Background: Autonomic Nervous System (ANS) activity can be considered as a landmark of brain function, reflecting the overall central nervous system regulatory ability. Both branches of the ANS, sympathetic and parasympathetic, act synergistically on the sinus node of the heart. The inhibitory effect on Heart Rate (HR) is conveyed by the vagus (i.e. the parasympathetic branch), whereas the excitatory effect is conveyed by the sympathetic branch. The combined effects of these two branches result in fluctuations which modulate the HR around its steady baseline.
The objective of this study was to investigate the fetal and neonatal cardiac neural activity before birth and after delivery. To carry out this goal we used tools that have been developed by our Medical Physics group in order to conduct a reliable time-dependent spectral analysis of the fetal and neonatal HR fluctuations.
Study population: 12 cases of healthy pregnancies were examined. Each baby was first tested prior to birth, nearing the end of gestation (between week 38 and 41) and twice after birth: after 24 hours, and again after 72 hours. The duration of each recording was 40 minutes. Neonates were tested during sleep. We intend to expand the study population (up to at least 40 cases).
Data acquisition: Neonatal recording included direct ECG signals and direct respiration data. As opposed to neonates, the fetus is not accessible to direct measurements, and only very few technologies, which can achieve the required beat-to-beat accuracy, are available. Our medical physics group has developed a unique and complex algorithm which allows for non-invasive on-line fetal ECG detection. The fetal recordings are conducted with the aid of a Fetal ECG and Heart Rate Monitor (FEMO) system. The FEMO is a small and rapid computer based non-invasive system which detects the superimposed maternal and fetal ECG signal from the maternal abdomen. It then separates and processes the maternal and fetal signals independently. The application requires only a single lead, and is recorded on-line on a PC with an A/D converter.
Signal processing: A Continuous Wavelet Transform (CWT) was applied to assess the time-dependent power spectrum of fetal and neonatal HR fluctuations. An accurate selection of the frequency ranges allowed for the creation of dynamic parameters for the estimation of the autonomic states.
Results: We divided the frequency range into Low Frequency (LF) and High Frequency (HF) domains. The results demonstrate a significant difference between fetal and neonatal spectral power values in both frequency ranges (One-Way ANOVA, p<0.05). There was no significant difference between the mean values for the LF/HF ratio (named: the sympatho-vagal ratio).
Discussion: In this study we introduce a non-invasive technique which enables investigation of both fetal and neonatal HR fluctuations. The link between these HR fluctuations and the ANS enables us to examine the two main autonomic branches (sympathetic and parasympathetic) before and after delivery.
The significant increase in the values of the average power spectrum, in both frequency ranges, indicates that both branches of the autonomic system are more active after birth. It is possible that the increase in the sympathetic branch is related to the external stimuli the neonate is exposed to after birth. The increase in the activity of the parasympathetic branch is expected, and relates to the beginning of self respiration.
There was no significant difference in the sympatho-vagal ratio between the mature fetus group and the neonates. In prior research we have shown that for young fetuses the sympatho-vagal ratio is higher than that of mature fetuses. These results may indicate that the ANS is “arranging itself” in preparation for the end of gestation, and that the average balance between the two autonomic branches does not change significantly after delivery. The current work expands this research, allowing for a typical average value for the sympatho-vagal ratio prior to and after delivery to be established. This will allow prediction of possible fetal and neonatal distress. Furthermore, we assume that ANS activity levels reflect more global central nervous system maturity, and therefore are effective for the investigation of the brain development.
M. David, M. Hirsch, J. Karin, E. Toledo, and S. Akselrod. An Estimate of Fetal Autonomic State by Time-Frequency Analysis of Fetal Heart Rate Variability. Journal of Applied Physiology 102: 1057-1064, 2007.
