OUTPUT OF THE FETAL RESPIRATORY CENTRE IN-UTERO

Abstract

Dawes, using indirect methods described the presence of fetal respiratory movements up to 40% of the time with a frequency of 1 to 4 Hz. We undertook to monitor fetal respiratory centre output in-utero more directly by simultaneous recording of tracheal pressure & phrenic & diaphragmatic electrical activity in awake sheep. One week was allowed for recovery following surgery. Four fetuses were studied from 132 days to 145 days gestation and a total of 29.4 hours of recording analyzed in detail. Phrenic nerve activity was found in all fetuses and was coupled with diaphragmatic activity and tracheal pressure changes. In contrast to newborn and adult animals, fetal phrenic nerve activity was not phasic but consisted of episodic bursts interspersed by quiescent periods lasting up to 30 minutes. There were two types of bursts: a) a short brief burst with an average duration of 267 msec, b) a more prolonged burst with an average duration of 1.9 sec. Type A burst occurred 1313 times and type B 462 times during 29.4 hours; thus fetal respiration occurred during a total of 20.3 minutes or approximately 1% of the time. Continuous respiration was seen on 2 occasions just prior to fetal death. These studies indicate that normally the fetal respiratory centre during late fetal life is only minimally active in-utero. Continuous intrauterine respiration is probably associated with fetal asphyxia. Supported by MRC (Canada) & Children's Hospital Research Fdn.