EVIDENCE FOR A TONIC COMPONENT OF THE HERING-BREUER INFLATION REFLEX IN THE TERM NEONATE

Abstract

Animal studies have shown that an increase in FRC slows respiration, increases expiratory duration and is vagally mediated. To explore whether a similar response occurs in humans, we studied respiratory timing in 10 sleeping, healthy term neonates on days 1, 2 and 3, before and after a change in lung volume accomplished by adding a continuous positive airway pressure(CPAP) of 3 and 6 cm H2O. Using a mask and flowmeter with constant flow to reduce deadspace, we measured inspiratory time(Ti), expiratory time(Te), total respiratory cycle duration (Ttot), and tidal volume. Comparative movements of upper and lower rib cage were recorded via mercury strain gauges and transcutaneous(Tc) PO2, monitored via a skin electrode. Increasing lung volume caused a significant prolongation of Te% (100xTe/Ttot) on all 3 days(mean data in table). As CPAP was raised from 0 to 6. respiratory rate fell from 47 to 42(NS),58 to 42(p<.005) and 50 to 36(p<.005) on days 1, 2, and 3 respectively. CPAP at 6 cm caused a mean increase in FRC of 19±7cc without significant change in tidal volume and there was no change in TcPO2 when compared to the control period. The direction of paradoxical rib cage movement was unaltered by CPAP. These observations suggest that the significant rise in Te% at increased lung volume may be secondary to the stimulation of a tonic inflation reflex.