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Despite the use of neuromuscular blocking drugs in anesthesia practice being routine since at least 1942, there has only recently been a focus on ensuring that patients do not remain paralyzed at the conclusion of an anesthetic. To combat partial paralysis after anesthesia, quantitative twitch monitoring should be performed – but what is the most effective technique in the clinical setting? Andrew Bowdle, MD, PhD, FASE, Professor of Anesthesiology and Pharmaceutics, Laura Cheney Professor in Anesthesia Patient Safety, Department of Anesthesiology, University of Washington, weighs in on electromyography versus acceleromyography for twitch monitoring.

When performing quantitative twitch monitoring, acceleromyography, which uses an accelerometer sensor to measure movement on the thumb, is historically the most commonly used monitor. In comparison, the relatively newer electromyography twitch monitor does not require movement – one of a number of advantages that may represent the future of twitch monitoring.

Both acceleromyographic and electromyographic technologies are quantitative neuromuscular blockade monitors, which offer the potential benefit of avoiding residual neuromuscular blockade and its adverse respiratory consequences by assuring that patients are adequately recovered at the end of the anesthetic, Dr. Bowdle explained. In addition, quantitative neuromuscular blockade is useful for titrating neuromuscular blocking drugs to obtain whatever level of neuromuscular blockade is desired for a particular situation, he said. “Specifically, quantitative neuromuscular blockade monitors can measure the post-tetanic count, and thereby measure deep or profound neuromuscular blockade when there are no twitches in the train-of-four (the train-of-four ratio is the ratio of the fourth to the first twitch).”

Dr. Bowdle has previously published multiple studies investigating the benefits of acceleromyography versus electromyography, building an expertise on the topic (Anaesthesia 2020;75:1133-35). “We are strongly in favor of routine quantitative twitch monitoring as a standard of care,” Dr. Bowdle shared. “There is a lot of evidence supporting the routine use of quantitative twitch monitoring as a best practice.”

With acceleromyography, depolarization of the ulnar nerve results in contraction of the adductor pollicis, moving the thumb and producing an acceleration detected by the sensor. While this assessment of twitch has been the basis for many commercially available twitch monitors for many years, the technology has two major problems: one, 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 for unparalyzed train-of-four ratio, which should be equal to 1.0, is often greater than 1.0 and may be as high as 1.6 when measured by acceleromyography. Without an obtained baseline, it is impossible to know what train-of-four ratio represents in recovery from neuromuscular blockade in an individual patient. For the acceleromyograph to be used properly, there must be a baseline, unparalyzed train-of-four ratio, and subsequent train-of-four ratios must be normalized by dividing the train-of-four ratio by the baseline train-of-four ratio.

Electromyography solves both of these limitations since it directly measures muscle action potential, requiring no movement for measurement to be made. In addition, a baseline train-of-four ratio is not required.

Though electromyography is not a new technology, it has only recently been made widely available for clinical use. There are now at least five commercially available electromyograph monitors: three are standalone and two are components of physiologic monitoring systems. A comparison study of the TwitchView Monitor and the GE Healthcare Neuromuscular Transmission module performed by Dr. Bowdle et al. using train-of-four count and train-of-four ratio measurements found that the GE electromyograph monitor is prone to erroneously interpreting artifact – especially signal artifact from ulnar nerve stimulation – as twitch data. This error results in the GE monitor frequently counting a larger train-of-four count than either the TwitchView monitor or palpation. Problems with artifact interpretation during use of the GE monitor were also noted in a study by Todd et al. (Anesth Analg 2014;119:323-31).

In a further analysis by Dr. Bowdle in a 2024 study published in Anesthesiology, three acceleromyography monitors, three electromyography monitors, and a mechanomyograph were used to measure the train-of-four ratio (Anesthesiology May 2024). Historically, mechanomyography is the accepted laboratory reference standard for twitch measurement, using a force transducer to directly measure the isometric force of contraction of the thumb. Mechanomyography does not have the shortcomings of either acceleromyography or electromyography, with a baseline train-of-four ratio not significantly exceeding 1 and minimal susceptibility to electrical noise. However, the mechanomyograph is used primarily for research and very seldom used for routine clinical practice. The results of the study showed that electromyography monitors tested were substantially more accurate and precise than acceleromyography monitors and were comparable to mechanomyography.

Despite the evidence, there are limitations. The susceptibility to noise interference for electromyographic monitors presents a significant challenge for the instrument’s design, with management of noise and differentiation of noise from signal as a paramount concern. If noise is not effectively managed, such as electrical noise in the OR, it could be misinterpreted as compound action potentials, overestimating the twitch response. However, overly aggressive noise reduction could cause the monitor to underestimate twitches and lack sensitivity, leading the anesthesiologist to overestimate the depth of paralysis. Therefore, each new electromyography monitor should be validated in comparison to mechanomyography.

When measures have been put in place to effectively manage electrical noise, Dr. Bowdle said, electromyography is the clear choice for superior twitch monitoring. “Electromyographic monitors should be utilized whenever possible, as eventually acceleromyographic monitoring is likely to disappear,” he said. “Health care systems currently utilizing acceleromyography should plan to phase these monitors out and replace them with electromyography.”

Though replacing monitors could seem financially prohibitive, Dr. Bowdle explained that while the per-patient cost for electromyography may appear greater due to the cost of single-use electrode array versus the reusable sensors in acceleromyographic monitoring, health care systems should consider not simply the cost of an electrode array, but the overall cost-effectiveness of monitoring. “Cheaper monitoring that is not as effective may not be advantageous in the long run,” he said. “In addition, reusable sensors require careful cleaning to avoid cross-contamination with pathogens between patients, causing a risk of infection.”

Ultimately, the superiority of electromyography for twitch monitoring is clear. “It’s hard to argue in favor of acceleromyography when the performance of the monitors is not nearly as good as most electromyography monitors,” Dr. Bowdle said. “Electromyography has substantially better accuracy and precision than acceleromyography.”