VENTILATORY AND METABOLIC RESPONSES TO ACUTE HYPEROXIA IN NEWBORNS

Hyperoxia has previously been found to increase metabolic rate (oxygen consumption [VO2] and CO2 production [VCO2]) in newborn mammals. We asked whether the same occurs in the newborn infant. Breathing pattern was measured in 25 full-term infants, 1 to 2 days of age, from the spirometric record obtained with a pneumotachograph attached to a face mask. Concentrations of O2 and CO2 were continuously measured at the mouth; VO2 and VCO2 were computed as the product of VE and the difference between inspired and expired concentration of the respective gases, 5 min of air (Fl(O2) = 0.21) and 5 min of O2 (Fl(O2) = 1). A blas flow through the mask and pneumotachograph delivered the inspired gas and eliminated the effects of the instrumental dead space. In neither case did measurements st 1 min significantly differ from those taken at 5 min. In hyperoxia VE increased in 22 of the 25 infants, in average +18% (p < 0.001, paired two-talled t test). Because of a rise in tidal volume (+35%, p < 0.001) and a decrease in breathing rate (-11%, p < 0.005) alveolar ventilation (VA) increased by about 58% (p < 0.001). VO2 and VCO2 increased by 25% and 17%, respectively (p < 0.001). The rise in VO2 was too large to be explained by the greater respiratory work of the hyperventilation, whereas that of VCO2 was not large enough to fully explain the increase in VA. we conclude that in newborn humans, as in other newborn species, the normoxic metabolic rate seems to be limited by the availability of O2. In hyperoxia, the rise in metabolism contributes to, but does not completely explain, the increase in ventilation.