Glottic closing force in an anesthetized, awake pig model: Biomechanical effects on the laryngeal closure reflex resulting from altered central facilitation

Reflex sphincteric closure of the larynx, essential to lower airway protect ion, is most efficiently achieved through strong reflex adduction by both v ocal cords. Because the conversion of a bilaterally evoked response to a un ilaterally evoked one appears anesthesia-dependent, we hypothesized that ce ntral facilitation is an essential component of a bilateral adductor reflex and that its disturbance could result in weakened sphincteric closure. Six adult 50 kg pigs were used in this study. During electrical stimulation of the internal branch of the superior laryngeal nerve (iSLN) using bipolar p latinum-iridium electrodes. the force of the evoked glottic closure respons e was measured using a pressure transducer positioned between both vocal co rds, while electromyographic evoked response was recorded from both thyroar ytenoid muscles under varying levels [0.5-1.0 minimal alveolar concentratio n (MAC)] of isoflurane anesthesia. The force of glottic closure appeared le ss under deep anesthesia. even with bilateral stimulation of the iSLN, than under light anesthesia with unilateral stimulation, As anesthetic levels a pproached 1.0 MAC, the glottic closing force decreased to 52-72% of the for ce measured under 0.5 MAC light anesthesia. Although it is generally unders tood that alteration of central facilitation by deepening anesthesia abolis hes the crossed adductor reflex, the biomechanical effects of altered centr al facilitation on force differentials have never been previously demonstra ted. Precise understanding of this effect may improve the prevention of asp iration in patients emerging from heavy sedation or under prolonged psychot ropic control.