The effects of lung distension, oxygenation, and gestational age on fetal behavior and breathing movements in sheep

Abstract

Lung distension with 100% O2 at a continuous positive airway pressure of 30 cm H2O may induce continuous fetal breathing movements (FBM) in sheep. The objectives of this study were 1) to investigate the relative roles of lung distension and oxygenation and 2) to test the hypothesis that FBM can be induced during labor, when normally they are greatly reduced or absent. We studied 13 chronically instrumented, unanesthetized fetal sheep between 128 and 144 d of gestation (term = 147 ± 2 d). Each fetus was instrumented to record sleep states, diaphragm electromyogram, blood pressure, arterial pH, and blood gas tensions. The fetal lungs were distended via an in situ endotracheal tube with four different concentrations of O2 (0, 21, 50 and 100%) at a continuous positive airway pressure of 10, 20 and 30 cm H2O in a randomized order. No change in any recorded physiologic variable was observed at 129 ± 1 or 132 ± 1 d of gestation. At 135 ± 1 and 138 ± 1 d, in response to a continuous positive airway pressure of 30 cm H2O and 100% O2, pH decreased (p = 0.0004 and 0.005, respectively) and arterial O2 tension increased (p = 0.004 and 0.02, respectively). However, increases in 1) breathing time, 2) breathing time/low-voltage electrocortical activity ratio, 3) duration of arousal, and 4) length of single breathing epochs were observed only at 138 ± 1 d. Lung distension with N2 resulted in a decrease in FBM. Six fetuses were studied during labor. Significant increase in total breathing time, stimulation of breathing during quiet sleep, and increased duration of arousal were observed with an increase in fetal arterial O2 tension. The onset of breathing always coincided with the onset of arousal. We conclude that fetal maturity, an increase in fetal arterial O2 tension, and a change in state from quiet sleep toward arousal are necessary conditions for the induction of FBM. © 1991 International Pediatric Research Foundation, Inc.