Role of chemoreceptors in effects of chronic hypoxia

Effects of chronic hypoxia on chemoreceptors and chemoreflexes in the fetus and neonate are reviewed. The effects are discussed in relation to those in the adult, in which there is an apparent paradox between the increase in chemoreflex gain during acclimatisation to the hypoxia of altitude vs the 'blunting' reported during chronic hypoxia. The possible location and importance of "chemoreceptors" in the brainstem is discussed. In the neonate, chronic hypoxia blunts the ventilatory response to acute hypoxia, an effect largely due to blunting of chemoreceptor sensitivity. Whether this is mediated via a reduction or delay in the normal process of postnatal resetting of hypoxia sensitivity is not known. In addition, there is evidence that the effects on neonatal chemoreceptors are reversible on return to normoxia. Such differences from the adult suggest that special processes, affected by chronic hypoxia, operate in the neonate. In the fetus there is no direct information on the effect of chronic hypoxia on chemoreceptor responses, but it appears that chemoreflexes are augmented, as may occur in the adult during acclimatisation to hypoxia. Recent evidence suggests that arterial chemoreceptors play an important role in fetal cardiovascular and endocrine responses to prolonged hypoxaemia. This will be a productive avenue for future research.