Respiratory sensitivity before and after birth.

A description is first given of respiratory activity in the fetus and its control. Evidence suggests that when the fetus makes respiratory movements, it is in a state comparable to REM sleep in the newborn and adult and that in the alternating periods of apnoea, it is in quiet sleep. It does not appear that the respiratory movements are normally regulated by chemical or reflex, e.g. Hering Breuer, inputs though they are enhanced by CO2 and depressed by hypoxia. In the apnoeic periods, breathing movements are virtually impossible to elicit by chemical or reflex means. Evidence from examination of peripheral inputs indicates that: the carotid body chemoreceptors are inhibited at receptor level, stimulation of the aortic chemoreceptors affects the circulation only and although pulmonary stretch receptors are active and are excited by inflation of the fetal lung, such inflation does not affect discharge in medullary respiratory units or phrenic nerve. Since there is no real evidence of immaturity of the respiratory system in late gestation and since chemical and most reflex inputs appear to provide an adequate stimulus, it is most probable that the periods of apnoea are caused by an inhibitory process, possibly of supra-pontine origin which acts close to medullary respiratory units and effectively inhibits the operation of the automatic component. This inhibitory process may operate periodically; or continuously and be periodically overridden in REM sleep. After birth, breathing is normally continuous and sensitive to lung inflation, CO2 and after a variable delay, to hypoxia. This may be due to the lifting of the inhibitory process allowing activation of the automatic component. However, there is evidence that even in normal, full term infants, full maturation of the automatic component is not complete until about three months of age and in the meanwhile, breathing tends to be imperfectly regulated and subject to damped oscillations when disturbed.