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Apnoea remains an important problem in the prematurely born infant. Apnoeas can be divided into central, obstructed and mixed apnoea. Apnoeas frequently occur in association with periodic breathing, as well as isolated events. It remains unclear whether there is a common mechanism for all these types of disturbance in normal respiratory activity or whether there are additional factors which determine the type of apnoea. HYPOTHESES The hypotheses explored in this thesis were that isolated central apnoea was due to reduced chemoreceptor sensitivity, in mixed apnoea there was additional activation of the costophrenic inhibitory reflex and apnoea associated with periodic respiration was due to instability of respiratory control, with periods of over ventilation leading to apnoea. METHODS Polygraphic techniques were used to identify the frequency, type and length of apnoeic periods in infants born at/or less than 33 weeks gestation who were otherwise healthy sequential studies were undertaken to determine the pattern of apnoea and how it changed with time. Chemoreceptor sensitivity was assessed by measuring the ventilatory response to an added respiratory dead space of twice the anatomical dead space. The rate of response to the added dead space was assessed to determine if it related to the apnoea types and in particular was reduced in periodic respiration. The respiratory response to an imposed airway obstruction was used to determine the strength of the Hering Breuer and costophrenic inhibitory reflex, as respiratory efforts against an obstruction would induce chest wall distortion. RESULTS Twenty-eight infants with a mean gestation of 29+4 weeks and birth weight was of 1.39kg, were studied on 71 occasions. The prevalence of apnoea in this preterm population was nearly 100%, with the more immature infants at greater risk. Ninety percent of apnoeas (>5secs) were central, 9% mixed and 1 % obstructive; however, 50% of apnoeas over 14s had an obstructive component. Thirty-three percent of apnoeas were associated with a desaturation, the longer the apnoea the greater degree of desaturation. Apnoea associated with periodic respiration accounted for 72.3% of all apnoeas and periodic respiration increased with postmentrual age. In ninety-three percent of the studies there was at least one episode of periodic breathing. This relationship was not related to postmentrual age. A significant relationship was found between the rate of periodic central apnoeas and chemoreceptor sensitivity, as examined by the response to added dead space, related to a increased reliance on chemoreceptors in this group of infants The rate of response to the added dead space however shortened with increasing post menstrual age and that there was a significant inverse relationship between this and the rate of central periodic apnoeas. No significant relationship was found between the response to external obstruction and the rate of apnoea. Although an active Hering Breuer reflex was found in all infants, this was related to both to the gestational and postmenstrual age of the infants. CONCLUSIONS Apnoeas associated with periodic respiration are the commonest form of apnoea. As demonstrated in this thesis these occur most frequently in infants with the most rapidly responding chemoreceptors, these data support the hypothesis that this pattern of respiration is due to an underdamping of the chemoreceptor response. No evidence was found to support the hypothesis that mixed apnoea was associated with activation of the costophrenic inhibitory reflex.

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