Intrapartum fetal heart rate monitoring: evaluation of a standardized system of interpretation for prediction of metabolic acidosis at delivery and neonatal neurological morbidity

Objective: To assess the ability of the intrapartum fetal heart rate interpretation system developed in 2008 by the National Institute of Child Health and Human Development (NICHD) to predict fetal metabolic acidosis at delivery and neonatal neurological morbidity. Methods: We analyzed the intrapartum fetal heart rate tracings of 314 singleton fetuses at >= 37 weeks using the NICHD three-tier system of interpretation: Category I (normal), Category II (indeterminate) and Category III (abnormal). Category II was further divided into Category IIA, with moderate fetal heart rate variability or accelerations, and Category IIB, with minimal/absent fetal heart rate variability and no accelerations. The presence and duration of the different patterns were compared with several clinical neonatal outcomes and with umbilical artery acid-base balance at birth. Results: The mean values of pH and base excess decreased proportionally as tracings worsened (p<0.001). The duration of at least 30 min for Category III tracings was highly predictive of a pH <7.00 and a base excess <= -12 mmol/L. The same was true for the duration of Category IIB tracings that lasted for at least 50 min. Conclusions: Our study demonstrates that the interpretation of fetal heart rate tracings based on a strictly standardized system is closely associated with umbilical artery acid-base status at delivery.