The relationship between oxygen saturation and pH in umbilical blood: Implications for intrapartum fetal oxygen saturation monitoring

OBJECTIVE: Our purpose was to evaluate the relationship between umbilical arterial blood oxygen saturation determined by hemoximetry (Sao2) and umbilical arterial blood pH and base excess determined by blood gas analysis to establish a ″critical threshold” for fetal preductal arterial oxygen saturation determined by reflectance pulse oximetry (Spo2). STUDY DESIGN: Umbilical artery and vein blood specimens were obtained at delivery. Blood gas analysis and hemoximetry were performed. Polynomial regression analysis and receiver-operator characteristic curves were calculated for umbilical arterial blood Sao2 and theoretic preductal arterial blood Sao2 versus umbilical arterial blood pH and base excess. RESULTS: A total of 1101 paired umbilical artery and vein specimens were obtained. When the umbilical arterial blood Sao2 was ≥30%, umbilical arterial blood pH was ≥7.13 in 99.0% (388/392) of cases and <7.13 in 1.0% (4/392) of cases. When umbilical arterial blood Sao2 was <30%, umbilical arterial blood pH was ≥7.13 in 91.4% (648/709) of cases and <7.13 in 8.6% (61/709) of cases. CONCLUSIONS: From these analyses, it appears that an Spo2 cutoff value of 30% would be reasonable in clinical trials of intrapartum fetal pulse oximetry. (Am J Obstet Gynecol 1996;175:682-7.)