1223 THE EFFECTS OF LUNG INFLATION ON THE VARIOUS PHASES OF THE RESPIRATORY CYCLE IN PRETERM INFANTS

Abstract

Traditionally the Hering-Breuer reflex has been assessed by the presence and length of apnea and/or prolongation of expiratory time (Te) during sustained lung inflation. In recent years a decrease in inspiratory time (Ti) during lung inflation has also been suggested as an indicator of the inflation reflex. Both Te and Ti have not been analyzed systematically in neonates with stratified increase in lung volume. We decided, therefore, to examine the effects of sudden and sustained lung inflation on Ti, Te , and duration of apnea in preterm infants. Ten babies (G.A. 31±1 wk; B.W. 1500±110g) were studied. Lung inflation was obtained by applying negative pressure around the chest using a negative pressure incubator. Response time of the system was less than .3 sec. After a control period, we increased lung volume (FRC) by 15%, 25%, 35%, and 60%. We measured Ti, Te, apnea and FRC before and during lung inflation. We found a significant increase in Ti, Te, and duration of apnea with an increase in FRC by as little as 15% (p<0.01). These results suggest: 1)preterm infants show significant increase in Te and duration of apnea during sustained lung inflation indicating potent Hering-Breuer reflex; 2)contrary to expectations, Ti increased significantly with lung inflation. We speculate that changes in Ti and Te during lung inflation are not exclusively vagal dependent and may reflect changes in chest wall reflexes or in respiratory muscle performance.