1701 PROGRESSIVE ONSET OF SPONTANEOUS AND INDUCED FETAL BREATHING

Abstract

Intratracheal pressure (Pmax) and respiratory drive (dP/dt) from the occluded, liquid filled trachea of term fetal lambs in utero were measured for each breath during the onset of fetal breathing. Progressive breathing responses at the onset of fetal breathing were observed (1) during spontaneous breathing, (2) during sciatic nerve stimulation, (3) during induced hypercapnia by Fetal CO2 Tests (Moss and Scarpelli, J. Appl. Physiol. 47:527, 1979) and (4) following naloxone administration. These responses were characterized by linear increase of both Pmax and dP/dt for 6.8 ± 0.3 breaths (x ± SEM) over 13.9 ± 1.7 seconds, following which these parameters became stable. The rate of rise of Pmax and dP/dt versus both breath number and absolute time 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 or somatosensory induced “arousal” and by chemical stimulation, as well as on release from natural (endorphin) inhibition. (Supported by NIH HL 00688 (RCDA) and HL 23995).