Mivacurium Neuromuscular Block at the Adductor Muscles of the Larynx and Adductor Pollicis in Humans

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Abstract
Background: Laryngeal muscles must be paralyzed for tracheal intubation. Time to peak effect (onset time) is shorter and intensity of blockade is less at laryngeal muscles compared with the adductor pollicis. The authors' aim in this study was to determine the neuromuscular effects of mivacurium at the laryngeal adductor muscles and the adductor pollicis. Methods: In 22 adults, anesthesia was induced and maintained with propofol and alfentanil. The force of contraction of the adductor pollicis was recorded, and the laryngeal response was evaluated by measuring the pressure change in the cuff of a tracheal tube positioned between the vocal cords after train-of-four stimulation. Mivacurium (0.07 mg.kg-1 or 0.14 mg.kg-1) was given intravenously (10s). Results: With 0.07 mg.kg-1 mivacurium, onset time was 151 +/- 40 s(mean +/- SD) at the larynx and 241 +/- 79 s at the adductor pollicis, respectively (P < 0.005). Maximum block was 78 +/- 18% and 95 +/- 8%, respectively (P < 0.002), and time to 90% recovery was 11.1 +/- 2.9 min and 23.3 +/- 7.6 min, respectively (P < 0.001). With 0.14 mg.kg-1 mivacurium, onset time also was more rapid at the vocal cords (137 +/- 20 s) than at the adductor pollicis (201 +/- 59 s, P < 0.01). Maximum block was 90 +/- 7% and 99 +/- 1% (P < 0.005), and time to 90% recovery was 16.4 +/- 4.9 min and 27.4 +/- 7.8 min, respectively (P < 0.01). Conclusions: With mivacurium, onset and recovery are faster at the laryngeal muscles, but block is less intense than at the adductor pollicis. A dose greater than 0.14 mg.kg-1 mivacurium is necessary to ensure complete relaxation at the vocal cords.

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