Metabolic and ventilatory rates in newborn kittens during acute hypoxia

Newborns respond to acute hypoxia with a brief increase in ventilation (VE) followed by a decline toward the normoxic value. We asked to what extent this biphasic pattern is determined by changes in metabolic rate. In newborn awake kittens within the first week after birth we measured VE by the barometric method, oxygen consumption (VO2) and CO2 production (VCO2) by manometric techniques during air breathing and after 10 min of 10% O2 breathing. In hypoxia, VO2 and VCO2 dropped (about 45% and 35%, respectively) with a slight decrease in body temperature. VE was as in normoxia and, from the changes in breathing pattern, a drop in alveolar ventilation was calculated. Since the drop in VCO2 exceeded that in calculated alveolar ventilation (VA), a decrease in PACO2 (estimated at about 5 mm Hg) was expected. This decrement was confirmed by measurements of transcutaneous PCO2, which dropped an average of 3.3 mm Hg in the hypoxic kittens, even when VE was below the normoxic value. Hence, despite the decline in VE and VA during the biphasic response, the newborn continued to hyperventilate with respect to CO2 production, lowering PCO2. The strategy of dropping VO2 instead of maintaining the energetic requirements by increasing VE was not determined by mechanical constraints, since all kittens hyperventilated when exposed to CO2, either in air or hypoxia. Several implications of the tight coupling between ventilation and metabolism are discussed, including a possible role in the genesis of periodic breathing in hypoxia.