Effect of asphyxia on respiratory activity in fetal sheep

Hypoxia in fetal sheep depresses respiratory activity. To determine if this effect is counterbalanced by hypercarbia we studied the effects of two levels of asphyxia produced by occlusions of the maternal uterine artery. Moderate asphyxia (PaO₂ 16.8 ± 1.6 (SEM) PaCO₂ 48.9 ± 1.0 torr) produced no changes in the percent time fetal breathing movements occupied each hour which ranged from 25.6 ± 7.0 to 32.4 ± 6.2%. However, a more marked asphyxia (PaO₂ 12.0 ± 0.3, PaO₂ 57.0 ± 1.6) resulted in a decrease in fetal respiratory activity to 8.7 ± 3.7% during the first hour. This depression was sustained over the next 2 h but by the 5th hour breathing had returned to 26.2 ± 7.3%. We concluded that hypercarbia can offset the respiratory inhibition of acute moderate hypoxia, but not that of a more marked lowering of PaO₂ in fetal sheep. Severe asphyxia causes an initial inhibition of respiration which is followed by a return to normal respiratory activity.