Kunihiro Okamura

Tohoku J. Exp. Med., 221: 287-298, 2010. In the version of this article published in the August issue, 2010, the affiliation number of the tenth author (Shigeru Tsuchiya) was mistyped (page 287). The correct affiliation is shown above. In addition, the lettering of ‘ZT’ is a misprint for ‘CT’ in panels A, B and C of Figures 6 and 7 (pages 294 and 295).

A fetal electrocardiogram (ECG) provides clinically significant information concerning the electrophysiological states of a fetus. Since the first attempt by Cremer in 1906, for about 100 years nobody has been able to non-invasively extract the fetal ECG popularly used in clinical situations. The low amplitude of the fetal signals and the high background noise and the non-linearity of the ECG signals mainly have been interfering with the noninvasive extraction of the fetal ECG. The aim of this paper is to introduce a new nonlinear filtering method to extract the fetal electrocardiogram detected from the mother's abdominal wall. This method consists of two steps: The rough extraction step performed by blind source separation (BSS) with references and the precise extraction step with fast non-linear state space projection (FNSSP) based on a nonlinear state space projection (NSSP), which needs several complicated conditions or has limitations. So, it takes a lot of time to calculate. Our new method does not require such conditions, and it has about a thirty-fold speedup when compared with the previous method. Our new method has been successfully applied to on-line recordings with fetal components and will be useful for monitoring the fetal electrocardiogram in clinical applications.

The physiological significance of spectral and fractal components of spontaneous heart rate (HR) variability in the fetus remains unclear. To examine the relationship between circadian rhythms in different measures of HR variability, R-R interval time series obtained by fetal ECGs were recorded continuously over 24 h in five pregnant sheep at 116–125 days gestation. Conventional measures of short-term (STV) and long-term variability (LTV), low-frequency (LF; 0.025–0.15 cycles/beat) and high-frequency (HF; 0.2–0.5 cycles/beat) spectral powers, the LF-to-HF ratio, and fractal dimension values were calculated from 24-h ECG recordings and quantified every 60 min. STV, LTV, and LF and HF spectral powers were minimal during the day but increased significantly to their highest values at night. We found a significant positive correlation between these measures, whereas the cosinor method showed significant similarity between their circadian rhythm patterns. Fetal R-R intervals also exhibited fractal structures. Fetal HR variability had a fractal structure, which was similar between day and night. These results suggested that the circadian rhythms exhibited by STV and LTV during the day were mainly due to changes in frequency components rather than to fractal components of fetal HR fluctuation.