PSAFE: Extraction of Faint Fetal PPG from Non-Invasively Acquired Mixed PPG Signals

Traditional approaches to intrapartum fetal monitoring, based on interpretation of fetal heart rate (FHR) tracings, have high false positive rates for detection of fetuses at risk of birth asphyxia. Transabdominal Fetal Pulse Oximetry (TFO) promises to supplement FHR trace interpretation through non-invasive sensing of fetal blood oxygen saturation $\left({\text{fSpO}}_2\right)$ from photoplethysmography (PPG) signals acquired through the maternal abdomen. However, the acquired signals, referred to as mixed PPG, contain contributions from both maternal superficial tissue layers and fetal tissue, as well as other noise sources. We propose Phase-Synchronized Averaging for Fetal Signal Enhancement (PSAFE), a novel algorithm that leverages fetal heart phase information to align and average mixed PPG segments. Evaluation using in-vivo data collected from pregnant ewe models demonstrate that PSAFE yielded a 43.4% reduction in mean absolute error, and a 25.9% improvement in correlation for ${\text{fSpO}}_2$ estimation, compared to a leading alternative approach.