SUPPRESSION OF THE FETAL RESPIRATORY RESPONSE IN SHEEP: AN EFFECT OF PHYSIOLOGIC HYPOXIA?

Abstract

The respiratory center of the in situ fetus is responsive only to extremes of acidemia and hypercarbia. Recent observation by Frady (Peds:50, 219, 1972) suggest that apnea in the immature newborn is related to hypoxia.

To test the thesis that physiologic fetal hypoxia might suppress the medullary responses we placed chronic catheters in the aorta and ventral lateral medulla of 10 in situ 130 day lamb fetuses.

Respiratory exscursions were monitored via tracheal cannulation. Infusion of buffered artificial CSF between a pH of 7.20 and 7.35 resulted in no change in spontaneous respiratory activity in the normoxic fetus (P_aO₂—25 TORR). When the ewe was placed in a hyperbacric chamber and the pressure and F_IO₂ adjusted to maintain fetal P_aO₂—100 TORR, infusion of artifical CSF in the same pH range resulted in a fetal respiratory rate between 20 and 80 and a response curve similar to that of the newborn lamb. Thus it appears that physiologic hypoxia may serve to depress the medullary response to hydrogen ions in the immature fetal lamb.

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