Rabbette, Patricia Sara; (1993) The Hering-Breuer reflex in infancy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

One of the most important adaptations to extrauterine life that must occur at birth is the establishment of a regular pattern of breathing. Although most infants achieve this without difficulty, abnormalities of respiratory control account for a number of clinical problems in the developing child. The work described in this thesis is concerned with the development of one aspect of respiratory control in healthy infants, with emphasis focused on the influence of postnatal age on the Hering-Breuer Lung Inflation Reflex (HBR). The HBR is a vagally-mediated reflex known to contribute to the establishment and control of regular breathing in the immediate newborn period. However, it is traditionally held that this reflex ceases to make any contribution to the control of tidal breathing beyond the first week of life. There is little evidence to support this contention, and the effects of postnatal development on this reflex are unclear. The hypothesis examined in this research was that the HBR does not diminish in strength during the first two months of life. The aims were to determine, firstly , whether a vagal contribution to the control of breathing, as evidenced by the presence of the HBR, persisted during this time; and secondly, whether there was any change in reflex strength with increasing postnatal age. In addition, the effects of sedation on reflex strength were assessed prior to serial measurements of the HBR in sedated infants during the first year of life. The potential influence of sleep-state, lung volume, and tidal breathing parameters was also assessed, as potential factors influencing the variability of reflex response. The methodological approach to, and findings from studies undertaken to achieve these objectives are discussed. Particular attention has been given to study design to avoid the potential problems encountered by previous workers in this field, with respect to sample size and study population, measurement conditions and analytical approach. The airway occlusion technique was used to assess changes in respiratory timing during stimulation and/or abolition of stretch receptor activity using brief end-inspiratory or end-expiratory airway occlusions. The strength of the HBR was assessed from the relative increase in respiratory time during occlusion, compared to values during spontaneous breathing. Findings from paired measurements in and 4-8 weeks of life indicate that, the HBR persists beyond the neonatal the first 2 months of life. Direct comparison of results from 33 unsedated infants and 33 infants sedated with Triclofos sodium, indicate that this sedation does not influence the strength of the HBR. Paired measurements performed at 6 weeks and 1 year in 25 infants reveal that a physiologically significant HBR persists in infants throughout the first year of life, but diminishes in strength during this time. The measured reflex response was found to be volume-dependent within individual infants. However, differences in lung volume between infants did not wholly account for the inter and intra-subject variability observed. The interpretation of maturational changes is complicated by associated changes in respiratory pattern and lung mechanics during the first year of life. However, a number of respiratory parameters assessed could not explain individual maturational changes. Despite a reduction in respiratory frequency and an increase in total respiratory system compliance during the first year, there was no evidence that these changes were causally associated with changes in reflex activity with age. The findings of the described in this thesis thus provide strong evidence for independent effect of maturation on HBR activity in healthy infants. The physiological and clinical significance of this reflex as a mechanism of respiratory control in infants is discussed, and implications for future research outlined.

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