Foetal ECG and maternal respiration

The fECG can be ten to twenty times smaller than the maternal ECG in amplitude, and wider in bandwidth, ranging between 0.05 Hz to 250 Hz (because of QRS duration shortening caused by the higher HR)⁵⁴. For this reason, previous research proposed minimum sampling frequencies between 1 kHz to 2 kHz and an analogue-to-digital quantization resolution of 16 bits²⁶. For this dataset, the bio-potentials were recorded with a Porti7 portable physiological measurement system (TMSi, The Netherlands). The system features simultaneous sampling up to 2048 Hz on the available input channels; however, the input bandwidth is limited by the internal digital decimation filter to approximately 550 Hz (0.27 × the sampling frequency). The analogue front-end is dc-coupled (contains no high-pass filter) with a 300 mV peak-to-peak amplitude range. Therefore, the digitisation at 22 bits provides a 71.5 nV amplitude resolution. Active cables shielding and high input impedance, equal to 1012Ω, reduces the needs of hard skin preparation and power-line interference. The device features 32 simultaneously sampled channels: 24 unipolar (all of the unipolar channels are acquired with respect to their average because it implements an average reference amplifier on these channels only), four bipolar (for differential recordings) and four auxiliary (also bipolar, with an extended dynamic range, used for sensors requiring polarisation). Different versions of the adopted device were previously used for the same purpose by other researchers⁵⁵ and more recently for advanced studies on foetal scalp electrode recordings and optimisation⁵⁶.

To date, there is no standard or widely accepted consensus for non-invasive fECG electrode placement. A recent review⁵⁷, studied twenty abdominal electrode configurations, four of which used 32 or more electrodes, and the rest used less than 16. For this study, to ensure maximum versatility of the dataset and in accordance with the most popular sensor placement schemes⁵⁷, a set of 30 signal electrodes plus a ground reference was placed as shown in Fig. 4.

The chosen electrodes positioning for the NInFEA dataset: (a) front and back positioning scheme and (b) image of a real recording (only the abdominal channel and respiration belt are visible).

This configuration can be mapped to lower-dimensional schemes by spatial sub-sampling because of its high spatial redundancy, as shown in Figs. 5 and ​and6.6. Moreover, a redundant number of electrodes also improves the chance of recording good-quality signals from multiple channels, even when the ultrasound probe placement introduces artefacts in the electrodes in its proximity. The chosen electrode configuration includes:

22 electrodes connected to unipolar channels, covering a large area of the maternal abdomen, avoiding iliac crests and rib regions (electrodes number 1–22, Fig. 4),

two further electrodes connected to unipolar channels, placed on the maternal back (electrode number 23–24, Fig. 4),

six electrodes for three bipolar channels, positioned on the maternal thorax, for recording the maternal ECG (to capture three non-coplanar maternal ECG signals, as required by some fECG extraction algorithms and adaptive filtering schemes^58–61), electrode number 25–27, Fig. 4),

one reference (signal ground) electrode, positioned on the right maternal hip.

Replicability of state-of-the-art electrode positioning with the proposed dataset. In order: (a) Adapted from⁷⁷, (b) Adapted from⁷⁸, (c) Adapted from⁷⁹, (d) Adapted from⁸⁰, (e) Adapted from⁸¹, (f) Adapted from⁶⁰, (g) Adapted from⁴¹, (h) Adapted from¹⁸,(i) Adapted from⁸², (j) Adapted from⁸³. (The reader is suggested to check the electronic version of this picture since colours were used to identify the used (green) and unused (red) electrodes from the proposed setup to replicate the one taken from the literature.) Reproduced with permission.

Replicability of state-of-the-art electrode positioning with the proposed dataset. In order: (a) Adapted from⁸⁴, (b) Adapted from⁸⁵, (c) Adapted from⁸⁶, (d) Adapted from⁸⁷, (e) Adapted from⁸⁸, (f) Adapted from⁸⁹, (g) Adapted from⁹⁰, (h) Adapted from⁵⁵, (i) Adapted from^91,92. (The reader is suggested to check the electronic version of this picture since colours were used to identify the used (green) and unused (red) electrodes from the proposed setup to replicate the one taken from the literature.) Reproduced with permission.

Considering the FDA/CE cleared non-invasive fECG devices present on the market, the number of channels collected in this dataset is significantly greater. In fact, except for the Meridian M110 Fetal Monitoring System (MindChild Medical, North Andover, MA, USA), which uses patches with 27 electrodes and the ground, other commercial devices use patch systems with less than six electrodes: Monica AN24 (Monica Healthcare, Nottingham, UK), Monica Novii Wireless Patch System (Monica Healthcare, Nottingham, UK), PUREtrace and Nemo Fetal Monitoring System (Nemo Healthcare, Veldhoven, The Netherlands) and the Wearable 5-Channel ECG Chip to Monitor Fetal Heart Rate and Mobility (Imec, Leuven, Belgium, and BloomLife, San Francisco, CA | Genk, Belgium). Figure 7 shows how the electrode positioning from the devices available on the market (characterised by a number of electrodes compatible with the dataset) are nearly reproducible with our setup. Remarkably, the highest number of electrodes of the dataset is meant to provide unavailable features to the scientists working on fECG extraction and foetal cardiac physiology. For instance, it can be useful for studies on the optimisation of electrode placement, the recovery of hidden information on the cardiac axis, the assessment of algorithms for the solution of the inverse problem and lead reconstruction by geometric transforms.

Replicability of electrode positioning used in the market-available wearable devices with the proposed dataset. In order: (a) Adapted from Monica AN24 (Monica Healthcare, Nottingham, UK), (b)Monica Novii Wireless Patch System (Monica Healthcare, Nottingham, UK), (c) PUREtrace (Nemo Healthcare, Veldhoven, The Netherlands), (d) Nemo Fetal Monitoring System (Nemo Healthcare, Veldhoven, The Netherlands), (e) Wearable 5-Channel ECG Chip to Monitor Fetal Heart Rate and Mobility by Imec and Bloomlife. (The reader is suggested to check the electronic version of this picture since colours were used to identify the used (green) and unused (red) electrodes from the proposed setup to replicate the one taken from the literature).

Small electrodes were chosen considering the large number of abdominal channels. BlueSensor N electrodes (Ambu, Denmark) have been developed for neonates, have a highly conductive liquid gel that reduces the skin-electrode contact impedance. Moreover, the offset connector helps to reduce the cable movement artefacts. Prior to electrode attachment, a mild skin treatment was performed on the maternal abdomen using NuPrep (Weaver and Company, USA), an abrasive gel enhancing the conductivity by reducing the skin contact impedance.

A piezo-resistive respiration belt was placed around the maternal chest and was connected to one of the auxiliary inputs of Porti7. The utilisation of the respiration belt enables further physiological studies and improves signal processing on the electrophysiological signals, e.g. removal of the baseline drift caused by thoracic wall movements during inspiration and expiration.