Background: Upper airway collapsibility is known to increase under anesthesia. This study assessed how this increase in collapsibility evolves during slow Propofol induction and how it relates to anesthesia-induced changes in upper airway muscle activity and conscious state.
Methods: Nine healthy volunteers were studied. Anesthesia was induced with Propofol in a step-wise manner (effect-site concentration steps of 0.5 microg x ml(-1) from 0 to 3 microg x ml(-1) and thereafter to 4 microg x ml(-1) and 6 microg x ml(-1) [target-controlled infusion]). Airway patency was maintained with continuous positive airway pressure. Pharyngeal collapsibility was assessed at each concentration by measuring critical pressure. Intramuscular genioglossus electromyogram and anesthetic depth (bispectral index score) were monitored throughout. Loss of consciousness was defined as failure to respond to loud verbal command.
Results: Loss of consciousness occurred at varying Propofol effect-site concentrations between 1.5 and 4.0 microg x ml(-1). Initially genioglossus electromyographic activity was sustained with increases in Propofol concentration, increasing in some individuals. At or approaching loss of consciousness, it decreased, often abruptly, to minimal values with an accompanying increase in critical pressure. In most subjects, bispectral index score decreased alinearly with increasing Propofol concentration with greatest rate of change coinciding with loss of consciousness.
Conclusions: Slow stepwise induction of Propofol anesthesia is associated with an alinear increase in upper airway collapsibility. Disproportionate decreases in genioglossus electromyogram activity and increases in pharyngeal critical closing pressure were observed proximate to loss of consciousness, suggesting that particular vulnerability exists after transition from conscious to unconscious sedation. Such changes may have parallels with upper airway behavior at sleep onset.