Upper airway stability and respiratory muscle activity during inspiratory loading in full-term neonates.

To investigate the role of genioglossus and posterior cricoarytenoid (PCA) activity in stabilizing the extrathoracic airway (ETA) of full-term infants during inspiratory flow-resistive loading (IRL), 10 unsedated full-term infants were evaluated in quiet sleep. IRLs were randomly imposed (L2, 125 cmH2O.l-1.s; L3, 250 cmH2O.l-1.s). Ventilation, total respiratory resistance (a correlate of ETA resistance), and moving time averages of PCA, submental activity of the genioglossus (SM), and diaphragm electromyogram were obtained. Results revealed no phasic activity in the SM during baseline breathing or with either IRL. Phasic PCA activity was always observed; burst duration increased with L2 and L3 (P < 0.01) and commenced earlier in relation to the onset of inspiratory airflow with both loads (P < 0.05). PCA activity always preceded that of the diaphragm and invariably outlasted it other than with L3. The upper airway negative pressure changes induced by IRL were insufficient to recruit SM activity; other potential stimuli such as transcutaneous PO2, transcutaneous PCO2, and pulmonary stretch receptor activation (increase in tidal volume) remained unchanged. Ventilation decreased with both loads (L3: P < 0.01), esophageal and mouth pressures increased (P < 0.01), and inspiratory time and inspiratory time divided by total time were both prolonged (P < 0.01). Total respiratory resistance remained unchanged with L2 but increased with L3 (P < 0.01). We concluded that ETA narrowing may be induced in full-term infants during quiet sleep with moderately large-sized IRL and that it is not entirely ameliorated by activation of the SM or PCA or by arousal.