Volume of activation of the Hering-Breuer inflation reflex in the newborn infant

Although the Hering-Breuer inflation reflex (HBIR) is active within tidal b reathing range in the neonatal period, there is no information regarding wh ether a critical volume has to be exceeded before any effect can be observe d. To explore this, effects of multiple airway occlusions on inspiratory an d expiratory timing were measured throughout tidal breathing range using a face mask and shutter system. In 20 of the 22 healthy infants studied, ther e was significant shortening of inspiration because the volume at which occ lusion occurred rose from functional residual capacity (FRC) to end-inspira tory volume [14.9% reduction in inspiratory time (per ml/kg increase in lun g volume at occlusion)]. All infants showed a significant increase in expir atory time [17.1% increase (per ml/kg increase in lung volume at occlusion) ]. Polynomial regression analyses revealed a progressive increase in streng th of HBIR from FRC to similar to4 ml/kg above FRC. Eighteen infants showed no further shortening of inspiratory time and 10 infants no further length ening of expiratory time with increasing occlusion volumes, indicating maxi mal stimulation of the reflex had been achieved. There was a significant re lationship between strength of HEIR and respiratory rate, suggesting that H EIR modifies the breathing pattern in the neonatal period.