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 (T_e) during sustained lung inflation. In recent years a decrease in inspiratory time (T_i) during lung inflation has also been suggested as an indicator of the inflation reflex. Both T_e and T_i 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 T_i, T_e , 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 T_i, T_e, apnea and FRC before and during lung inflation. We found a significant increase in T_i, T_e, 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 T_e and duration of apnea during sustained lung inflation indicating potent Hering-Breuer reflex; 2)contrary to expectations, T_i increased significantly with lung inflation. We speculate that changes in T_i and T_e during lung inflation are not exclusively vagal dependent and may reflect changes in chest wall reflexes or in respiratory muscle performance.

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