The Fetal Reserve Index Significantly Outperforms ACOG Category System in Predicting Cord Blood Base Excess and pH: A Methodological Failure of the Category System

Abstract

Objective:Electronic fetal monitoring (EFM) has been used extensively for almost 50 years but performs poorly in predicting and preventing adverse neonatal outcome. In recent years, the current “enhanced” classification of patterns (category I-III system [CAT]) were introduced into routine practice without corroborative studies, which has resulted in even EFM experts lamenting its value. Since abnormalities of arterial cord blood parameters correlate reasonably well with risk of fetal injury, here we compare the statistical performance of EFM using the current CAT system with the Fetal Reserve Index (FRI) for predicting derangements in base excess (BE), pH, and pO2in arterial cord blood.Methods:We utilized a research database of labor data, including umbilical cord blood measurements to assess patients by both worst CAT and last FRI classifications. We compared these approaches for their ability to predict BE, pH, and pO2in cord blood.Results:The FRI showed a clear correlation with cord blood BE and pH with BE being more highly correlated than pH. The CAT was much less predictive than FRI ( P < .05). The CAT II cases had FRI scores across the spectrum of severity of FRI designations and as such provide little clinical discrimination. The PO2was not discriminatory, in part, because of neonatal interventions.Conclusions:The Fetal Reserve Index (FRI) provides superior performance over CAT classification of FHR patterns in predicting the BE and pH in umbilical cord blood. Furthermore, the CAT system fails to satisfy multiple fundamental principles required for successful screening programs. Limitations of CAT are further compounded by assumptions about physiology that are not consistent with clinical observations.

Changing Perspectives of Electronic Fetal Monitoring

“…Category II is too far to the left-giving a high sensitivity but too many false positives labor starts with the occurrence of contractions. In our studies, for control patients (good outcomes), the average length of labor was 11.3 h, while for those patients whose infants developed CP, the average length of labor was 17.7 h. There were several other differences in the average time to initial appearance of EFM abnormalities and the order of deterioration of EFM variables, and there was progressive and relatively predictable loss of reassuring characteristics of EFM parameters [4,[21][22][23][24][25][26][27][28][29][30]. With traditional overall assessment of the FHR tracing, we noted, as internal benchmarks for our studies, both the point when the fetus became "no longer reassuring (which we define as Point A)" and then the point at which it became "injured (which we define as Point B)."…”

“…While almost all the CP cases reached both Points A and B, only 30% of CP cases reached CAT III, and when this did occur, it happened later than Point B in every case, and most often in the 2nd stage of labor, within 20 min of delivery [21]. Such considerations have been discussed more extensively in our previous publications [4,[21][22][23][24][25][26][27][28][29][30]. Points A and B require sophisticated interpretation of EFM and consequently are not included in our proposed methodology that will be described later.…”

“…We have previously written on many of the basic principles of screening and argued that EFM as it is currently being used falls short of satisfying them [4,[21][22][23][24][25][26][27][28][29][30]. In spite of being introduced into widespread practice about 50 years ago, there is still a wide confusion as to precisely what EFM is trying to accomplish, and how well it does it [20].…”

“…We have analyzed patients who entered labor with CAT I tracings and delivered a baby with CP without any apparent cause other than labor [21][22][23][24][25][26]. We have analyzed the degree of abnormality of the individual EFM parameters and the timing and duration of abnormalities during the course of labor and delivery.…”

Electronic Fetal Monitoring–Prevention or Rescue?

“…), it is unreasonable to consider that the metabolic status or the tissue oxygen reserve of each fetus, or the time to decompensation is the same. Indeed, "Category II" pattern may reflect a normal healthy fetus, but it does not exclude either fetal acidosis or fetal neurological injury (10)(11)(12)(13).…”