Real-time fetal monitoring using photoacoustic measurement of placental oxygen saturation in a rabbit hypoxia model.

Introduction: Ensuring adequate fetal oxygenation is an essential aim of fetal monitoring. The purpose of this study was to establish a basic technique for real-time measurement of blood oxygen saturation of the placenta by photoacoustic (PA) technique as a new fetal monitoring method.

Methods: The hypoxia model established in our previous study was applied to 7 pregnant rabbits. Three phases were induced: normal phase, hypoxia phase, and recovery phase. Three methods were simultaneously used for real-time fetal monitoring: fetal heat rate (FHR) monitoring, oxygen saturation (SO) measurement by near-infrared spectroscopy (SO), and placenta SO measured by PA technique (SO). The maternal hypoxia was assessed by skin SO measured by PA technique (SsO), and arterial blood SO by blood gas analysis (SaO).

Results: The average of SO in normal phase was 52.6 ± 13.9 %. The averages of SO, SO, and SO in the seven rabbits changed in parallel from the normal phase to hypoxia phase. In the recovery phase, the SO rose in parallel with recovery of SaO. There was lag in increase of the FHR compared to the change in the other values. In the detailed analysis of PA signals from the labyrinth and decidua, a unique change in oxygen saturation was seen in one case.

Discussion: Results of this study showed that sensitivity of our novel PA technique in detecting tissue hypoxia was similar to near-infrared spectroscopy (NIRS). As an advantage, unlike NIRS, monitoring with PA technique was unaffected by ischemia and surface changes in oxygen saturation because of its higher spatial resolution. We conclude that PA technique provides more accurate information about fetal blood placenta than NIRS. Ultrasound imaging, combined with oxygen saturation monitoring by PA technique, would improve fetal monitoring and fetal diagnosis in the future.