Effect of selective hypoglossal nerve stimulation on tongue movements and pharyngeal airflow mechanics

These experiments examined the influence of hypoglossal nerve stimulation on tongue movements and airflow mechanics of the isolated pharyngeal airway of anesthetized rats. Airflow was generated by rapidly lowering the hypo-pharyngeal pressure with a vacuum pump. The maximal rate of airflow (VImax), the naso-pharyngeal pressure associated with flow limitation (Pcrit), and pharyngeal resistance were monitored while nerve branches innervating tongue protrudor and retractor muscles were stimulated either simultaneously or independently. Resistance was calculated both upstream and across the flow limiting segment (FLS) of the pharyngeal airway. When the protrudor and retractor muscles were co-activated, the tongue retracted, and VImax increased by 44% (p<0.05). Independent protrudor muscle activation caused tongue protrusion and a 61% increase in VImax (p<0.05). Co-activation resulted in a more negative Pcrit (peak decrease of 44%, p<0.05), but independent activation of either protrudor or retractor muscles failed to change Pcrit significantly. Activation of protrudor muscles reduced resistance across the FLS by 129% (p<0.05), but none of the interventions significantly influenced resistance upstream of the FLS. These results indicate that co-activation of protrudor and retractor tongue muscles results in a stiffer pharyngeal airway as the antagonistic muscles work against one another. In contrast, unopposed tongue protrusion dilates, but does not stiffen, the pharyngeal airway.