Noninvasive fetal electrocardiography: an overview of the signal electrophysiological meaning, recording procedures, and processing techniques

Abstract

Background: Noninvasive fetal electrocardiography (fECG), obtained positioning electrodes on the maternal abdomen, is important in safeguarding the life and the health of the unborn child. This study aims to provide a review of the state of the art of fECG, and includes a description of the parameters useful for fetus clinical evaluation; of the fECG recording procedures; and of the techniques to extract the fECG signal from the abdominal recordings.

Methods: The fetus clinical status is inferred by analyzing growth parameters, supraventricular arrhythmias, ST-segment variability, and fetal-movement parameters from the fECG signal. This can be extracted from an abdominal recording obtained using one of the following two electrode-types configurations: pure-abdominal and mixed. Differently from the former, the latter also provides pure maternal ECG tracings. From a mathematical point of view, the abdominal recording is a summation of three signal components: the fECG signal (i.e., the signal of interest to be extracted), the abdominal maternal ECG (amECG), and the noise. Automatic extraction of fECG includes noise removal by abdominal signal prefiltration (0.5-45 Hz bandpass filter) and amECG cancellation.

Conclusions: Differences among methods rely on different techniques used to extract fECG. If pure abdominal electrode configurations are used, fECG is extracted directly from the abdominal recording using independent component analysis or template subtraction. Eventually, if mixed electrode configurations are used, the fECG can be extracted using the adaptive filtering fed with the maternal ECG recorded by the electrodes located in the woman thorax or shoulder.

Keywords: abdominal ECG; fetal monitoring; noninvasive fetal ECG.

Figure 1

Simultaneous direct (scalp) fECG recording (upper panel) and abdominal recording (lower panel), showing both maternal and fetal ECG components.

Figure 2

PRISMA flow diagram indicating the process that had let to publications selection.

Figure 3

Electrode configurations for abdominal recordings. Pure abdominal configurations consider (A) four, (B) six, (C) ten, (D) thirteen, and (E) thirty two electrodes, respectively. Instead mixed configurations consider (F) eight, (G) nine, and (F) fourteen electrodes.

Figure 4

Automatic fECG extraction from an abdominal recording (aLead) consisting in an initial prefiltration and subsequent maternal ECG (amECG) cancellation.

Figure 5

Example of an artifact generated during the amECG cancellation (by subtraction) from faLead due to a nonperfect synchronization between estimated amECG and real amECG in correspondence of a high‐frequency QRS complex.