The prediction of fetal acidosis by means of intrapartum fetal pulse oximetry

Abstract

Objectives: The study's objectives were to verify a threshold value for fetal arterial oxygen saturation as the critical boundary for fetal compromise during labor and to investigate a method of predicting acidosis caused by hypoxemia. Study Design: In a multicenter study involving 3 German obstetric centers, a total of 400 deliveries were monitored by fetal pulse oximetry (Nellcor-Puritan-Bennett Model N-400 Oxygen Saturation Monitor and FS-14 Sensor; Nellcor, Inc, Pleasanton, Calif). The durations of low (≤30%), medium (31%-60%), and high (>60%) fetal arterial oxygen saturations during the measurement were compared between neonates with a pH <7.15 versus ≥7.15 and a base excess <−12 mmol/L versus >−12 mmol/L in the umbilical artery post partum and in neonates with an Apgar score <7 versus ≥7 by Mann-Whitney U test. In 121 of the pulse oximetry measurements the durations of low, medium, and high fetal arterial oxygen saturations were measured from one fetal scalp blood sampling to the next and correlated with the change of scalp blood pH between samplings. Multiple regression analysis was performed to estimate the expected change of pH between 2 fetal scalp blood samplings, and receiver operating characteristic analysis was done to define a minimum duration of low fetal arterial oxygen saturation values to exclude or predict a significant decline of pH. Results: Neonates with a 1-minute Apgar score <7 differed from those with 1-minute Apgar score ≥7 significantly in the duration of low fetal arterial oxygen saturation but not in the durations of medium and high fetal arterial oxygen saturations. The duration of low fetal arterial oxygen saturation had been significantly longer in children with pH <7.15 or base excess <−12 mmol/L in the umbilical artery compared with those with a pH ≥7.15 or base excess ≥−12 mmol/L. The duration of high fetal arterial oxygen saturation was significantly shorter for children with a pH <7.15 or base excess <12 mmol/L than for those with a pH ≥7.15 or base excess ≥12 mmol/L. There was no difference in the groups with respect to the duration of medium fetal arterial oxygen saturation values. The duration of low fetal arterial oxygen saturation proved to be the best predictor of a decline of scalp pH between 2 fetal scalp blood samples. The pH declined significantly with a longer duration of low fetal arterial oxygen saturation (0.02 per 10 minutes). No decrease of pH by more than 0.05 was observed unless fetal arterial oxygen saturation had remained at ≤30% for ≥10 minutes. Conclusion: An arterial oxygen saturation of 30% was confirmed as the critical boundary for fetal compromise during labor. The development of acidosis seems to be predictable by the duration of hypoxemia, as indicated by fetal arterial oxygen saturation ≤30%. (Am J Obstet Gynecol 1999;180:73-81.)