In this paper we characterize fetal sympatho-vagal balance by studying the correlation, time shifts, and ratios, between very-low-frequency (VLF), low-frequency (LF), and high-frequency (HF) components in heart rate (HR) variability over time. The HR is obtained from fetal magnetocardiographic (fMCG) recordings at various gestational periods. The frequency components are separated from the HR signal and pre-processed with the Length Transform (LT). The LT represents a temporarily smoothed time course of the data and has the property of highlighting the variability information of the signals over fixedlength windows. Therefore, we compute the correlations, time shifts, and ratios using the transformed data in order to avoid miscalculations and provide accurate quantitative measures. Our results show accordance with previous studies in sympathetic and parasympathetic control systems, and have the potential of characterizing fetal development.
IntroductionRecently, it has been of great interest to monitor the development of the fetus using magnetocardiographic techniques in order to characterize normal development as well as to identify possible pathologies [1], [2]. Many efforts have been placed in studying fetal heart rate (HR) variability, in particular time and frequency domain variables that are important markers for fetal well-being [3]- [5]. Assessments of fetal HR and fetal behavior are nowadays cornerstones of clinical fetal evaluation and neurodevelopmental research.Fetal magnetocardiography (fMCG) is an excellent signal source for assessment of fetal HR, providing a precise signal that can be detected throughout the last half of pregnancy and often earlier [6]. It is generally believed that clinical fetal HR monitors based on Doppler ultrasound lack the precision required to accurately assess beat-tobeat variations.Although HR is affected by many factors, the utility of fetal HR monitoring is based on the dominant contribution of the autonomic nervous system. Absent the influence of nerves and hormones, the SA node will spontaneously discharge at a nearly constant rate. Parasympathetic, or vagal, tone causes HR to rise, while sympathetic tone causes HR to fall. Thus, HR is regulated primarily by balancing sympathetic and vagal tone [7].Accurate quantification of the total vagal component in the HR is evidently vital for assessing the relative sympatho-vagal balance. Some studies suggest that very-low-frequency (VLF), low-frequency (LF) and highfrequency (HF) oscillations characterizing the HR have some relationship with HR variability [8], [9]. However, there is not a reliable quantitative measure of such relationship yet. In [10] and [11], the ratio of the LF to HF components of the fetal HR is proposed as a measure of the sympatho-vagal balance, while in [9] the ratio of the VLF to LF is the one considered. These measures also rely on computing the correlations and time shifts between the different frequency components of the fetal HR. However, these calculations are very sensitive to signal-to...