Respiratory responses to single and episodic hypoxia during development: mechanisms of adaptation.

The respiratory responses of the developmental subject to single and repeated episodes of hypoxia are distinct. During a single exposure, the fetus responds with an arrest of breathing activity, and the neonate, with excitation followed by depression (the biphasic response). Mechanisms under active consideration include chemosensory resetting, hypometabolism, prevalence of inhibitory neurotransmitter/modulator influence, and supramedullary regulation of control functions. When exposed to recurrent episodic hypoxia, neonates respond with relative hypoventilation, i.e. tolerance to a subsequent hypoxic stimulus. Whereas the investigation of processes responsible for this tolerance is at its infancy, studies using chronic hypoxia appear to be a useful guide. So far, altered interstitial neuromodulator levels and central markers of programmed neuronal death are harbingers of future research in this field. The clarification of the mechanisms involved in response to recurrent episodic hypoxia during development will be of fundamental value and may be useful for the eventual treatment and/or prevention of harmful central respiratory-related processes.