Abstract
The mechanisms which ensure fetal apnea and the initiation of respiration at birth are poorly understood. Direct study of the fetal respiratory center has not been previously undertaken. We have mapped the inspiratory center of the exteriorized fetal sheep near term using stereotaxic placement of a stimulating electrode in the region of the medulla. Ventilatory efforts were monitored by connecting the trachea to a liquid plethysmograph. The results were compared to a similar mapping of the respiratory center of the newborn lamb. The fetal respiratory center was quite diffuse, extending from the obex to the lower pons and 2.5 mm on either side of the midline. In contrast, the respiratory center of the newborn lamb was more localized, extending only 6 mm rostral to the obex. In the unanesthetized fetus the threshold current required to initiate respiration was 0.15 ± .06 ma. Stimulation of a similar area in the unanesthetized newborn lamb caused apnea at 0.33 ma. Pentobarbital increased the threshold current in the fetus to as high as 1.96 ± .40 ma. In contrast pentobarbital decreased the apneic threshold in the newborn lamb to .21 ± .03 ma. It is concluded that the respiratory center is not actively inhibited during the latter period of gestation and therefore cannot account for respiratory inactivity in utero. The nature of the respiratory response to electrical stimulation of the brain stem and the influence of pentobarbital appears to depend on the presence or absence of rhythmic respiration.
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Chernick, V., Sklenovsky, A., Havlicek, V. et al. Initiation of respiration by direct electrical stimulation of the fetal brain stem. Pediatr Res 5, 428 (1971). https://doi.org/10.1203/00006450-197108000-00237
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DOI: https://doi.org/10.1203/00006450-197108000-00237