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 (S 2). 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 O2CTG; 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 O2CTG devices were positioned on the fetal scalp, and recordings were compared with directly determined fetal arterial pH, P 2, and S 2 values. Results: Maternal Sa 2 and Pa 2 ranged from 102% to 16% and 110 to 18 mm Hg, respectively. Fetal Sa 2 and Pa 2 ranged from 76% to 12% and 28 to 8 mm Hg, respectively. There was excellent correlation between direct fetal Sa 2 and scalp S 2 (r2 = 0.90; scalp S 2 = 0.79 Sa 2 + 6.89). With an Sa 2 of <30% as the cutoff point for assessment of fetal compromise, scalp S 2 measurements had a 94% ± 10% specificity and a 94% ± 10% positive predictive value. Conclusion: (1) Preliminary studies of the Spiral O2CTG sensor demonstrated high correlation of scalp S 2 with fetal Sa 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. (Am J Obstet Gynecol 2000;183:1549-53.)