The progressive onset of spontaneous and induced fetal breathing.

Intratracheal pressure (ITPmax) and respiratory drive (dITP/dt) from the occluded, liquid-filled trachea of term fetal lambs in utero were measured for each breath at the onset of fetal breathing (1) during spontaneous breathing, (2) during sciatic nerve stimulation, (3) during induced hypercapnia, and (4) following naloxone administration. These responses were characterized by linear increase of ITPmax and dITP/dt following which these parameters became stable. The rate of rise of ITPmax and dITP/dt was lowest and similar during spontaneous breathing and sciatic stimulation, but increased incrementally with hypercapnia and naloxone. Mechanical factors could not account for these responses in the liquid-filled lung, nor did appreciable chemical changes occur during this period. These results suggest that progressive breathing responses at the onset of fetal breathing may stem from gradual recruitment of central respiratory neurons, and that the rate of rise of such recruitment depends on facilitation by natural arousal, somatosensory stimulation and hypercapnia as well as on release from natural endorphin inhibition.