Respiratory plasticity following intermittent hypoxia: developmental interactions.

Intermittent hypoxia (IH) is the most frequent form of hypoxia occurring in the developing mammal. On one hand, the maturational process of neural, mechanical, pulmonary, and sleep state-dependent factors will favor the occurrence of IH during early postnatal life. On the other hand, it has also become clear that hypoxia, even when short lasting, can modify subsequent respiratory responses to hypoxia and induce a variety of genes whose consequences will persist for much longer periods than the duration of the hypoxic stimulus itself, i.e., functional and adaptive plasticities. The dynamic interactions between the overall duration and recurring frequency of IH, the severity of IH, and the level of neural maturity at the time of IH will modify the ventilatory, metabolic, and cardiovascular responses to hypoxia. We propose that the earlier IH will occur in the developmental course the more likely that the physiological responses to an ulterior hypoxic challenge will be altered even into adulthood. At this point in time, a critical examination of the field would suggest that the short-term alterations of the hypoxic ventilatory response (HVR) of the developing mammal to IH are qualitatively similar to those of the adult and display a biphasic pattern, namely, initial enhancement of the HVR followed by a reduction in HVR. However, the short- and long-term effects of IH on the modulation of neurotransmitter release, receptor binding and expression, intracellular signaling cascades, transcriptional regulation, and gene expression as a function of animal maturity are almost completely unknown. Further delineation of such complex responses to IH may permit the formulation of interventional strategies aiming at reducing the overall vulnerability of the young infant and child to apnea and sudden death.