Assessment of fetal scalp oxygen saturation determination in the sheep by transmission pulse oximetry

OBJECTIVE: Electronic fetal heart rate monitoring has an unacceptable false -positive nonreassuring rate, which results in an excess of operative inter ventions. As a more objective measure of fetal oxygenation, fetal ;scalp pu lse oximetry has been used to assess fetal blood oxygen saturation (SO2). T he current devices use reflectance oximetry, which has inherent limitations . These include varying depths of signal penetration, variation with positi on, and potential for optical interference. In this study we evaluated a ne wly developed transmission pulse oximetry device consisting of transmitter and receiver diodes mounted within the coil of a standard scalp electrode ( Spiral O(2)CTG; Respironics Inc, Marietta, Ga). STUDY DESIGN: Six pregnant ewes at 127 to 135 days' gestation (term, 145 da ys' gestation) were anesthetized, intubated, and prepared with a femoral ar tery catheter. Fetuses were prepared with brachial artery and jugular vein catheters. Maternal inspired oxygen fraction was titrated from 21% to 3%. O ximetry O(2)CTG devices were positioned on the fetal scalp, and recordings were compared with directly determined fetal arterial pH, Pop, and So, valu es. RESULTS: Maternal Sao(2) and Pao(2) ranged from 102% to 16% and 110 to 18 m m Hg, respectively. Fetal Sao(2) and Pao(2) ranged from 76% to 12% and 28 t o 8 mm Hg, respectively. There was excellent correlation between direct fet al Sao(2) and scalp So(2) (r(2) = 0.90; scalp So(2) = 0.79 Sao(2) + 6.89). With an Sao(2) of <30% as the cutoff point for assessment of fetal compromi se, scalp So, measurements had a 94% +/- 10% specificity and a 94% +/- 10% positive predictive value. CONCLUSION: (1) Preliminary studies of the Spiral O(2)CTG sensor demonstrat ed high correlation of scalp So(2) with fetal Sao(2). (2) Although potentia l inaccuracies remain, transmission oximetry may offer potential advantages in consistency, ease of application, and technology with respect to the cu rrent reflection oximeter devices.