Hoo, Ah-Fong; (1997) The influence of gestational and postnatal age on the maturation of the Hering-Breuer inflation reflex in infants. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

Introduction Preterm infants are known to be at greater risk of sudden death in infancy. The peak age for sudden infant death (SID) coincides with a time when young infants are undergoing many changes in patterns of breathing, sleep organisation and thermoregulation. As a result of these changes, or delays in normal maturational processes, some infants may be more susceptible to sudden death during early infancy. It has been shown that the activity of the vagally mediated Hering-Breuer inflation reflex (HBIR), which is thought to play an important part in controlling the rate and depth of breathing in newborns, remains unchanged in fullterm infants from birth to 6 weeks of life (Rabbette et al, 1991a) but diminishes at some time between 6 weeks and 1 year of age (Rabbette et al, 1994). However, there remains uncertainty as to exactly when reflex activity begins to decline in fullterm infants and whether the maturational pattern differs in preterm infants. Aims The hypotheses examined in this research were that the HBIR activity begins to decline by about 3 months of age, when the infant is undergoing other important physiological changes, and that the pattern of maturational decline in HBIR activity is delayed in preterm infants. The aim was, therefore, to determine the independent effects of gestational and postnatal age on the strength and maturation of HBIR in early infancy in healthy fullterm and preterm infants. Methods Using the end-inspiratory occlusion technique, serial measurements of HBIR activity and respiratory system compliance were made in 25 preterm and 27 fullterm infants at matched postnatal and postconceptional ages (where postconceptional age (PCA) = gestational age (GA) + postnatal age (PNA)) during the first 6 months of life. Results The results from this study suggest that gestational age does not influence HBIR activity at birth. However, the subsequent pattern of maturation is influenced by preterm delivery. At 40 weeks PCA (i.e. term equivalent), HBIR activity and respiratory rate were higher in preterm than fiillterm infants, which may reflect lower, unstable lung volumes and/or increased metabolic rate in the former group. At 4 months PNA, HBIR activity remained significantly higher in preterm infants, but decreased to values found in fullterm infants when measurements were repeated approximately 4 months after the expected rather than actual date of delivery. Discussion and conclusion It has been reported that infants of short gestation who succumb to sudden infant death tend to do so at a later postnatal age when compared with fullterm infants. Several recent physiological studies have suggested that there may be a window of increased vulnerability between 10-20 weeks PNA in fullterm infants. Findings from this study clearly demonstrate that maturation of respiratory patterns and timing differ between preterm and fullterm infants. In addition, these results suggest that the window of increased vulnerability may extend over a more prolonged period in preterm infants, such that interpretation of respiratory measurements from infants must take gestational age into account during the first 6 months of life.

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