Cardiovascular fluctuations and transfer function analysis in stable preterm infants.

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 degrees (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.