Characterization and developmental aspects of anoxia-induced gasping in the rat

With increasing postnatal age, mammals display diminished tolerances for prolonged exposures to severe oxygen deprivation. Similarly, duration and efficiency of gasping, a unique mechanism for enhancing survival after anoxia-induced apnea, are also affected by postnatal age. We hypothesized that maturational patterns of anoxia-induced gasping may encompass more than a single monophasic phenomenon. Each of the putative phases of the gasping response may underlie unique characteristics which could be of relevance to survival capability. To study these issues, adult rats and rat pups at 2-3, 5, 10, 15, and 25 days of age underwent anoxic exposures with 100% N2 in a barometric chamber. In pups aged < 25 days but not thereafter, following an age-dependent period of central apnea, an initial gasping phase characterized by vigorous and frequent periodic bursts of a large inspiratory effort preceded and followed by expiration excursions emerged (phase I). This phase was followed by a period of relative respiratory silence of variable duration with occasional, interspersed phase I-like gasps (phase II). Finally, a third phase easily recognized by the onset of frequent inspiratory-only gasping efforts developed (phase III). The amplitude of phase III inspiratory gasps progressively diminished until their complete cessation. Although overlap between gasping phases was present, a marked age dependency in both duration and gasping frequency within each phase occurred. We conclude that anoxia-induced gasping responses in rat pups < 25 days old are triphasic in nature, exhibit defined phase-locked periodicities and respiratory effort patterns, and undergo significant maturation.