This study clarified that the iPREFACE(30) has the highest capacity for predicting fetal acidemia, suggesting that it may be useful in clinical practice. Additionally, the iPREFACE(10), iPREFACE(30), and iPREFACE(60) correlated significantly with the pH, BE, and lactate in the umbilical cord arterial blood, suggesting that hypoxia caused by repeated decelerations during labor and delivery may lead to fetal acidemia and accumulation of lactate from anaerobic metabolism.
The observed difference between the iPREFACE(10) and iPREFACE(30) suggests that a short evaluation time is insufficient for predicting fetal acidemia. Although there was no significant difference between iPREFACE(30) and iPREFACE(60), the 30-min assessment is generally preferable in clinical practice to reduce maternal discomfort.
In a report analyzing prenatal CTGs of 35 infants with HIE, the median time from the development of pathological CTGs to delivery was 145 min [19]. The time-lapse may increase the risk of fetal HIE during the 60-min scoring assessment, which further supports the usage of the 30-min scoring time to determine the timing of delivery before the fetus develops HIE.
We hypothesize that prolonged assessment does not enhance the ability to predict fetal acidemia because there is no significant difference between iPREFACE(30) and iPREFACE(60). We speculated that recovery of fetal damage during the less frequent deceleration periods could account for the similarity. Fetal acidemia caused by umbilical cord occlusion reportedly normalizes to pH ≥ 7.25 between 20 and 30 min after resolution [20]. If the frequency of umbilical cord occlusion exceeds the recovery of fetal damage, comprehensive damage will occur; however, if umbilical cord compression becomes less frequent and the recovery exceeds it, the umbilical artery pH is expected to normalize. In a study of fetal sheep, 1 min of complete umbilical cord occlusions every 2.5 min for 4 h resulted in severe metabolic acidemia, whereas occlusion every 5 min for 4 h resulted in mild metabolic acidemia[14]. Therefore, when assessing the integration of fetal damage due to decelerations over an extended duration, fetal damage recovery occurring within that period may reduce the ability to predict fetal acidemia.
The correlations between iPREFACE(10), iPREFACE(30), and iPREFACE(60) and the pH, BE, and lactate levels in umbilical artery blood were consistent with those in a separate study on fetal sheep, which reported a significant decrease in fetal arterial pH as a result of decelerations caused by repeated umbilical cord occlusion and accumulation of lactic acid in the brain and throughout the body, due to anaerobic metabolism [21]. The primary cause of fetal acidemia is hypoxia due to decreased uteroplacental circulation. Historically, only late decelerations were believed to cause fetal hypoxia; however, all decelerations at every stage have been observed to cause hypoxia [14]. Since iPREFACE(10), iPREFACE(30), and iPREFACE(60) have significant correlations with the pH, BE, and lactate in umbilical cord arterial blood in this study, in the human fetus, repeated decelerations are believed to cause fetal acidemia due to hypoxia and lactate accumulation due to anaerobic metabolism. Furthermore, increased damage to the fetus from repeated umbilical cord occlusions increases the likelihood of progression from respiratory to metabolic acidemia.
A limitation of our study is that there were only two cases with an umbilical artery blood pH < 7.0, which is the threshold for clinically significant acidemia [22, 23]. Furthermore, only one case met Low’s definition of hypoxia associated with brain injury, with an umbilical artery blood pH < 7.0 and BE <-12 mmol/L at delivery [24, 25]. This was potentially due to the fact that the study population comprised only full-term singleton vaginal deliveries without fetal abnormalities. Since 75% of neonates with neurological adverse events have been reported to have umbilical artery blood pH < 7.1 [26], we selected pH < 7.2 as the cutoff value to detect early-stage fetal asphyxia. Additional research is required to study multiple cases of emergency cesarean sections, including those with an umbilical artery blood pH < 7.1.
In conclusion, the predictive capacity of the iPREFACE score for fetal acidemia was highest in the group with a CTG scoring time of 30 min immediately before delivery, suggesting that this scoring time may be applicable in clinical practice.