Age-related changes in the ventilatory response to inspired CO 2 in neonatal rats

Abstract

The objective of this study was to determine whether there is an age-related ventilatory response to transient increases in inspired CO 2 in unanesthetized rat pups. Using plethysmography, ventilatory responses to 30 sec of 0, 2, 4, 6, and 8% inspired CO 2 were measured in 21 rat pups from two litters. Recordings were made 1, 2, 3, 5, 7, 9 and 12 days after the day of birth (day 0). On day 1 there was a significant dose-related decrease in mean ventilatory frequency in response to each of the inspired CO 2 concentrations. On day 2 there was no significant change in breathing frequency in response to 2 or 4% CO 2 and a significant increase in frequency in response to 6 and 8% CO 2. On days 3, 5, 7, 9 and 12 there was generally a significant increase in frequency in response to each of the inspired CO 2 concentrations. Tidal volume was not significantly affected by the CO 2 stimuli on any of the test days. Minute ventilation exhibited a significant decrease, on day 1, in response to 6 and 8% CO 2. Litter, sex or weight of the rat pups was not correlated with the ventilatory depressions observed on day 1. These results show that in neonatal rats the ventilatory response to inspired CO 2 is age-related and indicates a possible link between upper airway CO 2 chemoreceptors, an inhibition of breathing, and SIDS.