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-te rm infants during inspiratory flow-resistive loading (IRL), 10 unsedat ed full-term infants were evaluated in quiet sleep. IRLs were randomly imposed (L2, 125 cmH(2)O.1(-1).s; L3, 250 cmH(2)O.1(-1).s). Ventilati on, total respiratory resistance (a correlate of ETA resistance), and moving time averages of PCA, submental activity of the genioglossus (S M), and diaphragm electromyogram were obtained. Results revealed no ph asic 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 o f inspiratory airflow with both loads (P < 0.05). PCA activity always pre ceded that of the diaphragm and invariably outlasted it other than with L3. The upper airway negative pressure changes induced by IRL we re insufficient to recruit SM activity; other potential stimuli such a s transcutaneous Po-2, transcutaneous Pco(2), and pulmonary stretch re ceptor activation (increase in tidal volume) remained unchanged. Venti lation decreased with both loads (L3: P < 0.01), esophageal and mouth pressures increased (P < 0.01), and inspiratory time and inspiratory t ime divided by total time were both prolonged (P < 0.01). Total respir atory resistance remained unchanged with L2 but increased with L3 (P < 0.01). We concluded that ETA narrowing may be induced in full-term in fants during quiet sleep with moderately large-sized IRL and that it i s not entirely ameliorated by activation of the SM or PCA or by arousa l.