Consumer reports: Quantitative neuromuscular blockade monitoring
Myron Yaster MD and Debra Faulk MD
We’ve discussed the need for quantitative neuromuscular blockade monitoring to prevent residual neuromuscular block in several previous PAADs. 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. Essentially, there are two main types of commercially available quantitative neuromuscular block monitors; acceleromyographs and electromyographs. Now that the use of quantitative monitoring is supported by ASA and ESA guidelines,1,2 which monitor should you and your group purchase? Today’s PAAD by Wedemeyer et al.3 and its accompanying editorial by Brull and Fuchs-Bader4 is essentially a Consumer Reports evaluation and cutting to the chase, electromyography is the clear winner. Myron Yaster MD
Editorial
Brull SJ, Fuchs-Buder T. Accuracy and Precision of Acceleromyography, Electromyography, and Mechanomyography: Time to Rethink What We Know. Anesthesiology. 2024 Aug 1;141(2):204-207. doi: 10.1097/ALN.0000000000005054. PMID: 38980163.
Original article
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 Aug 1;141(2):262-271. doi: 10.1097/ALN.0000000000005051. PMID: 38728090.
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 (TOFr). A decreasing train-of-four ratio 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 may 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.1,5
There are basically 2 types of clinically available quantitative monitors, acceleromyography (AMG)- and electromyography (EMG)-based monitors. While mechanomyography (MMG) has been considered the ‘gold standard’ comparator, it is no longer commercially available. In the current study, Wedemeyer et al. utilized a laboratory-built mechanomyography as “truth” in comparing the accuracy of acceleromyography- and electromyography-based quantitative monitors. Before going any further, let’s review how these devices work and differ.
“AMG 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 thumb must move freely, such that the arm cannot be tucked at the patient’s side. Another important limitation of AMG is that the baseline, unparalyzed train-of-four ratio often exceeds 1.0, a phenomenon known as “overshoot” or “inverse fade.” In order to address overshoot, experts have recommended “normalizing” the acceleromyographic train-of-four ratio by dividing this result by the baseline train-of-four ratio obtained prior to administration of neuromuscular blocking agents. However, the validity of normalization has never been formally tested with a large set of train-of-four ratio measurements.”3
EMG on the other hand operates by measuring compound muscle action potentials and therefore does not require unencumbered movement of the monitored muscle (adductor pollicis). Baseline stabilization is unnecessary, and the monitor calibration requires less than a minute. Overshoot is not as common nor as pronounced as seen with AMG, and the estimated baseline TOFr with EMG is essentially 1.3
Wedemeyer et al. “evaluated the accuracy (how close the measured train-of-four was to 1.0) and precision (how variable repeated measurements were) of TOFr measurements of three commercially available electromyography-based monitors, three commercially available acceleromyography-based monitors, and a laboratory-built mechanomyograph. The hypothesis was that electromyography and mechanomyography would be more accurate and precise modalities than acceleromyography and that normalization of acceleromyograph TOF would reduce overestimation of the TOFr due to overshoot.”3
An important feature of this study is that none of the patients in the study received neuromuscular blocking agents, thus, the expected train-of-four ratio was 1.0. This was noted to be both a limitation and a strength of the study by the authors. The true comparative performance of these monitors during neuromuscular block cannot be known, but the study design allowed collection of many measurements when the true TOFr is known to be 1. The authors found that “All of the acceleromyograph monitors produced overshoot in the train-of-four ratio (estimated means, 1.10 to 1.13) and substantial variability (gauge SDs, 0.07 to 0.18). Normalization of the train-of-four ratio measured by AMG improved the estimated mean for each device (0.97 to 1.0), but the variability was not improved (gauge SDs, 0.06 to 0.17). EMG and MMG produced minimal overshoot (estimated means, 0.99 to 1.01) and substantially less variation (gauge SDs, 0.01 to 0.02). For EMG and MMG, 0.3% of all train-of-four ratios were outside of the range 0.9 to 1.1. For AMG, 27 to 51% of normalized train-of-four ratios were outside the range of 0.9 to 1.1.”3
The authors conclude that the “three currently available AMG monitors produced overshoot and substantial variability that could be clinically significant. Normalization corrected the overshoot in the average results but did not reduce the wide variability. Three EMG monitors measured the TOFr with minimal overshoot and variability, similar to a MMG.”3 But what does this mean for the anesthesiologist looking to incorporate quantitative monitoring into their practice according to currently recommended guidelines?
The results of the study support that currently available EMG devices provide information that is interchangeable with the gold standard MMG. This may facilitate the adoption of quantitative monitoring in routine clinical care as recommended and supported by recent guidelines (reference ASA and ESIAC).1,2 From our perspective, if you are buying a quantitative monitor, EMG is the modality shown to have greater accuracy, precision, reproducibility, and ease of use compared to AMG. For pediatric anesthesiologists, practical considerations including that EMG can be utilized with minimal set up time and when the arms are tucked to the patient’s sides (the most common patient positioning for pediatric surgery) also supports its use over that of AMG-based quantitative monitors.
The accompanying editorial from Brull and Fuchs-Buder4 does not emphasize preference for one modality over another, but they do point out that use of AMG monitors should be done with knowledge of their limitations. Most notably is that the currently available AMG monitors studied by Wedemeyer et al. demonstrated notable reverse fade which can significant impact clinical decision making. They saw 42% of baseline TOF ratios obtained with AMG monitoring exceeded 1.1, or 110%. This means that for a recovery threshold of 0.9 to be reached, the actual TOFr reading would need to be 1.0. For example, the normalization process would entail the following calculation: 1.1 (raw baseline TOFR) X 0.9 (desired recovery threshold TOFr) = 1.0 (raw recovered TOFr). Even if this conservative measure were assumed (ref Blobner , if all patients were extubated at a TOFr 1.0, 42% of patients would be extubated at a TOFr < 0.9 and residual weakness. This is why normalization and knowledge of this limitation with AMG monitoring is critical to understand.
Whether the choice is EMG or AMG, we are excited by the advancements in quantitative monitoring and whole-heartedly support adoption of this technology to support the safe use and antagonisms of neuromuscular block in our patients.
What do you think? Send your thoughts and comments to Myron who will post in a Friday reader response.
References
1. Thilen SR, Weigel WA, Todd MM, et al. 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(1):13-41. (In eng). DOI: 10.1097/aln.0000000000004379.
2. Fuchs-Buder T, Romero CS, Lewald H, et al. Peri-operative management of neuromuscular blockade: A guideline from the European Society of Anaesthesiology and Intensive Care. European journal of anaesthesiology 2023;40(2):82-94. (In eng). DOI: 10.1097/eja.0000000000001769.
3. Wedemeyer Z, Michaelsen KE, Jelacic S, et al. 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(2):262-271. (In eng). DOI: 10.1097/aln.0000000000005051.
4. Brull SJ, Fuchs-Buder T. Accuracy and Precision of Acceleromyography, Electromyography, and Mechanomyography: Time to Rethink What We Know. Anesthesiology 2024;141(2):204-207. (In eng). DOI: 10.1097/aln.0000000000005054.
5. Murphy GS, Brull SJ. Quantitative Neuromuscular Monitoring and Postoperative Outcomes: A Narrative Review. Anesthesiology 2022;136(2):345-361. (In eng). DOI: 10.1097/aln.0000000000004044.