The ventilatory response of the premature newborn infant to increased PACO2 is less than that of the term infant, but increases with postnatal and gestational age. The term-infant's response to CO₂ has the same slope as the adult's response (scaled according to body weight) but is shifted to the left, reflecting a relative metabolic acidosis. The infant's response appears to be more variable than the adult's; however, whether this variability has a physiological basis or reflects the limitations of monitoring is unclear. The infant's ventilatory drive depends on PaO2 as well as on PaCO2 and pH. The term-infant's response to hypoxia and hyperoxia is biphasic. That is, when made hypoxic, the infant first hyperventilates, then decreases his ventilation to the point of hypoventilating; when made hyperoxic, the infant's response is the inverse of the response to hypoxia, with hypoventilation occurring first, followed by hyperventilation. These responses are paradoxical in that they tend to increase the stimulus. Such a situation could be dangerous in the case of hypoxia were it not for the hypercarbic response; ie, a steady state is reached in which the decrease in ventilatory drive caused by hypoxia is balanced by the increase in drive elicited by the resultant hypercarbia. It has been reported that the infant's paradoxical response to hypoxia changes to the adult response by 3 weeks of age; however, rather than a reversal of response, this may be a lengthening of time constants and a decrease in magnitude of the hypoxic ventilatory depression. The infant's ventilatory response to changes in PaO2 is quite similar to the adult's response, with the main differences being quantitative rather than qualitative.
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