Maturation of Fetal Cardiac Autonomic Control as Expressed by Fetal Heart Rate Variability

Prof. Asher Gotesman, The faculty of Exact Sciences, Physics, Tel-Aviv University

Objective

Establish quantitative parameters to measure fetal brain maturation, as derived from by the Fetal Heart Rate Variability (FHRV). Applying adequate algorithms with the ability to reveal the activity of the cardiovascular control, which should reflect the Fetal Autonomic Nervous System (ANS) development, as a function of gestational age.

Methodology and Achievements

• The recordings were conducted with the aid of a Fetal ECG and Heart Rate Monitor (FEMO) system, a device based on an algorithm developed by the Medical Physics group at Tel Aviv University.
• We examined 35 cases of healthy pregnancies which we divided into three groups according to gestational age: Group 1 - 23±2 weeks, Group 2 - 32±1 weeks, and Group 3 - 39±1 weeks.
• The FHRV was analyzed by a time-frequency decomposition based on a Continuous Wavelet Transform (CWT).
• In 8 cases, we also performed ultrasound imaging for fetal breathing motion, along with simultaneous fetal ECG monitoring.
• We used spectral analysis techniques in order to detect the fetal respiratory motion from Fetal Heart Rate (FHR).
• We developed an algorithm for measurement of fetal 'breathing' frequency using B-Scan Ultrasound. This enables us to validate the detected fetal respiratory motion from the FHR.

Results and Discussion
Spectral analysis techniques enable a non-invasive insight into the operational mode of the ANS. The investigation of FHR fluctuations is essential, especially when dealing with the development of the fetal ANS. The FEMO algorithm along with the CWT have proven to be sensitive tools which enable us to monitor the fetus in its natural surroundings and perform quantitative estimations of the fetal ANS.

We have shown that independently of the gestational age most of the FHR power spectrum is concentrated in the Very Low Frequency (VLF) range (0.02-0.08Hz) and in the Low Frequency (LF) range (0.08-0.2Hz). In addition, there is power in the High Frequency (HF) range that correlates with the frequency range of fetal respiratory motion (0.4-1.7Hz). In the intermediate frequency range (0.2-0.4Hz), the power is significantly smaller.

The changes in the average power spectrum in relation to gestation time were carefully and quantitatively examined. The results imply that the power variation as a function of time is similar in the LF and VLF domains. In both cases the average power is greater in group 2 when compared with group 1, and the average power of group 3 is not significantly different than that of group 2. The VLF/LF ratio values of group 2, on the other hand, are significantly lower than those of group 1, i.e. the change in power in the LF domain that occurs between the middle of the second trimester (1st group) and the beginning of the third trimester (2nd group) is more pronounced when compared with the power in the VLF domain.

There might be two possibilities regarding autonomic innervation in the period between the middle of the second trimester and the beginning of the third trimester: 1) The parasympathetic activity may increase in this period of the pregnancy. 2) The frequency domain in which sympathetic activity is reflected may widen, since it is possible that an increase in the sympathetic component of the LF band occurred, along with the power increase in the VLF band.

The first possibility is strengthened by the fact that the average spectral power in the HF domain increases with gestational age. It is known that in adults the vagal system is modulated by respiration at the level of cardiac vagal motor neurons. Assuming that fetal respiration movements also affect the vagal system, one can assume that parasympathetic activity increases with gestational age.

The average power of the 3rd group in the VLF domain is not significantly different from that of the 2^nd group. This result indicates some stabilization in the sympathetic activity during the third trimester of pregnancy.

In summary, the results imply that there is a clear neural organization during the last trimester of the pregnancy, and that the sympatho-vagal balance is gradually reduced with gestational age.