Development of Chemoreflexes Affecting Breathing: Influence of Chronic Hypoxia

Abstract

The effects of acute hypoxia on fetal breathing (see Dawes 1984) and neonatal breathing (see Haddad and Mellins 1984; Rigatto 1984) are well established. This information comes from the effects of exposure of the fetus or neonate to an episode of hypoxia lasting from a few minutes to about 1 h. It is of relevance to the understanding of the processes operating during a brief hypoxic or asphyxic epidode, e.g. during the reductions in uteroplacental blood flow occurring with the uterine contractions of labour or during a period of apnoea in the neonate. This knowledge may, however, not shed much light on the processes operating during chronic or repeated acute hypoxia, e.g. in certain types of fetal growth retardation, in right to left shunting (e.g. when the ductus arteriosus remains patent) or in recurrent apnoea post-natally. The processes operating during acute and chronic hypoxia may be utterly different. The question of fetal adaptation during chronic hypoxia and its clinical relevance have been discussed recently (see Dawes et al. 1990), so in this review we will concentrate on the effects of chronic hypoxia on chemoreflexes acting on breathing. This is not to underestimate the importance of chemoreflexes acting on the circulation under such circumstances, but our emphasis arises from the relative lack of knowledge of that area (but see Bocking et al. 1986, 1988 a, b).