Dissociation of electromyogram and mechanical response in sleep apnoea during propofol anaesthesia

Pharyngeal collapsibility during sleep is believed to increase due to a decline in dilator muscle activity. However, genioglossus electromyogram (EMG) often increases during apnoeas and hypopnoeas, often without mechanical effect. 17 patients with obstructive sleep apnoea were anaesthetised and evaluated from termination of propofol administration to awakening. Genioglossus EMG, flow and pharyngeal area (pharyngoscopy) were monitored. Prolonged hypopnoeas enabled evaluation of the relationships between genioglossus EMG and mechanical events, before and after awakening. Additional dilator muscle EMGs were recorded and compared to the genioglossus. Electrical stimulation of the genioglossus was used to evaluate possible mechanical dysfunction. Prolonged hypopnoeas during inspiration before arousal triggered an increase in genioglossus EMG, reaching mean +/- SD 62.2 +/- 32.7% of maximum. This augmented activity failed to increase flow and pharyngeal area. Awakening resulted in fast pharyngeal enlargement and restoration of unobstructed flow, with marked reduction in genioglossus EMG. Electrical stimulation of the genioglossus under propofol anaesthesia increased the inspiratory pharyngeal area (from 25.1 +/- 28 to 66.3 +/- 75.5 mm(2); p<0.01) and flow (from 11.5 +/- 6.5 to 18.6 +/- 9.2 L.min(-1); p<0.001), indicating adequate mechanical response. All additional dilators increased their inspiratory activity during hypopnoeas. During propofol anaesthesia, pharyngeal occlusion persists despite large increases in genioglossus EMG, in the presence of a preserved mechanical response to electrical stimulation.