Improving noninvasive fetal pulse oximetry using N-layer analysis techniques for extraction of deep-layer absorption changes from silicone-based phantoms

In this study a simple and reproducible phantom design was created to mimic transabdominal fetal photo-plethysmography (PPG) changes for the purpose of developing a fetal pulse oximetry sensor for use during labor and delivery. Custom phantoms were created that included variation in fetal depth, optical properties of both maternal and fetal layers, and variation in fetal pulse oxygen saturation (fSpO2). Data were collected using a spatially resolved continuous wave spectroscopy device with four source-detector distances (2.15 - 9.5 cm) and four wavelengths (730 - 850 nm). The effects of each phantom variable on the fetal PPG and ratio of ratios (RoR) were quantified using a thirty-two run factorial design with two levels. This study demonstrated that the fetal PPG can be resolved at a fetal depth of 4 cm, with the size of the fetal PPG most impacted by fetal depth. The effect of the fetal depth is corrected for using an analytical method to calculate mean partial pathlength. RoR results are not impacted by the phantom variables, although the instrumental error increased for fetal depths 4 cm. Future steps are discussed for correcting the detected fetal PPG signal for the influence of the maternal and fetal properties.