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 pressure within the region. Neuromuscular load compensation was evaluated 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 cmH2O.l-1.s at 1 l/min). Minute ventilation, changes in esophageal pressure (Pes) and proximal airway pressure, and moving time averages of posterior cricoarytenoid (PCA), submental genioglossus (SM), and diaphragm (DIA) electromyograms were obtained during BL and 1 and 5 min of IRL. Total respiratory resistance was calculated from pressure and flow changes and was used to estimate ETA narrowing: there was an increase in total respiratory resistance from 90 +/- 15 to 120 +/- 34 and 151 +/- 86 cmH2O.l-1.s after 1 and 5 min of IRL, respectively (P < 0.05, 1-min IRL vs. BL), in association with a sustained decline in minute ventilation (P < 0.05) and increases in Pes and proximal airway pressure (P < 0.05). Phasic PCA activity was always present, but its duration was only transiently prolonged 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 infants during IRL. The decline in Pes from 1 to 5 min of IRL occurred despite continuing increases in peak and average activities of the DIA moving time average, which may reflect an onset of DIA fatigue. The transient prolongation of phasic PCA activity and occasional recruitment of SM activity with sustained loading explain, in part, the ETA instability detectable by moderate IRL in sleeping preterm infants.