An Estimate of Fetal Autonomic State by Time Spectral and Nonlinear Analysis of Fetal Heart Rate Variability

Abstract

In this study we present a noninvasive method that enables the investigations of the fetal heart rate (FHR) fluctuations. The system is designed to measure the fetal heart rate variability for the evaluation of autonomic nervous system (ANS) and to investigate its development as a function of the gestational age and body mass index (BMI). Our Medical Engineering group has designed & developed a real time new method to assess instantaneous fetal heart rate variation (beat to beat) during gestation period with automated analysis of FHR variability using Doppler ultrasound method. We studied 64 maternal abdominal Doppler ultrasound signals. From these, we identify 41 ceases of pregnancies that we divided into three groups according to gestational age: Group A, 26-29 ± 1 wk (7th month pregnancy); Group B, 30-33 ±1wk (8th month pregnancy); and Group C, 34 onwards ± 1 wk (9th month pregnancy), whose body mass index (BMI) ranging from 20 to 37. The method consists in three steps: Doppler envelope filter, variable threshold detector and non retriggerable monostable multivibrator having adjustable pulse width for heartbeats detection. We defined a coefficient of variance (CVRR) as an index of parasympathetic activity, and a low frequency/high frequency (LF/HF) ratio as a sympathetic activity. The R–R interval variability which shows the variation between consecutive heart beats, change over time was eventually adopted to calculate for time-domain, frequency domain and non linear analysis. In the frequency domain analysis power spectral density (PSD) of the RR series is calculated by analyzing powers and peak frequencies of different frequency bands. The value of CVRR in the normal pregnancy group displayed a clearly increasing trend with gestational age (y = 0.383x + 13.16; R² = 0.009) (one-way ANOVA: P = 0.75655). The value of CVRR in the normal pregnancy group displayed a clearly decreasing trend with BMI. In contrast, the LF/HF ratio in the normal pregnancy group displayed a slight increasing trend over the gestational period. The value of LF/HF ratio in normal pregnancy group displayed clearly decreasing trend with body mass index (BMI) y = - 0.013x + 0.965; R² = 0.040 (one-way ANOVA: P = 0.081282). Analysis based on the time domain, frequency domain and non linear analysis of heart rate variability enable an evaluation of autonomic nervous system (ANS) activity. It is also observed that the parasympathetic nervous activity increased with gestational age in the normal pregnancy group and heart rate variability was found to be diminished in case of fetus from obese mother which is indicative of problems in the ANS activity. The dispersion of both short (SD1) & Long term (SD2) is smaller in normal weight mother fetus. The different fHRV parameters evaluated show a particular range for identification of autonomic maturation in the developing fetus.