Physio-optical considerations in the design of fetal pulse oximetry sensors

Pulse oximetry is being investigated as a potentially useful tool for assessing the well-being of the fetus during labor and delivery. Two commonly made simplifying assumptions in the basic mathematics of pulse oximetry, however, strongly influence the accuracy of the measurement if not accounted for in the sensor's design. Accurate oximeter performance requires a good overlap in light penetration at the wavelengths utilized in order to minimize the effects of tissue heterogeneity. Furthermore, path length changes arising from variations in tissue absorbance must be balanced at the two wavelengths or else calibration bias may result. The effects of light scattering within the tissue need to be considered over the range of oxygen saturation in which best operation of the system is desired. For oximeters designed to monitor air-breathing patients at high saturation, the conventional choice of emitter wavelengths optimizes both accuracy and measurement sensitivity. For the lower oxygen saturations typical in the fetus during labor, numerical modeling suggests that significantly better accuracy can be expected when a far red and near infrared emitter pair are used, even though the saturation measurement sensitivity is somewhat reduced.