HYPERBARIC OXYGENATION INCREASES AROUSAL AND BREATHING MOVEMENTS IN FETAL LAMBS

Oxygenation produced by distending the lungs with 100% O-2 increases t he occurrence of arousal and fetal breathing movements (FBM), particul arly during non-rapid-eye-movement (NREM) sleep, in fetal sheep of gre ater than or equal to 135 days of gestation. We studied the breathing and behavioral responses to a rise in arterial Po-2 (Pa-o2) without lu ng 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% O-2 at 3 atmosphere absolute pr essure spontaneously (group I, n = 5, 129 +/- 1 days of gestation) or with mechanical ventilation to control fetal arterial Pco(2) (group 2, n = 7, 131 +/- 1 days of gestation). Hyperbaric oxygenation raised fe tal Pa-o2 by 20 Torr in both groups. During hyperbaric oxygenation, th e occurrence of arousal increased severalfold in both groups. The occu rrence of FBM increased during arousal in both groups, during rapid-ey e-movement sleep in group I, and during NREM sleep in group 2. The tim ing of diaphragmatic activity during arousal and the variability of di aphragmatic activity during NREM sleep were different than those in ra pid-eye-movement sleep. We conclude that oxygenation without lung dist ension increases the occurrence of arousal and of FBM, principally dur ing arousal and NREM sleep, in fetuses of less than or equal to 135 da ys of gestation.