A problem of maternal and fetal QRS complexes overlapping in fetal heart rate estimation

Indirect fetal electrocardiography is a rapidly developing monitoring method. Bioelectrical signals being recorded from maternal abdomen contain more information than mechanical heart activity measurement based on Doppler ultrasound signals. The main problem relies on extraction of basic useful component - fetal electrocardiogram - from a signal containing dominant maternal electrocardiogram and muscles activity artifacts. The algorithms of maternal component suppression via subtraction are commonly used. Although regarding their complexity, these algorithms have limited applicability in systems for long-term monitoring of fetal heart variability. The simplest solution relies on blanking procedure which is based on replacement of maternal QRS complexes by isoline values. However, in case of overlapping of fetal and maternal QRS complexes, this approach leads to partial loss of information about fetal heart rate activity. This paper presents the evaluation of how often such episodes of overlapping of both complexes takes place considering the physiological signal limits.