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GENERAL ARTICLES

Assessing Neuromuscular Block at the Larynx: The Effect of Change in Resting Cuff Pressure and a Comparison with Video Imaging in Anesthetized Humans

University Department of Anaesthesia, University Hospital, Queen's Medical Centre, Nottingham, United Kingdom

Address correspondence and reprint requests to Keith Girling, University Department of Anaesthesia, University Hospital, Queen's Medical Centre, Nottingham NG7 2UH, UK. Address e-mail to Keith.Girling{at}nottingham.ac.uk

Neuromuscular block (NMB) at the larynx has been assessed by measuring the cuff pressure (CP) in an endotracheal tube (ETT) placed between the vocal cords. In this study, we evaluated the decrease in resting cuff pressure (RCP) after the administration of rocuronium and the effect of this decrease on the assessment of NMB, and we compared CP measurement with an alternative technique, video imaging (VI). In 20 patients, NMB was determined at the hand by mechanomyography and at the larynx initially by CP and subsequently by VI, recording images using a fiberoptic bronchoscope via a laryngeal mask. Train-of-four stimuli were applied at both sites. After baseline measurements, the ETT was replaced, and rocuronium was infused to achieve a steady-state 50% (n = 10) or 75% (n = 10) block at the hand. CP measurements were recorded before and after restoration of RCP to prerocuronium pressure, followed by further VI measurements. The mean RCP decreased from 21 ± 4 to 12 ± 5 mm Hg after rocuronium. At 50% block at the hand, the CP estimate of block at the larynx with reduced RCP was 62% ± 18%, and that after restoring RCP was 29% ± 13%; VI estimated 27% ± 14% block. At 75% block at the hand, CP and VI estimated 52% ± 11% and 46% ± 9% block, respectively (RCP maintained). We conclude that RCP decreases after the administration of rocuronium, that restoring RCP significantly alters CP estimates of NMB, and that VI is in agreement with CP measurement if RCP is maintained at prerelaxant values.

Implications: In this study, we show that a muscle relaxant-induced decrease in resting tension at the larynx may confound the assessment of neuromuscular block by cuff pressure measurement. The preliminary data suggest that video imaging may provide a suitable alternative to cuff pressure measurement to assess neuromuscular block at the larynx.

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