“…electromyography has a number of advantages over acceleromyography and may represent the future of twitch monitoring.”
Neuromuscular blocking drugs have been used in anesthesia practice since at least 1942 and have become a routine component of many if not most general anesthetics. Despite this, only in recent times have we focused on ensuring, with a high degree of certainty, that patients do not remain partially paralyzed at the conclusion of an anesthetic. Numerous recent studies have documented that some patients are in fact partially paralyzed for a period of time after anesthesia, sometimes to their detriment.1 We know that subjective twitch monitoring with a peripheral nerve stimulator is not sufficiently sensitive for detection of clinically significant fade in the train-of-four.2 Sugammadex, a highly effective reversal agent for aminosteroid nondepolarizing neuromuscular blocking drugs, and improved quantitative neuromuscular blockade monitors (so-called twitch monitors) have been mobilized in an effort to prevent residual paralysis.
In this issue of Anesthesiology, Nemes et al. have compared a relatively new, commercially available quantitative twitch monitor that utilizes electromyography, with a more commonly used quantitative twitch-monitoring technology, acceleromyography.3 This is valuable because electromyography has a number of advantages over acceleromyography and may represent the future of twitch monitoring.
Acceleromyography utilizes a tiny sensor called an accelerometer, a common component contained in nearly every smart phone, which is usually attached to the thumb (fig. 1). Depolarization of the ulnar nerve results in contraction of the adductor pollicis, which flexes the thumb, producing an acceleration detected by the sensor. The force of thumb movement (which is related to the depth of neuromuscular blockade) is directly proportional to acceleration (force = mass × acceleration). While acceleromyography has been used to quantitatively assess twitch for many years and is the basis for many commercially available twitch monitors, there are two major problems with acceleromyography. The first problem is that the thumb must be entirely free to move, which precludes monitoring the hand that has been tucked at the patient’s side during surgery. The second problem is that the baseline, unparalyzed train-of-four ratio (the ratio of the fourth to the first twitch of a train-of-four), which should theoretically be equal to 1, is often greater than 1, and may be as high as 1.6 when measured by acceleromyography (for reasons that are not entirely clear).4,5 Unless a baseline, unparalyzed train-of-four ratio is obtained, it is impossible to know what train-of-four ratio represents recovery from neuromuscular blockade in an individual patient, when acceleromyography is used.
Electromyography theoretically solves both of these limitations of acceleromyography. Electromyography directly measures the compound action potential of the adductor pollicis muscle (or another intrinsic muscle of the hand). No movement is required for this measurement to be made. The hand can be tucked at the patient’s side without any significant effect on the electromyogram. In addition, a baseline, unparalyzed train-of-four ratio is not required, because with electromyography, unlike with acceleromyography, the baseline, unparalyzed train-of-four ratio does not significantly exceed 1.6
Why, then, should we not rush to adopt any one of the several electromyographic twitch monitors that are now commercially available? The answer to this question requires some additional understanding of electromyography technology. The muscle compound action potentials that constitute the electromyogram are relatively small, especially in comparison to the electrical noise that is ubiquitous in an operating room; the same is true for the electrical signals of the electroencephalogram. This poses a significant challenge for the engineer designing an electromyograph (or an electroencephalogram). Management of noise, and differentiation of noise from signal, are paramount. If noise is not effectively managed, it can be misinterpreted as compound action potentials. On the other hand, if noise reduction algorithms are too aggressive, the electrical signal from the electromyogram may be lost along with the noise, causing the monitor to underestimate twitches and lack sensitivity. Because of this, a commercially available electromyograph should be “validated” to determine how reliably it recognizes twitches, while not misinterpreting noise. How should validation best be accomplished?
Historically, the accepted standard for measurement of twitch is mechanomyography.7 A mechanomyograph is an instrument that directly measures the isometric force of contraction of the thumb, using a force transducer. Assuming that the force transducer is sensitive and accurate, mechanomyography does not have the shortcomings of either acceleromyography or electromyography. When stimulated, the thumb presses against the force transducer, but does not translate in space, and the baseline, unparalyzed train-of-four ratio does not significantly exceed 1, as it often does with acceleromyography. The mechanomyograph is not susceptible to electrical noise, as with the electromyograph. This is why mechanomyography is the accepted standard for twitch measurement. A mechanomyograph (fig. 1) is a somewhat cumbersome instrument that has been used primarily for research, and very seldom for routine clinical practice. Currently, mechanomyography is not commercially available (to the best of our knowledge), and historically this has often been the case. Therefore, investigators have constructed their own mechanomyographs from commercially available force transducers.8 The performance of the force transducer is easily verified by the application of a known force to the mechanomyograph. In our opinion, the performance of commercially available twitch monitors should be compared to mechanomyography, using mechanomyography as the reference standard. There are examples of such studies over the years.6,9,10
In the current study by Nemes et al.,3 mechanomyography was not used. Instead, an acceleromyograph was used for comparison to the electromyograph. In our opinion, this is not ideal, but the study is nevertheless well-executed, enlightening, and valuable. Nemes et al. confirmed numerous previous studies that have demonstrated that baseline, unparalyzed train-of-four ratio determined by acceleromyography may exceed 1.4,5 They also found that the electromyograph that was tested performed better when the twitch amplitude was greater, during more shallow depth of neuromuscular blockade, than when the twitch amplitude was smaller, at deeper depth of neuromuscular blockade. They discuss the possible causes of this, which they attributed to a filter intended to reduce noise. This illustrates the potential challenge to interpretation of electromyograph signals that we have mentioned above. The study by Nemes et al. also illustrates the importance of examining the performance of the twitch monitor not only when the twitch amplitude is relatively high (train-of-four ratio) but also when the twitch amplitude is relatively low (post-tetanic count and twitch count).
The implications of misinterpreting noise as twitch (lack of specificity) are quite different from not recognizing a twitch because it cannot be distinguished from noise (lack of sensitivity). For example, a monitor that reports the presence of one or more twitches when there are in fact no twitches could result in unrecognized residual neuromuscular blockade. A monitor that reports the absence of any twitches when in fact there are one or more twitches could lead the anesthesia provider to overestimate the depth of paralysis. The ideal clinical twitch monitor would resemble, as closely as possible, the accepted standard mechanomyograph.
In summary, Nemes et al.3 have contributed valuable data about the performance of a commercially available electromyograph. We believe that electromyography is the future of quantitative twitch monitoring because of the shortcomings of acceleromyography. However, it is important to understand that signal interpretation and noise management are critically important for electromyography, and the performance of commercially available electromyography-based twitch monitors should be validated, ideally in comparison to mechanomyography.
The authors have been scientific collaborators with Justine Hulvershorn, M.D., Ph.D. (Blink Device Company, Seattle, Washington), the inventor of an electromyograph-based twitch monitor.