Altered carotid body function by intermittent hypoxia in neonates and adults: relevance to recurrent apneas

Chronic intermittent hypoxia (CIH) is associated with recurrent apneas in adults and premature infants. It has been proposed that reflexes arising from the carotid bodies contribute to the autonomic abnormalities associated with CIH. The purpose of this review is to summarize recent studies on the effects of CIH on adult and neonatal carotid bodies. CIH exerts two major effects on the adult carotid body that includes sensitization of the hypoxic sensory response and induction of sensory long-term facilitation (LTF). In neonates CIH leads to sensitization of the hypoxic response but does not induce sensory LTF. The effects of CIH on carotid bodies develop over time and could be reversed in adults but not in neonates. CIH-evoked changes in the carotid body involve reactive oxygen species (ROS)-mediated signaling and transcriptional activation by hypoxia-inducible factor-1. Augmented chemoreceptor sensitivity to hypoxia increases the likelihood of unstable breathing perpetuating the effects of CIH, whereas sensory LTF may contribute to increased sympathetic tone and systemic hypertension associated with recurrent apneas.