Effect of location of the sensor on reflectance pulse oximetry

The influence of the location of the sensor on reflectance pulse oximetry during fetal monitoring in labour was investigated using the newborn infant as a model.

Seven healthy infants were studied between 19 and 48 hours after term delivery. Recordings of reflectance pulse oximetry were obtained from eight different sites on the infant's head. The relative changes in red to infrared light (R/IR) were determined. In pulse oximetry R/IR values are converted to arterial oxygen values by means of an empirically derived calibration curve.

Significantly lower R/IR values were found at the forehead compared with the fontanelle, the parietal and occipital position, and the temporal area. Conversion to oxygen saturation values revealed a difference of up to 13.4% in oxygen saturation between the forehead and the occipital area.

Assuming that the arterial blood oxygen saturation did not change substantially, our findings indicate that in reflectance pulse oximetry there is no unique relation between R/IR and arterial oxygen saturation. The differences in reflectance pulse oximetry at the various sites are explained by differences in optical properties (scattering and absorption) of the tissue underneath the sensor. These will affect the red and infrared light reaching the detectors in a different way, and consequently R/IR changes. Because during intrapartum monitoring exact positioning of the sensor on the fetal head is usually impossible, the accuracy of fetal reflectance pulse oximetry is impaired.