Several investigators have applied pulse oximetry technology to the fetus. Fetal oxygen saturation monitoring provides a direct, quantitative, and continuous measurement of the amount of oxygen present in fetal arterial blood. The fetus is a unique patient, both by being inaccessible to study and by having several mechanisms which enable it to survive at remarkably lower arterial oxygen saturation levels than found in either the new-born or the adult. Pulse oximetry sensors exist in either a transmission or a reflectance configuration. The former involves positioning of the red and infra-red light-emitting diodes on the opposite side of the vascular bed to the photo-detector and this is the arrangement that is usually used for neonates and adults. Fetal pulse oximetry requires the use of reflectance technology as digits or ears are not usually available to the obstetrician for placement of transmission sensors. Here the emitters and the detector are situated adjacent to one another, on the same skin surface and the percent of oxygen saturation is calculated from the ratio of red and infra-red light that scatters back to the skin surface. The Nellcor FS14 fetal sensor incorporates LEDs of 735 nm and 890 nm wavelengths. The depth of tissue penetration at both high and low saturation is very similar at each of these wavelengths and since their introduction the technology has improved with less artefact, greater reproducibility and greater continuity of signal. It is concluded that fetal oximetry may be a valuable second-line monitor for use in conjunction with the cardiotocograph but the clinical model of interpretation needs be subjected to a randomised controlled trial before it is used for routine clinical management.
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