Passive Fetal Monitoring by Advanced Signal Processing Methods in Fetal Phonocardiography

Fetal phonocardiography (fPCG) is a non-invasive technique for detection of fetal heart sounds (fHSs), murmurs and vibrations. This acoustic recording is passive and provides an alternative low-cost method to ultrasonographic cardiotocography (CTG). Unfortunately, the fPCG signal is often disturbed by the wide range of artifacts that make it difficult to obtain significant diagnostic information from this signal. The study focuses on the filtering of an fPCG signal containing three types of noise (ambient noise, Gaussian noise, and movement artifacts of the mother and the fetus) having different amplitudes. Three advanced signal processing methods: empirical mode decomposition (EMD), ensemble empirical mode decomposition (EEMD), and adaptive wavelet transform (AWT) were tested and compared. The evaluation of the extraction was performed by determining the accuracy of S1 sounds detection and by determining the fetal heart rate (fHR). The evaluation of the effectiveness of the method was performed using signal-to-noise ratio (SNR), mean error of heart interval measurement (<inline-formula> <tex-math notation="LaTeX">$\overline {|\Delta T_{i}|}$ </tex-math></inline-formula>), and the statistical parameters of accuracy (ACC), sensitivity (SE), positive predictive value (PPV), and harmonic mean between SE and PPV (F1). Using the EMD method, <inline-formula> <tex-math notation="LaTeX">$\text {ACC} > 95$ </tex-math></inline-formula>% was achieved in 7 out of 12 types and levels of interference with average values of <inline-formula> <tex-math notation="LaTeX">$\text {ACC} = 88.73$ </tex-math></inline-formula>%, <inline-formula> <tex-math notation="LaTeX">$\text {SE} = 91.57$ </tex-math></inline-formula>%, <inline-formula> <tex-math notation="LaTeX">$\text {PPV} = 94.80$ </tex-math></inline-formula>% and <inline-formula> <tex-math notation="LaTeX">$\text {F1} = 93.12$ </tex-math></inline-formula>%. Using the EEMD method, <inline-formula> <tex-math notation="LaTeX">$\text {ACC} > 95$ </tex-math></inline-formula>% was achieved in 9 out of 12 types and levels of interference with average values of <inline-formula> <tex-math notation="LaTeX">$\text {ACC} = 97.49$ </tex-math></inline-formula>%, <inline-formula> <tex-math notation="LaTeX">$\text {SE} = 97.89$ </tex-math></inline-formula>%, <inline-formula> <tex-math notation="LaTeX">$\text {PPV} = 99.53$ </tex-math></inline-formula>% and <inline-formula> <tex-math notation="LaTeX">$\text {F1} = 98.69$ </tex-math></inline-formula>%. In this study, the best results were achieved using the AWT method, which provided <inline-formula> <tex-math notation="LaTeX">$\text {ACC} > 95$ </tex-math></inline-formula>% in all 12 types and levels of interference with average values of <inline-formula> <tex-math notation="LaTeX">$\text {ACC} = 99.34$ </tex-math></inline-formula>%, <inline-formula> <tex-math notation="LaTeX">$\text {SE} = 99.49$ </tex-math></inline-formula>%, <inline-formula> <tex-math notation="LaTeX">$\text {PPV} = 99.85$ </tex-math></inline-formula>% a <inline-formula> <tex-math notation="LaTeX">$\text {F1} = 99.67$ </tex-math></inline-formula>%.