Basic principles and general problems of pulse oximetry at low oxygen saturations

Abstract

Many authors active in the field of fetal pulse oximetry, including our own group, primarily believed that the main objective would be the early detection of hypoxia (1). From the clinical experience our group has gained, as well as from communications with other groups active in this field, we now feel this is not the main objective. Instead, fetal pulse oximetry can be used to rule out fetal hypoxia. There are many cases where the CTG concerns the obstetrician — especially if he or she is not particularly experienced — while the fetal pulse oximetry measurement provides a reassuring, high reading of the fetal oxygen saturation, for example 65 %. If we assume a maximum possible error of ± 20 % absolute (the error is the difference between the actual O₂ saturation of the fetus and the value the instrument reads) then we can be confident that the oxygen saturation of the fetus in this example is between 45 and 85 %. 45 % is of course already rather low, however it is believed that such a level is still safe (3).