To the Editor:
I read with great interest the article by Herbstreit et al. 1describing the effect of residual neuromuscular blockade on upper airway collapsibility in humans. The authors observed that upper airway integrity was impaired when the train-of-four ratio was maintained at the level of 0.5 or 0.8 with rocuronium. In a former study in volunteers, I and others could not demonstrate any susceptibility to upper airway obstruction when the train-of-four ratio was maintained to 0.5 with an infusion of vecuronium.2In our study, decreased inspiratory negative pressure, which is known to exaggerate the possibility of upper airway collapse, was elicited by increasing ventilation during carbon dioxide rebreathing or by adding an inspiratory resistance. Breathing with pressure at airway opening held at a pressure from −5 to −40 cm H2O was also tested. No effect of partial neuromuscular blockade maintained at a train-of-four ratio of 0.5 on upper airway could be observed. When the upper airway pressure was progressively decreased to −40 cm H2O, no participant showed evidence of upper airway collapse or flow limitation at any time period. The main difference between the two studies is that subjects were breathing through a mouthpiece in our study2instead of a nasal mask in the study by Herbstreit et al. 1Therefore, the flow limitation observed by Herbstreit et al. 1may be related to resistance at the velopharynx rather than to an impaired compensatory response of the genioglossus muscle.
I agree with Herbstreit et al. 1when they state that the upper airway muscles are more susceptible to neuromuscular blocking agents than the diaphragm. However, because the diaphragm is the most resistant muscle to neuromuscular blocking agents among all skeletal muscles, this comparison is not valid to point out a particular susceptibility of upper airway muscles to neuromuscular blocking agents. For example, the geniohyoid muscle, which also contributes to upper airway integrity, was shown to be more susceptible than the diaphragm but less susceptible than the adductor pollicis to mivacurium in humans.3Finally, as suggested by Herbstreit et al. ,1further work on the function of other airway openers should be performed. In the future, for a rigorous evaluation of the effect of residual neuromuscular blockade on the function of upper airway muscles, the investigator should be blinded to the level of neuromuscular blockade.
Assistance Publique, Hospitaux de Paris et Universite Paris XII, Créteil, Cedex, France. firstname.lastname@example.org