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Department of Anesthesia, Centre Hospitalier de l’Université de Montréal, Montréal, Québec, Canada
To the Editor:
We read with interest the article by Girling et al. (1) presenting video imaging as a new method to determine neuromuscular block at the larynx. We would like to make some critical comments.
Movement of the bronchoscope during the period of monitoring might have an influence on the accuracy and reproducibility of the angles measured. Because the coefficient of variation in cord movement was only measured during supramaximal stimulation, it is not known what the coefficient of variation might be at another state of neuromuscular blockade, e.g., 50% blockade.
Changes in cuff resting pressure of the laryngeal mask during neuromuscular blockade might change the position of the larynx. Have they made comparative measurements and how might the images and the position of the vocal cords be altered by this?
Supramaximal stimuli were applied at a mean current of 74 ± 13 mA. Our own experiences suggest that this amount of current causes significant movement of the larynx and, possibly, the vocal cord angles.
Girling et al. (1) calculated intra- and interobserver variability in determining vocal cord angles. Because the mean angles of these measurements are not presented, evaluation of the Bland-Altman plots is somewhat limited. Furthermore, agreement of interobserver measurements should have been compared during the onset and offset of neuromuscular blockade.
Video imaging at the larynx and mechanomyography (MMG) of the hand are two fundamentally different methods of measurements. MMG measures isometric forces; video imaging measures the visual movement of the vocal cords without relation to the actual force generated. A comparable method at the hand would be to measure contraction of the adductor pollicis muscle via video imaging, e.g., determining different contraction angles.
In their discussion, Girling et al. (1) state that the advantage of video imaging is direct visualization of the neuromuscular blockade versus "blind" cuff pressure measurements. We cannot agree with this statement. The main difference between this new technique and MMG or electromyography (EMG) is the subjective principle of video imaging. It cannot be concluded that video imaging, because of the mere visualization of the vocal cord movement, is superior to electromyography or cuff pressure measurements in determining neuromuscular blockade. A proper evaluation of the video imaging technique would be to compare it with one of these methods. This seems technically impossible with the cuff pressure technique but is certainly possible with IM laryngeal EMG, as recently described (2).
Girling et al. (1) measured very similar onset of neuromuscular blockade after succinylcholine at the larynx and the adductor pollicis muscle. This is in concordance to a study by D’Honneur et al. (3) using EMG and in contrast to studies by Wright et al. (4) and Meistelman et al. (5) using MMG. Girling et al. (1) suggest the change in resting cuff pressure as a reason for this difference. Our own recent findings (6) using surface EMG at the larynx and the adductor pollicis muscle, however, support the findings by Wright et al. (4) and Meistelman et al. (5) and showed a significant difference in lag and onset time between larynx and adductor pollicis muscle. We suggest another explanation for these contrasting studies. Whereas in the latter studies the same technique was used to measure neuromuscular blockade at the larynx and the adductor pollicis muscle (MMG or EMG), D’Honneur et al. (3) and Girling et al. (1) have used different methods at the larynx and adductor pollicis muscle (EMG or video imaging versus MMG). Given the fact that EMG tends to measure onset times which are usually longer than MMG (7), this might well explain why no difference in onset time between larynx and adductor pollicis muscle was found by D’Honneur et al. (3) and Girling et al. (1).
We conclude that video imaging seems to be an interesting research tool to measure neuromuscular blockade at the larynx but needs more evaluating to estimate its role in neuromuscular research.
References
Queen’s Medical Centre, Nottingham, UK
In response:
We would like to thank Drs. Hemmerling and Donati for their interest in our recent article on video imaging at the larynx (1). Movement of the bronchoscope during the monitoring period may indeed influence the accuracy of the technique. Therefore, great care was taken through this study to ensure that the bronchoscope position was fixed such that the length introduced into the laryngeal mask airway; the rotation of the bronchoscope and position of the laryngeal mask/bronchoscope apparatus remained constant throughout the study.
We have only reported the coefficient of variation in cord movement during supramaximal stimulation in this study because the study design did not allow the coefficient of variation to be measured at 50% blockade. However, in our previous study (2), we compared cuff pressure measurement with video imaging at 50% blockade. The SD of the measurements made by the two techniques was strikingly similar and it is unlikely that the coefficient of variation is significantly greater at this level of block. It is possible that the laryngeal mask cuff pressure may have changed with time. However, there was no visible evidence of any change in the position of the bronchoscope through the study. In addition, because all of the measurements of cord movement were made relative to the baseline for the stimulation, any movement would have affected baseline and maximal contraction equally.
We did see significant movement of extralaryngeal structures to the stimulating currents used, but again the laryngeal mask/bronchoscope apparatus was fixed in its relationship to the larynx to avoid changes in the vocal cord angles. We considered it of prime importance to use a truly supramaximal stimulus and to attempt to reduce the effects of other potential confounding factors as much as possible. We also showed in this present study that we are not monitoring direct muscle stimulation because vocal cord movement was obliterated.
The Bland-Altman plot was not presented in full because this was considered to detract form the main theme of this study; however, the range of angles is given from 5 to 50 degrees. Perhaps it should have been made clearer in the text that the 10 maximal adduction images were taken to represent each of the periods: onset, maximum block, and offset of blockade.
We agree that mechanomyography and video imaging are two very different methods of measurement. We have considered examining the movement of the thumb with video imaging for comparison with the larynx, but as discussed in the article at some length, we not only acknowledge that the measurement techniques are different but that there are inherent differences between the two sites of measurement. This makes comparison of the two sites with the same techniques more difficult. In addition we have previously shown a very close relationship between the measurements at the larynx using the cuff pressure technique and video imaging if the baseline endotracheal cuff pressure is maintained (2). This we think is a "proper evaluation of video imaging" and although seemingly "technically impossible" has been published previously with due acknowledgment of the technical limitations encountered.
Finally, the difference in time to onset of block as measured by different techniques and by different authors is indeed very interesting. We consider that the principal advantage of video imaging in this situation is that it reflects most closely the end point used by most clinicians to assess conditions for tracheal intubation, i.e., vocal cord movement (3).
We would concur that video imaging seems to be an interesting research tool to measure neuromuscular blockade at the larynx and we concede that like many other such tools, it would benefit from further evaluation. However, we consider that video imaging has already been subjected to evaluation that is at least as stringent as that has been applied to other methods of assessment of neuromuscular blockade at the larynx.
We apologize for omitting an acknowledgement for financial support for this work from the NHS Executive Trent and the Special Trustees for the Nottingham University Hospitals from the original publication.
References
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