Hyperbaric oxygenation increases arousal and breathing movements in fetal lambs

Oxygenation produced by distending the lungs with 100% O₂ increases the occurrence of arousal and fetal breathing movements (FBM), particularly during non-rapid-eye-movement (NREM) sleep, in fetal sheep of ≥135 days of gestation. We studied the breathing and behavioral responses to a rise in arterial PO₂ (Pa(O₂)) without lung distension in fetuses between 128 and 132 days of gestation. Twelve fetuses were chronically instrumented to record FBM, behavioral state blood pressure, arterial blood gas tensions, and pH. Fetal Pa(O₂) was raised by having the ewe breathe 100% O₂ at 3 atmosphere absolute pressure spontaneously (group 1, n = 5, 129 ± 1 days of gestation) or with mechanical ventilation to control fetal arterial PCO₂ (group 2, n = 7, 131 ± 1 days of gestation). Hyperbaric oxygenation raised fetal Pa(O₂) by 20 Torr in both groups. During hyperbaric oxygenation, the occurrence of arousal increased severalfold in both groups. The occurrence of FBM increased during arousal in both groups, during rapid-eye-movement sleep in group 1, and during NREM sleep in group 2. The timing of diaphragmatic activity during arousal and the variability of diaphragmatic activity during NREM sleep were different than those in rapid-eye-movement sleep. We conclude that oxygenation without lung distension increases the occurrence of arousal and of FBM, principally during arousal and NREM sleep, in fetuses of ≤135 days of gestation.