RIP acceleromyography
Myron Yaster MD and Debra Faulk MD
Newton's Second Law of Motion is expressed as F = ma, where F represents the net force acting on an object, m is its mass, and a is its acceleration. This formula indicates that the force applied to an object is directly proportional to its mass and the resulting acceleration.
We’ve discussed the need for quantitative neuromuscular blockade monitoring to prevent residual neuromuscular block in several previous PAADs (August 15, 2024 https://ronlitman.substack.com/p/consumer-reports-quantitative-neuromuscular July 02/2024 https://ronlitman.substack.com/p/quantitative-neuromuscular-blockade-67e ). Mechanomyography has been considered the accepted gold standard of quantitative neuromuscular block monitors against which all other monitors are evaluated for accuracy, but is no longer commercially available and to be honest was more of a research tool than a practical clinical monitor. Essentially, there are two main types of commercially available quantitative neuromuscular block monitors; acceleromyographs (AMG) and electromyographs (EMG). Now that the use of quantitative monitoring is supported by ASA and ESA guidelines,1,2 which monitor should you and your group purchase? In the previous PAAD review of the paper by Wedemeyer et al.3 and its accompanying editorial by Brull and Fuchs-Buder4, which was essentially a Consumer Reports evaluation of quantitative monitors, it was pretty clear that electromyography was substantially better than acceleromyography. Today’s editorial by Bowdle et al.5 and the paper by Kopman and Brull6 essentially puts the final nail in the acceleromyography coffin. Myron Yaster MD
Editorial
Bowdle A, Jelacic S, Michaelsen K. Let's Say Goodbye to Acceleromyographic Twitch Monitoring. Anesthesiology. 2025 Jun 1;142(6):1002-1004. doi: 10.1097/ALN.0000000000005439. Epub 2025 May 13. PMID: 40358335; PMCID: PMC12077598.
Research letter
Kopman AF, Brull SJ. Etiology of Acceleromyographic Train-of-four Overshoot: A Hypothesis. Anesthesiology. 2025 Jun 1;142(6):1171-1173. doi: 10.1097/ALN.0000000000005377. Epub 2025 Apr 7. PMID: 40193198.
Before reading today’s PAAD, let’s cut to the chase: quantitative neuromuscular blockade monitoring to prevent residual neuromuscular block is now an ASA and ESA guideline. The most commonly used quantitative monitors in the last several decades have been AMG-based monitors, though electromyography is currently considered the gold standard by many experts in the field. Today’s article by Kopman and Brull provides insight into a known limitation of AMG monitoring: acceleromyography’s tendency to overshoot when measuring the train of four ratio (TOFr), producing baseline ratios greater (sometimes significantly) than 100%. As the accompanying editorial by Bowdle mentions, Kopman and Brull show that this limitation seems ‘baked into’ AMG monitoring. What’s the bottom line? When you and your practice are in the market to buy quantitative neuromuscular monitors, choosing a product based on electromyography is the way to go.5 Okay, read on…
As way of reminder: The train-of-four (TOF) stimulation involves the delivery of four brief electrical pulses to a peripheral nerve at the rate of 2 Hz and assessing the “twitches” that result. During the onset of block when four twitches are still present, the amplitude of the fourth twitch divided by that of the first can be measured quantitatively to permit the calculation of the train of four ratio or TOFr. A decreasing TOFr indicates greater depth of block. With increasing depth of block, sequential twitches in the TOF decrease in amplitude with the progressive disappearance of the fourth, then the third, then the second, and finally the first twitch. When the train-of-four count goes to zero, one should perform a post-tetanic count (PTC) to understand the depth of blockade. At a PTC of zero, the patient is in complete block. Recovery from neuromuscular block may occur spontaneously over time, or can be facilitated with antagonists such as neostigmine or sugammadex. Current recommendations are not to extubate until the TOFr is ≥ 90% as measured quantitatively at the adductor pollicis.
AMG-monitoring is based on Newton’s second law of motion (f=ma or force=mass times acceleration) and utilizes a piezoelectric sensor that measures tissue acceleration from nerve stimulation and resulting muscle contraction, ideally at adductor pollicis. A key limitation of acceleromyography is that the monitored muscle (ideally thumb motion due to adductor pollicis contraction) must move freely. Therefore, the arm cannot be tucked at the patient’s side which is especially relevant for the pediatric anesthesiologist. Another important limitation of AMG is that the baseline, unparalyzed TOFr often exceeds 1, a phenomenon known as “overshoot” or “inverse fade.” In order to address overshoot, TOFr results must be “normalized” by dividing this result by the baseline train-of-four ratio obtained prior to administration of neuromuscular blocking agents. For example, if the baseline TOFr is 120%, then adequate recovery to a TOFr ≥ 90% would be 90% of 120% or 108%! Recovery to a non-normalized AMG reading of 90% in this patient would represent a TOFr of 75%! Without normalizing TOFr results during recovery, inaccurate and likely underestimation of recovery will occur placing patients at risk of residual neuromuscular block. Unfortunately, this step may be commonly dismissed in a busy, fast-moving clinical practice, or can be difficult if NMB administration occurs unexpectedly in the middle of a case.
Why does overshoot occur? It has been previously proposed that this is an artifact created when the thumb does not return to its original position following the first stimulus and twtich response in the train-of-four. Strategies or devices to ensure the thumb returns to its initial position have claimed to address the problem of overshoot and some new AMG monitors will only report TOFr to a maximum value of 100% (i.e. ignore the problem). It has also been suggested that when using AMG-based monitors, the TOFr should be 100% rather than 90% to ensure adequate recovery. Unfortunately, this only works if the baseline TOFr is ≤ 110% and in the previous study by Wedemeyer et al. baseline AMG TOFr was greater than 110% in 2/3rds of study patients, suggesting a significant proportion of patients could suffer residual block with this strategy (3).
In an elegant re-analysis of older and unpublished data, Kopman and Brull provide a hypothesis to explain the mechanism of overshoot.6 “The likely explanation for the overshoot is an increase in the acceleration of the thumb twitch, between the first and the forth twitch in the train-of-four response. Neuromuscular block reduces both the force and acceleration of the thumb in response to ulnar nerve stimulation, but not necessarily in equal proportions as was previously assumed. Kopman and Brull have shown that in the absence of neuromuscular block, the acceleration increases slightly during successive twitches in the train-of-four response, despite the force remaining constant (as measured directly by mechanomyography). This is the fundamental flaw in acceleromyography—the relationship between force and acceleration is not constant during the train-of-four response”.4,6 If this limitation is indeed ‘baked in to acceleromyography’, then normalization can address correct interpretation of the results, but none of the previously mentioned strategies will successfully address overshoot.
EMG, by comparison, measures compound muscle action potentials and therefore does not require unencumbered movement of the monitored muscle. Overshoot is not as common nor as pronounced as seen with AMG, and the estimated baseline TOFr with EMG is essentially one, or 100%. “Acceleromyographic monitors are substantially less precise (repeated measurements had greater variability) and less accurate (measurements deviated more from the expected value) than electromyographic monitors.”3,5
We hope that all of you are getting on the quantitative neuromuscular blockade monitoring band wagon. While many experts in the field agree that EMG is the current gold standard in quantitative monitoring, we applaud the use of any quantitative means of assessing neuromuscular block! If the current quantitative monitors available to you are acceleromyographs, use them armed with this knowledge of their limitations and when ready to replace them, consider that EMG-based monitors are superior in many ways. If you are preparing or ready to introduce quantitative monitoring to your practice, consider that EMG is really the way to go!
Send Myron your thoughts and we’ll publish in a Friday reader response.
Conflict of interest
Dr. Brull the author of today’s paper declares intellectual property assigned to Mayo Clinic (Rochester, Minnesota); was a consultant for Merck & Co., Inc. (Rahway, New Jersey); is a principal, shareholder, and chief medical officer in Senzime AB (Uppsala, Sweden); is an unpaid member of scientific advisory boards for NMD Pharma (Aarhus, Denmark) and Takeda Pharmaceuticals (Tokyo, Japan); and is a member of the editorial boards for Anesthesiology; Journal of Clinical Anesthesia; Anesthesia, Critical Care & Pain Medicine; and Journal of Clinical Medicine. Dr. Kopman declares no competing interests. Note from Myron Senzime AB manufactures Tetragraph, a quantitative neuromuscular monitor using electromyography.
References
1. Thilen SR, Weigel WA, Todd MM, Dutton RP, Lien CA, Grant SA, Szokol JW, Eriksson LI, Yaster M, Grant MD, Agarkar M, Marbella AM, Blanck JF, Domino KB: 2023 American Society of Anesthesiologists Practice Guidelines for Monitoring and Antagonism of Neuromuscular Blockade: A Report by the American Society of Anesthesiologists Task Force on Neuromuscular Blockade. Anesthesiology 2023; 138: 13–41
2. Fuchs-Buder T, Romero CS, Lewald H, Lamperti M, Afshari A, Hristovska AM, Schmartz D, Hinkelbein J, Longrois D, Popp M, de Boer HD, Sorbello M, Jankovic R, Kranke P: Peri-operative management of neuromuscular blockade: A guideline from the European Society of Anaesthesiology and Intensive Care. Eur J Anaesthesiol 2023; 40: 82–94
3. Wedemeyer Z, Michaelsen KE, Jelacic S, Silliman W, Lopez A, Togashi K, Bowdle A: Accuracy and Precision of Three Acceleromyographs, Three Electromyographs, and a Mechanomyograph Measuring the Train-of-four Ratio in the Absence of Neuromuscular Blocking Drugs. Anesthesiology 2024; 141: 262–271
4. Brull SJ, Fuchs-Buder T: Accuracy and Precision of Acceleromyography, Electromyography, and Mechanomyography: Time to Rethink What We Know. Anesthesiology 2024; 141: 204–207
5. Bowdle A, Jelacic S, Michaelsen K: Let's Say Goodbye to Acceleromyographic Twitch Monitoring. Anesthesiology 2025; 142: 1002–1004
6. Kopman AF, Brull SJ: Etiology of Acceleromyographic Train-of-four Overshoot: A Hypothesis. Anesthesiology 2025; 142: 1171–1173