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

OBJECTIVES: The study's objectives were to verify a threshold value for fet al 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-P uritan-Bennett Model N-400 Oxygen Saturation Monitor and FS-14 Sensor; Nell cor, Inc, Pleasanton, Calif). The durations of low (less than or equal to 3 0%), medium (31%-60%), and high (>60%) fetal arterial oxygen saturations du ring the measurement were compared between neonates with a pH <7.15 Versus greater than or equal to 7.15 and a base excess <-12 mmol/L Versus >-12 mmo l/L in the umbilical artery post partum and in neonates with an Apgar score <7 Versus greater than or equal to 7 by Mann-Whitney U test. In 121 of the pulse oximetry measurements the durations of low, medium, and high fetal a rterial oxygen saturations were measured from one fetal scalp blood samplin g to the next and correlated with the change of scalp blood pH between samp lings. 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 decli ne of pH. RESULTS: Neonates with a I-minute Apgar score <7 differed from those with 1 -minute Apgar score greater than or equal to 7 significantly in the duratio n of low fetal arterial oxygen saturation but not in the durations of mediu m and high fetal arterial oxygen saturations. The duration of low fetal art erial 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 greater than or equal to 7.15 or base excess >-12 mmol/L. The du ration of high fetal arterial oxygen saturation was significantly shorter f or children with a pH <7.15 or base excess <12 mmol/L than for those with a pH greater than or equal to 7.15 or base excess greater than or equal to 1 2 mmol/L. There was no difference in the groups with respect to the duratio n of medium fetal arterial oxygen saturation values. The duration of low fe tal arterial oxygen saturation proved to be the best predictor of a decline of scalp pH between 2 fetal scalp blood samples. The pH declined significa ntly with a longer duration of low fetal arterial oxygen saturation (0.02 p er 10 minutes). No decrease of pH by more than 0.05 was observed unless fet al arterial oxygen saturation had remained at less than or equal to 30% for greater than or equal to 10 minutes. CONCLUSION: An arterial oxygen saturation of 30% was confirmed as the criti cal boundary for fetal compromise during labor. The development of acidosis seems to be predictable by the duration of hypoxemia, as indicated by feta l arterial oxygen saturation less than or equal to 30%.