Prolonged mild hypoxia alters fetal sheep electrocorticogram activity.

Objective: To assess the effects of prolonged mild hypoxemia on fetal brain electrocorticogram (ECoG) in late gestation.

Study Design: Fetal and maternal catheters were placed under general anesthesia and animals allocated at random to receive intratracheal maternal administration of either nitrogen (n = 8) or compressed air (n = 8). Five days after surgery (125 days' gestational age), nitrogen infusion was adjusted to reduce fetal brachial artery PO2 by 25%. The targeted decrease in fetal oxygenation was maintained for 5 days while fetal ECoG activity and fetal and maternal cardiovascular variables were continuously recorded. Data are presented as mean +/- SEM and were analyzed by two-way analysis of variance (ANOVA) or two-sample t test.

Results: Nitrogen infusion decreased fetal Po2 by 26% (20.5 +/- 1.7 versus 14.3 +/- 0.8) without changing fetal PCO2 or pH. Mild fetal hypoxemia was associated with fetal tachycardia and increased fetal blood pressure (P < .05). Fetal ECoG in hypoxic fetuses showed a significant decrease in the time spent in high voltage (HV) (P < .05) and an increase in the time spent in low voltage (LV) and in the number of low voltage events (P < .05). Also, a significant decrease in the proportion of 1-4 Hz and an increase in the proportion of 13-20 Hz frequencies was observed in LV events without a significant change in the frequency profile of HV events (P < .05).

Conclusion: Prolonged mild hypoxemia significantly altered fetal homeostasis as reflected by the sustained tachycardia and increased blood pressure. Fetal ECoG activity was affected significantly in a qualitatively and quantitative manner by mild prolonged hypoxemia.