ROLE OF RESPIRATORY MUSCLES IN UPPER AIRWAY NARROWING INDUCED BY INSPIRATORY LOADING IN PRETERM INFANTS

Extrathoracic airway (ETA) narrowing is induced in preterm infants by inspiratory flow-resistive loading (IRL), which reduces intraluminal p ressure within the region. Neuromuscular load compensation was evaluat ed over time in 10 infants [body wt 1.5 +/- 0.17 (SD) kg, gestational age 33 +/- 2.3 wk, age 12 +/- 5.2 days] during quiet sleep. Baseline ( BL) studies were followed by IRL (125 cmH(2)O.1(-1).s at 1 l/min). Min ute ventilation, changes in esophageal pressure (Pes) and proximal air way pressure, and moving time averages of posterior cricoarytenoid (PC A), submental genioglossus (SM), and diaphragm (DIA) electromyograms w ere obtained during BL and 1 and 5 min of IRL. Total respiratory resis tance was calculated from pressure and flow changes and was used to es timate ETA narrowing: there was an increase in total respiratory resis tance from 90 +/- 15 to 120 +/- 34 and 151 +/- 86 cmH(2)O.1(-1).s afte r 1 and 5 min of IRL, respectively (P < 0.05, 1-min IRL vs. BL), in as sociation with a sustained decline in minute ventilation (P < 0.05) an d increases in Pes and proximal airway pressure (P < 0.05). Phasic PCA activity was always present, but its duration was only transiently pr olonged with IRL (P < 0.05, 1-min IRL vs. BL). SM activity was present in only one infant during BL and was recruited in two additional infa nts during IRL. The decline in Pes from 1 to 5 min of IRL occurred des pite continuing increases in peak and average activities of the DIA mo ving time average, which may reflect an onset of DIA fatigue. The tran sient prolongation of phasic PCA activity and occasional recruitment o f SM activity with sustained loading explain, in part, the ETA instabi lity detectable by moderate IRL in sleeping preterm infants.