Breathing activity in fetal sheep during mechanical ventilation of the lungs in utero

Effects of mechanical ventilation of the lungs of fetal lambs in utero are described. Feasibility studies were performed acutely on four fetuses at 140-142 days gestational age. Five fetuses were prepared for chronic experiments at 122-128 days gestation. Mechanical ventilation was used to produce changes in fetal blood gases for periods of up to 24 h in acute preparations or for repeated periods of 3-9 h on consecutive days in chronic preparations. This technique permits the study of fetal lambs in utero under conditions which normally occur after birth without producing maternal changes or using anesthesia and whilst the umbilical circulation remains open. Four situations were examined using ventilation in these chronic preparations: (1) expansion of the lungs whilst maintaining fetal normoxia and normocapnia; (2) fetal normoxia and hypocapnia; (3) hyperoxia and normocapnia; (4) hyperoxia and hypocapnia. None of these situations produced continuous fetal breathing, either during ventilation or after switching off the ventilator. We conclude that they do not provide appropriate stimuli to mimic those which normally produce continuous breathing after birth.