Influence of chemoreceptor stimuli on genioglossal response to negative pressure in humans

Genioglossal muscle (GG) activity is modulated by both chemoreceptive and mechanoreceptive reflexes that help stabilize airway patency. We assessed the effects of blood gas changes, within the range encountered during mild obstructive apnea-arousal cycles, on GG activity and the GG reflex to upper airway negative pressure. Eighteen healthy adults were studied while awake under 5 conditions: (1) baseline (PET(CO₂) = 40 mm Hg, Sa(O₂) = 99%); (2) hypercapnia (PET(CO₂) = 45 mm Hg); (3) hypocapnia (PET(CO₂) = 35 mm Hg, induced via hyperventilation with an iron lung ventilator); (4) hypoxia (Sa(O₂) = 87%); and (5) hypercapnia plus hypoxia (PET(CO₂) = 45 mm Hg, Sa(O₂) = 87%). Measurements included airflow, choanal and epiglottic pressures (Pchoa and Pepi), upper airway resistance, phasic and tonic GG EMG, and the GG reflex to negative pressure (Pchoa = -12.5 cm H₂O). Ventilation increased from a baseline of 10.7 up to 22.7 L · min^-1 under conditions of altered blood gases. Peak inspiratory phasic GG EMG increased from 6.5 to 11.1% of maximal contraction but there were no significant changes in either tonic GG EMG (range, 4.3 to 5.8% of maximum) or magnitude of the GG reflex (range, 4.1 to 5.5% of maximum). Among conditions there was a high correlation between upper airway pressures and peak phasic GG EMG (Pchoa, r = 0.97, p < 0.01; Pepi, r = 0.87; p = 0.06). We conclude that in this range of blood gases: (1) the GG reflex to negative pressure is unchanged; (2) slow airway pressure changes throughout inspiration, generated either actively or passively, influence GG EMG activity; and (3) mechanoreceptive control of GG EMG can fully explain all changes in GG activity, suggesting that chemoreceptive inputs to GG are minimal, or are not simply summated with mechanoreceptor inputs.