Adaptive shut-down of EEG activity predicts critical acidemia in the near-term ovine fetus

Abstract

In fetal sheep, the electrocorticogram (ECOG) recorded directly from the cortex during repetitive heart rate (FHR) decelerations induced by umbilical cord occlusions (UCO) predictably correlates with worsening hypoxic-acidemia. In human fetal monitoring during labor, the equivalent electroencephalogram (EEG) can be recorded noninvasively from the scalp. We tested the hypothesis that combined fetal EEG - FHR monitoring allows for early detection of worsening hypoxic-acidemia similar to that shown for ECOG-FHR monitoring. Near-term fetal sheep (n = 9) were chronically instrumented with arterial and venous catheters, ECG, ECOG, and EEG electrodes and umbilical cord occluder, followed by 4 days of recovery. Repetitive UCOs of 1 min duration and increasing strength (with regard to the degree of reduction in umbilical blood flow) were induced each 2.5 min until pH dropped to <7.00. Repetitive UCOs led to marked acidosis (arterial pH 7.35 ± 0.01 to 7.00 ± 0.03). At pH of 7.22 ± 0.03 (range 7.32-7.07), and 45 ± 9 min (range 1 h 33 min-20 min) prior to attaining pH < 7.00, both ECOG and EEG amplitudes began to decrease ~fourfold during each FHR deceleration in a synchronized manner. Confirming our hypothesis, these findings support fetal EEG as a useful adjunct to FHR monitoring during human labor for early detection of incipient fetal acidemia.

Keywords: Acidosis; ECOG; EEG; FHR; Fetus; asphyxia; hypoxia; monitoring.

© 2015 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.

Figures

Figure 1

Acid-base status, arterial oxygen saturation, and glucose during the experiment’s baseline and umbilical cord occlusions (UCOs). Blood samples taken between the mild, moderate, and severe UCO series as well as at pH 2Sat; $, glucose; #, Lactate – all vs. baseline.

Figure 2

Example of an individual ECOG / EEG response to repetitive UCO. TOP: 60 min view of the adaptive brain shut-down pattern visible in ECOG and EEG in response to changes in arterial blood pressure (ABP) and fetal heart rate (FHR). BOTTOM: 10 min zoomed-in window of this pattern.

Figure 3

Cross-correlation function (CCF) analysis of the ECOG/EEG response to FHR decelerations. The baseline, mild, and moderate umbilical cord occlusion (UCO) series (denoted “other”) were compared with the ‘Severe’ UCO series when the adaptive brain shut-down was observed in all fetuses. The four leftmost plots represent the cross-correlation function between the smoothed ECOG/EEG amplitude and FHR with a 10 sec moving average with a delay ranging from −100 sec to 100 sec. For example, a time delay of 0 represents the correlation of the ECOG and FHR values at the same time. When a positive delay of 100 sec is used, the ECOG at time t is compared with the FHR at time t + 100. The two leftmost plots are the “other” groups, and the middle plot is the severe group when the adaptive brain shut-down was mostly observed in all fetuses. The upper plots are for the EEG/FHR correlation and the lower plots are for the ECOG/FHR. One can see that the amplitude of correlations is much higher in the severe group than in all other groups (P < 0.01 and P < 0.001, for EEG and ECOG, respectively), and that the amplitude is much larger in the EEG/FHR correlation than in the ECOG/FHR. The difference in the correlation amplitude is presented as a box plot in the two rightmost plots, with the red bar representing the median of the distribution and the box size represent the 25–75th percentiles. + signifies the outliers of the distributions (beyond the 90th percentile values). High amplitude of the correlation between EEG and FHR implies a strong synchronization between the two signals with an average delay of 30 sec.