Pulse oximetry during labour--does it give rise to hope? Value of saturation monitoring in comparison to fetal blood gas status.

OBJECTIVE

Purpose of this presentation is to show the diagnostic power of fetal pulse oximetry in comparison to the other blood gas parameters from fetal blood samplings (FBS). The distribution of saturation readings in acidotic fetuses and normally oxygenated fetuses should be established.

STUDY DESIGN

A fetal pulse oximetry system (N400, FS14) was evaluated in a strictly observational study design based on blinded saturation display and on continuous data storing. The investigation was performed on 170 fetuses with non-reassuring fetal heart rate (FHR)-tracings. Since pulse oximetry readings were not available for decision finding, the clinical management was based on electronic fetal monitoring and fetal blood samplings. The oxygen saturation from FBS or umbilical cord blood was measured by blood gas analysers with an integrated hemoximeter (Bayer 865; ABL 625, Radiometer) and biosensors measuring lactate as metabolic component. Out of the 170 cases 17 cases were defined as group of acidemia (pH(umb.art.) < 7.16 + BD<-9.4). The distribution of saturation readings and the duration of desaturation periods (in minutes and percentage of total monitoring time SPO2 below 30%) were determined. ROC curve analysis from FBS preceding delivery compared the diagnostic power of other blood gas parameters with oxygen saturation. The Wilcoxon test for uneven pairs was used.

RESULTS

The distribution of oxygen saturation in the normal group of fetuses differs significantly from the acidemic group. The correlation coefficient between both methods to determine oxygen saturation was r=0.66. A specific evaluation of the distribution of SPO2 shows an overestimation of pulse oximetry in the low range and an underestimation in the high range of saturation. ROC-curve analysis showed a good diagnostic power of lactate in comparison to the oxygen saturation measured by pulse oximetry or by hemoximetry.

CONCLUSIONS

The advantage of continuous fetal pulse oximetry surveilling the fetus under suspicion of hypoxia appears limited by the poor diagnostic power of the respiratory parameter saturation itself and by the impairment of the precision of the technology.