Is Abdominal Fetal Electrocardiography an Alternative to Doppler Ultrasound for FHR Variability Evaluation?

Abstract

Great expectations are connected with application of indirect fetal electrocardiography (FECG), especially for home telemonitoring of pregnancy. Evaluation of fetal heart rate (FHR) variability, when determined from FECG, uses the same criteria as for FHR signal acquired classically-through ultrasound Doppler method (US). Therefore, the equivalence of those two methods has to be confirmed, both in terms of recognizing classical FHR patterns: baseline, accelerations/decelerations (A/D), long-term variability (LTV), as well as evaluating the FHR variability with beat-to-beat accuracy-short-term variability (STV). The research material consisted of recordings collected from 60 patients in physiological and complicated pregnancy. The FHR signals of at least 30 min duration were acquired dually, using two systems for fetal and maternal monitoring, based on US and FECG methods. Recordings were retrospectively divided into normal (41) and abnormal (19) fetal outcome. The complex process of data synchronization and validation was performed. Obtained low level of the signal loss (4.5% for US and 1.8% for FECG method) enabled to perform both direct comparison of FHR signals, as well as indirect one-by using clinically relevant parameters. Direct comparison showed that there is no measurement bias between the acquisition methods, whereas the mean absolute difference, important for both visual and computer-aided signal analysis, was equal to 1.2 bpm. Such low differences do not affect the visual assessment of the FHR signal. However, in the indirect comparison the inconsistencies of several percent were noted. This mainly affects the acceleration (7.8%) and particularly deceleration (54%) patterns. In the signals acquired using the electrocardiography the obtained STV and LTV indices have shown significant overestimation by 10 and 50% respectively. It also turned out, that ability of clinical parameters to distinguish between normal and abnormal groups do not depend on the acquisition method. The obtained results prove that the abdominal FECG, considered as an alternative to the ultrasound approach, does not change the interpretation of the FHR signal, which was confirmed during both visual assessment and automated analysis.

Keywords: Doppler ultrasound; fetal electrocardiogram; fetal heart rate analysis; fetal outcome; fetal state assessment.

Figure 1

The screen illustrating the operation of the procedure for synchronizing the two signals, determined via the ultrasound (red FHR_U) and electrocardiographic (blue FHR_E) methods. (A) Presents the concept of automated synchronization via minimizing the mean error. The current value of the mean absolute difference MAD parameter is displayed on the gauge. Auto mode had automatically set up the value of the shift parameter to 9 samples forward. However, basing on a visual analysis of the signals, that shift was manually corrected using a slider—the FHR_U signal was shifted by 2 samples backward in relation to the FHR_E signal. (B) Shows the enlarged signal fragment from part A, but after a procedure for removing the sudden FHR changes. It allows for additional manual synchronization and final validation of the recording for further investigations.

Figure 2

Bland Altman plot showing the dependence of the mean value of the differences MD (Y axis) between corresponding instantaneous values of FHR_U and FHR_E, in relation to the average fetal heart rate in the recording (X axis), for each of the 60 pairs of signals. The values are expressed in beats per minute—bpm.

Figure 3

An example of a 12-min fragment of signal pair—result of an indirect comparison to evaluate the impact of the FHR signal acquisition method on clinically relevant parameters, determined by a computer-aided fetal monitoring system. The autocorrelation technique, commonly used in the US method to determine the signal periodicity, is often not able to follow the rapid decrease of FHR_U signal related to deceleration, which results in signal loss episodes. This, in turn, causes that the deceleration is not recognized, because it does not meet the established criteria of amplitude and duration. Graphic markers of the analysis results illustrate the signal loss (above the curve), the estimated FHR baseline (line fitted on FHR curve) and detected deceleration episodes (horizontal bars under the curve).