Selection of optimal electrode positions for ambulatory pregnancy monitoring

Non-invasive pregnancy monitoring has become increasingly relevant in order to prevent complications. Fetal heart-rate (fHR) monitoring and estimation of uterine activity are important means to assess fetal health during pregnancy and delivery. Recent advances in signal processing technology enable pregnancy monitoring using abdominal electrocardiogram (ECG) and electrohysterogram (EHG) recordings [1], [2]. The large number of electrodes required in current noise-robust solutions, however, leads to high power consumption and reduced patient comfort, hampering unobtrusive ambulatory monitoring. To enable pregnancy monitoring in an ambulatory setting, the number of electrodes has to be reduced without compromising signal quality. Therefore, the influence of abdominal electrode positioning on the SNR of various signals of interest for pregnancy monitoring is explored based on measurements. Three measurements on women at full gestation just prior to labor (w39d4 ± 12d), with a total length of 1 hour, were performed using an electrode grid consisting of 12 electrodes. The electrodes are organized in a triangular fashion, which allows for analysis of the recorded signals in six different directions at three different inter-electrode scales, resulting in a total of 18 bi-polar signals for each measurement. To define the ability of all electrode pairs to capture the electrophysiological characteristics of interest, a signal quality criterion for both the fetal and maternal ECG as well as the EHG is defined based on the SNR. The SNR of both ECG signals is defined as the power of the QRS segment divided by the RMS power of the remaining signal. In case of the fetal ECG (fECG), the maternal ECG (mECG) was first removed using the method described in [1]. For the EHG, the signal and noise power are defined as the RMS signal in the 0.34 – 0.8 Hz frequency band in time segments during and in between contractions, respectively. Results show that the SNR of both the fetal and maternal ECG increasing with increase inter-electrode distance and are strongly dependent on measurement direction. In the case of mECG, a horizontal measurement orientation is preferred, while the optimal measurement direction for fECG changes depending on the orientation of the fetus with respect to the electrode grid. Contrary to the ECG, the SNR of the EHG shows an optimum for an electrode distance of 8 – 12 cm, while the optimal electrode orientation changes for every contraction. An electrode grid allowing for optimal detection of both ECG and EHG signals, therefore, should contain inter-electrode distances of both 8 and 16 cm with various orientations. According to our results, a triangular measurement setup with 6 electrodes, therefore, enables recording of mECG, fECG, and EHG signals with a SNR of 12.3 dB, 6.1 dB, and 9.8 dB, respectively.