To the Editor:
We strongly support the implementation of quantitative twitch monitoring to guide neuromuscular blocker administration and to ensure that patients have recovered adequately before extubation, and we applaud the efforts of Weigel et al. and others to do so. Weigel et al. used electromyographic quantitative twitch monitoring preferentially for patients with tucked arms while using raw (nonnormalized) acceleromyographic quantitative twitch monitoring for all other patients, apparently in part to spend less money on electromyographic electrode arrays. Although saving money is laudable, limitations inherent to acceleromyographic monitoring should be considered. Acceleromyography and electromyography cannot be considered as interchangeable technologies differing only in cost, as suggested by Weigel et al. This approach may result in unrecognized residual neuromuscular blockade in the patients monitored with raw acceleromyography data. Numerous studies of acceleromyography have confirmed that unparalyzed, baseline train-of-four ratio exceeds 1.0 (and may reach 1.6) in approximately 20 to 30% of patients. It is important to realize that some acceleromyograph monitors display the raw train-of-four results, including the StimPod device (used by Weigel et al.) and the TOF-Watch-SX, whereas others may only display a maximum value of 1.0, or a value that is adjusted (for example, the TOF-Watch-S displays the ratio of T4 to T2 instead of T4 to T1 when T2 > T1); it is important for users to be aware of the manner in which their acceleromyograph displays train-of-four data. This is in contrast to electromyography or mechanomyography, which seldom record a baseline train-of-four ratio greater than 1.0. Because of the potential “overshoot” of baseline train-of-four ratio, an unparalyzed, baseline raw train-of-four ratio should be routinely measured when using acceleromyography. This requires obtaining a train-of-four ratio after the induction of anesthesia, before the administration of a neuromuscular blocking drug, a vital step that many practicing anesthesiologists find inconvenient and challenging to perform. The raw train-of-four data should then be “normalized” by dividing the raw train-of-four ratio by the baseline train-of-four ratio. For example, if the baseline train-of-four ratio is 1.3, and the raw train-of-four ratio is 1.2, then the normalized train-of-four ratio is 1.2/1.3 = 0.9. Recovery from neuromuscular blockade in this case requires a raw train-of-four ratio of 1.2, not 0.9. While some have suggested using a raw, nonnormalized train-of-four ratio of 0.95 or 1.0 as the target for reversal or recovery when using acceleromyography; in some patients, even a raw train-of-four ratio of 1.0 will not be sufficient, as illustrated in the example just described.
An important limitation of the study by Weigel et al. is that the train-of-four ratios were self-reported by anesthesia providers. Self-reported data are less reliable than data collected by direct observation (by persons conducting the research) or by importing machine data directly into the electronic patient record and may overestimate adherence, particularly when anesthesia providers are aware that their performance is being tracked. It is impossible to know what proportion of documented train-of-four ratios were actually based on quantitative twitch measurements.
It is an important caveat that visually counting twitches, as mentioned by Weigel et al., is not reliable, whereas counting twitches manually by palpation can compare favorably with mechanomyography when the proper technique is used. When counting twitches manually, thumb movement must be palpated while finger movement is physically restrained. Movement of the entire hand or fingers by direct stimulation of forearm muscles can easily produce the false appearance of ulnar nerve–stimulated thumb twitches, and an incorrect conclusion may be drawn that the quantitative twitch monitor is not properly counting twitches.
Finally, Weigel et al. mentioned TwitchView “array failures.” We have performed validation studies of the TwitchView EMG-based twitch monitor in comparison with mechanomyography for determination of the train-of-four ratio and for counting twitches. Subsequently, we have used approximately 25,000 TwitchView arrays in our clinical practice. In our experience, array failures have been rare.