276 INFLUENCE OF LUNG FUNCTION AND REFLEX ACTIVITY ON THE SUCCESS OF PATIENT TRIGGERED VENTILATION

During patient triggered ventilation (PTV) each of the infant's respiratory efforts will trigger a positive pressure inflation, providing that the respiratory effort is of sufficient magnitude to exceed the critical trigger level. It is likely therefore that the nature of the infant's lung function and respiratory reflex activity will influence the success of PTV. The aim of this study was to test that hypothesis. 20 premature infants (median gestational age 29 weeks) in the recovery stage of respiratory distress were studied at a median postnatal age of 2.5 days. Lung function was assessed by measurement of compliance using a single breath technique. Reflex activity was assessed by measurement of the strength of the Hering Breuer reflex, indicated by the degree of prolongation of expiration following end inspiratory occlusion. PTV was considered to have failed if the infant became apnoeic or required an increased level of respiratory support. PTV failed ultimately in 6 infants, compliance of the respiratory system of those 6 infants did not differ significantly from the rest of the cohon, but the Hering Breuer reflex was significantly weaker (p<0.01). Although the infants in whom PTV failed compared to those in whom it succeeded were significantly more immature and of lower birthweight (p < 0.01), multiple regression analysis demonstrated that reflex activity was independently associated with PTV failure after accounting for gestational age. birthweight and compliance. We conclude that failure of PTV is more likely in immature infants who have a weak Hering Breuer reflex.