Genioglossus Muscle Function During Propofol Induction
The genioglossus muscle undergoes a critical and often abrupt decrease in activity during propofol induction, particularly at or near loss of consciousness, leading to increased upper airway collapsibility and potential obstruction. 1
Mechanism of Genioglossus Suppression
During propofol induction, the genioglossus demonstrates a characteristic biphasic response:
Initial preservation phase: Genioglossus electromyographic (EMG) activity is initially sustained or may even increase as propofol effect-site concentrations rise from 0 to approximately 1.5-4.0 μg/mL 1
Critical transition at loss of consciousness: At or approaching loss of consciousness, genioglossus activity decreases abruptly to minimal values, with a corresponding sharp increase in pharyngeal critical closing pressure (Pcrit) 1
Alinear relationship: The increase in upper airway collapsibility during propofol induction follows an alinear pattern, with disproportionate vulnerability occurring at the transition from conscious to unconscious sedation 1
Clinical Significance for Airway Patency
The genioglossus is the primary upper airway dilator muscle responsible for maintaining airway patency:
Dominant role in airway resistance: Among upper airway muscles, genioglossus contraction produces the most significant reduction in supraglottic resistance (from 7.9 ± 0.6 to 0.4 ± 0.1 cmH₂O·L⁻¹·sec) and is the only muscle that significantly increases critical closing pressure 2
Dual mechanism of action: The genioglossus maintains airway patency both by dilating the supraglottic airway and by stiffening its walls, making its relaxation the main impediment to airflow 2
Propofol-Specific Effects vs. Other Anesthetics
Propofol demonstrates distinct effects on genioglossus function compared to volatile anesthetics:
Marked suppression with propofol: Propofol produces significantly lower tonic and phasic genioglossus activity compared to isoflurane at equivalent anesthetic depths 3
Respiratory drive dependency: The impairment of phasic genioglossus activity during propofol is closely correlated with the level of respiratory depression rather than the depth of anesthesia per se 3
Potential for recovery: The suppressed genioglossus activity during propofol can be reversed when respiratory drive is increased through hypoxia or hypercapnia 3
Practical Implications for Airway Management
Without muscle relaxants, propofol-induced muscle relaxation is incomplete, and the genioglossus suppression creates particular vulnerability to upper airway obstruction. 4
Key clinical considerations:
Timing of maximum vulnerability: The period immediately following loss of consciousness represents the highest risk for airway obstruction due to abrupt genioglossus suppression 1
Muscle relaxant considerations: When airway obstruction occurs during propofol induction without muscle relaxants, administration of neuromuscular blocking agents is recommended to facilitate airway management 4
Propofol for laryngospasm: Despite genioglossus suppression, propofol (0.25-0.8 mg/kg) remains effective in treating laryngospasm in 77% of cases, though muscle relaxants are highly recommended for complete laryngospasm 4
Regional Muscle Activity Patterns
Posterior predominance: The posterior region of the genioglossus demonstrates greater EMG activity (11.10 ± 0.99% regional maximum) compared to the anterior region (7.95 ± 0.57%) during normal breathing 5
Positional effects: Genioglossus activity increases in the supine position (10.42 ± 0.90%) compared to upright (8.63 ± 0.73%), reflecting increased work to maintain airway patency 5