EFFECT OF MEDULLARY LESIONS, VAGOTOMY AND CAROTID-SINUS DENERVATION ON FETAL BREATHING

Chronically prepared fetal sheep were subjected to bilateral surface l esions of the Area ''S'' on the ventrolateral medulla and/or to periph eral chemoreceptor denervation by section of the vagus, sinus or both nerves. Sino-aortic denervation or Area ''S'' lesions reduced the inci dence of fetal breathing (FB) for several days. Area ''S'' lesions als o disrupted the pattern of FB; diaphragmatic EMG activity initially wa s mostly tonic and then of very high frequency, up to 7 Hz. Incidence and pattern of FB generally recovered by 7 days, but mean Ti was reduc ed in Area ''S'' lesioned fetuses (0.14+/-0.01 sec) compared to nonles ioned fetuses (0.19+/-0.01 sec) (P<0.0001). Respiratory sensitivty to CO2 Was variable but not different between control, denervated, and Ar ea ''S'' lesioned groups. Eight of eight fetuses with Area ''S'' lesio ns were unable to initiate breathing at birth, but three sham operated fetuses were born normally. These data suggest that the classical per ipheral and central chemoreceptors have a negligible influence on the control of FB, and that breathing activity in the fetus is mediated by a different mechanism than during postnatal life.