2 Hz) and high-frequency band (HF defined as the frequency band between the 10th and 90th centiles of the individual respiratory frequency). The LF/HF ratio reflects the sympathovagal balance. The mean frequency (±SD) of LF peaks was centered at 0.07 ± 0.02 Hz. The mean frequency (±SD) of the individual HF band was 0.82 ± 0.21 Hz. The LF/HF ratio in the R-R interval series [median, 29; interquartile range (IQR), 16–40] was higher than in the SBP series (median, 8; IQR, 4–14). The gain between R-R interval and SBP fluctuations (median, 4.2 ms/mm Hg; IQR, 2.4–5.0) in the LF band was higher than in the HF band (median, 1.7 ms/mm Hg; IQR, 1.4–3.0). SBP fluctuations lead R-R interval fluctuations in the LF band with a median phase difference of +96° (IQR, 67–132). At LF the fluctuations in SBP precede changes in R-R interval with a time delay of 3.8 s. These observations indicate a dominant role of the sympathetic system in stable preterm infants in comparison with published adult values. Cross-spectral analysis allows a test for tracking the development of the sympathetic system in neonates.