Effects of hyperoxia on ventilatory and metabolic rates of newborn mice.

Newborn mammals of medium or large sized species have ventilatory rates, expressed per kg body weight, larger than adults of corresponding size, while newborns of the smallest species do not. We hypothesized that the oxygen consumption of the smallest newborns is limited by the supply of oxygen and reasoned that if this were the case, an increase in Po2 of the inspired air should decrease their ventilation/oxygen consumption (VE/Vo2) ratio. We exposed 1-2 days old newborn mice for 5 min to 21% O2 in N2 or 100% O2, then measured their breathing pattern, by flow plethysmography, and Vo2 with an isovolume closed system. During hyperoxia the VE/Vo2 ratio dropped in average 36%, since VE decreased in 14 out of 18 animals and Vo2 increased in all the animals tested. The drop in VE was due to a prolongation of the expiratory time, with no changes in inspiratory time or tidal volume. During expiration, interruptions of the expiratory flow and tendency to maintain the lung inflated, a characteristic of neonatal respiration, were more pronounced with 100% O2 than 21% O2 breathing. We conclude that the resting metabolic rate of newborn mice is limited by the supply of oxygen; when Po2 is raised, metabolism increases and ventilatory rate decreases in favor of a breathing pattern aimed to preserve lung volume elevated.