Fetal breathing: induction in utero and effects of vagotomy and barbiturates.

Breathing movements were induced in the mature fetal lamb in utero by (1) both electrical stimulation and manipulation of the sciatic nerve, (2) stimulation of cutaneous receptors, and (3) KCI injection into the peripheral circulation. The breathing patterns that were produced included the "respiratory reflex", sustained regular breathing lasting up to three hours, and short bursts of irregular breathing. The latter resembled the spontaneous irregular breathing of the fetus in situ as reported by others. It is suggested that the intensity of peripheral somatic stimulation is an important determinant of the resultant breathing activity, ie, low intensity produces irregular breathing and high intensity, sustained regular breathing. The breathing responses were not affected by bilateral cervical vagotomy, but were inhibited completely by maternal administration of pentobarbital and of quantities of thiopental and methohexital equal to or less than the doses used clinically.