Abstract
STUDY OBJECTIVES: Subjects with a collapsible upper airway must activate their pharyngeal dilators sufficiently in response to increasing chemical drive if they are to maintain airway patency without arousal from sleep. Little is known about the response of pharyngeal dilators to increasing chemical drive in these subjects. We wished to determine, in obstructive apnea patients, the response of the genioglossus to increasing chemical drive and the contribution of mechanoreceptor feedback to this response. DESIGN: Physiological study. SETTING: University-based sleep laboratory. PATIENTS: 20 patients with obstructive apnea. INTERVENTIONS: Genioglossus activity was monitored during overnight polysomnography on optimal continuous positive airway pressure (CPAP). Intermittently, inspired gases were altered to produce different levels of ventilatory stimulation. CPAP was then briefly reduced to 1.0 cm H(2)O (dial-down), inducing an obstruction. MEASUREMENTS AND RESULTS: Without mechanoreceptor feedback (i.e., on CPAP) the increase in genioglossus activity as ventilation increased from 6.1 ± 1.4 to 16.1 ± 4.8 L/min was modest (ΔTonic activity 0.3% ± 0.5%maximum; ΔPhasic activity 1.7% ± 3.4%maximum). Genioglossus activity increased immediately upon dial-down, reflecting mechanoreceptor feedback, but only when ventilation before dial-down exceeded a threshold value. This threshold varied among patients and, once surpassed, genioglossus activity increased briskly with further increases in chemical drive (1.1% ± 0.84%GG(MAX) per L/min increase in V(E)). CONCLUSIONS: In sleeping obstructive apnea patients: (1) Mechanoreceptor feedback is responsible for most of the genioglossus response to chemical drive. (2) Mechanoreceptor feedback is effective only above a threshold chemical drive, which varies greatly among patients. These findings account in part for the highly variable relation between pharyngeal mechanical abnormalities and apnea severity.