Effects of Hypoxia and Acidity on the Initiation of Breathing in the Fetal Lamb at Term

Abstract

DESPITE their immense importance, the factors initiating respiration at birth are incompletely understood. The classic theory has been that respiration is initiated as a result of sensory stimulation of the baby as it passes through the birth canal and encounters the cooler, external environment. If this fails to elicit respiration, then asphyxic stimulation, acting through peripheral chemoreceptors, may be activated as an emergency mechanism to restore the depressed infant by induction of gasps. Since all infants undergo a degree of asphyxiation at birth, it was believed that a study of the effects of fall in fetal Po₂ and pH and rise in fetal Pco₂ on the initiation of respiration, under conditions where the fetus was not subjected to other stimulation, might indicate whether asphyxic stimulation is an emergency mechanism or actually a physiologic part of the initiation of breathing. Gas mixtures were administered to term ewes,