Correlation between genioglossal and diaphragmatic responses to hypercapnia during sleep

Oropharyngeal patency during sleep is dependent on the dilating force of the genioglossus, the main tongue protrusor muscle. We measured the ventilatory (V̇I), diaphragmatic (EMGdi), and genioglossal (EMGgg) electromyographic responses to CO₂ in awake and sleeping goats; ΔV̇I/ΔPET(CO₂) decreased from awake (0.85 ± 0.087 L/min/mm Hg) to NREM sleep (0.69 ± 0.069) to REM sleep (0.57 ± 0.078, p < 0.005 versus awake). There were minimal decreases in ΔEMGdi/ΔPET(CO₂) and EMGdi at a PET(CO₂) of 55 mm Hg from awake to NREM, but significant decrease in EMGdi at a PET(CO₂) of 55 mm Hg during REM sleep (p < 0.025 versus NREM). Inspiratory EMGgg was only present above a PCO₂ threshold, which was similar for each state (49.3 ± 2.3 mm Hg PET(CO₂) awake, 48.8 ± 2.4 during NREM, 49.5 ± 2.5 during REM), and ΔEMGgg/ΔPET(CO₂) tended to be less during both sleep states compared with that while awake (p = 0.10). At any PCO₂, inspiratory EMGgg was markedly inhibited during REM sleep when rapid eye movements were present (phasic REM). We conclude that there is disproportionate inhibition of the genioglossus relative to the diaphragm at low CO₂ tensions and at any PCO₂ during phasic REM sleep. This imbalance may predispose the upper airway to inspiratory occlusion during sleep.