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 interventions. As a more objective measure of fetal oxygenation, fetal ;scalp pulse oximetry has been used to assess fetal blood oxygen saturation (SO2). The current devices use reflectance oximetry, which has inherent limitations. These include varying depths of signal penetration, variation with position, and potential for optical interference. In this study we evaluated a newly 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 days' gestation) were anesthetized, intubated, and prepared with a femoral artery catheter. Fetuses were prepared with brachial artery and jugular vein catheters. Maternal inspired oxygen fraction was titrated from 21% to 3%. Oximetry O(2)CTG devices were positioned on the fetal scalp, and recordings were compared with directly determined fetal arterial pH, Pop, and So, values. RESULTS: Maternal Sao(2) and Pao(2) ranged from 102% to 16% and 110 to 18 mm Hg, respectively. Fetal Sao(2) and Pao(2) ranged from 76% to 12% and 28 to 8 mm Hg, respectively. There was excellent correlation between direct fetal 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 compromise, 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 demonstrated high correlation of scalp So(2) with fetal Sao(2). (2) Although potential inaccuracies remain, transmission oximetry may offer potential advantages in consistency, ease of application, and technology with respect to the current reflection oximeter devices.