Upper airway and diaphragm muscle responses to chemical stimulation and loading

Previous studies suggest that upper airway muscles in humans respond to changes in chemical respiratory drive in a fashion similar to the diaphragm. To test this hypothesis, in nine seated healthy subjects we monitored electromyographic (EMG) activity from the alae nasi (AN), genioglossus (GG), and diaphragm (DI) not only in response to progressive isocapnic hypoxia and hyperoxic hypercapnia but also to hyperoxic hypercapnia with and without an external inspiratory resistance (15 cmH2O X 1(-1) X s). There were linear increases in DI, GG, and AN in response to increasing chemical drive, but the AN in one subject and the GG in another subject did not respond to hypoxia or hypercapnia. In response to an inspiratory load, subjects decreased ventilation (P less than 0.01) and increased AN, GG, and DI EMG activity (P less than 0.05); however, one subject did not show an AN response to either increased chemical drive or ventilatory loading. We conclude that muscles of the upper airway and the diaphragm are generally activated in a parallel fashion as respiration is stimulated by hypoxia, hypercapnia, and increased inspiratory resistance.