Quantitative neuromuscular blockade monitoring: Placement makes a difference
Myron Yaster MD and Debra Faulk MD
Recently published guidelines by the American Society of Anesthesiologists (ASA)1 and the European Society of Anesthesiology and Intensive Care2 have made clear that only quantitative assessment of neuromuscular function with electromyography (EMG), mechanomyography, or acceleromyography at the adductor pollicis can accurately measure a train of four ratio (TOFr). As a 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 blockade 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 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 completely blocked.
Current recommendations are not to extubate until the TOFr is >90% as measured quantitatively at the adductor pollicis. Over the past 2 years, we’ve reviewed the topic of neuromuscular blockade, monitoring and antagonism in several PAADs. I am including some of our previous PAADs with their hyperlinks for those of you interested in a deeper dive.
PAAD 01/11/2023 Neuromuscular blockade Part 1 https://ronlitman.substack.com/publish/posts/detail/94707173?referrer=%2Fpublish%2Fposts%3Fsearch%3Dneuromuscular
PAAD 01/12/2023 Neuromuscular blockade Part 2 https://ronlitman.substack.com/p/neuromuscular-blockade-part-2
PAAD 07/15/2021 Neuromuscular blockade in pediatrics: some inconvenient truths https://ronlitman.substack.com/p/neuromuscular-blockade-in-pediatrics
PAAD 07/6/2022 “I don’t need no stinkin’ quantitative neuromuscular blockade monitor”…you probably do https://ronlitman.substack.com/publish/posts/detail/61905663?referrer=%2Fpublish%2Fposts%3Fsearch%3Dneuromuscular
PAAD 08/14/2023 Sugammadex reversal of neuromuscular blockade requires quantitative monitoring https://ronlitman.substack.com/p/sugammadex-reversal-of-neuromuscular
If we assume that quantitative neuromuscular blockade monitoring is essential, much like the use of pulse oximetry to assess oxygenation, in today’s PAAD, Tobias et al.3 wondered if the electromyographic (EMG) responses from the adductor pollicis (hand), the usual preferred monitoring site, and the flexor hallucis brevis (foot) muscles are equivalent if the hand is unavailable? Why would the hand be unavailable? We’ve all encountered, especially in young infants and children, that a single IV will knock out the real-estate of the entire limb for any other use. Alternatively, the upper extremity may be tucked and out of sight. If the upper extremities are not available, is it OK to move to the lower extremities for neuromuscular monitoring? Today’s article is important because, we actually don’t have a lot of evidence-based data on quantitative neuromuscular blockade monitoring in children to inform our practice.4, 5 The good news is that with the increasing availability of pediatric friendly, easy to use out of the box quantitative monitors, clinical research to guide practice, like this article by Tobias et al. is right around the corner. Indeed, quantitative monitoring of neuromuscular blockade is a potential research gold mine for many of you in academic medicine. Deb and I, and I hope you, are looking forward to seeing much more research in this area in the near future. Myron Yaster MD
Original article
Tobias JD, Epstein RH, Rice-Weimer J, Yemele Kitio SA, Brull SJ, Kalsotra S. Pediatric Intraoperative Electromyographic Responses at the Adductor Pollicis and Flexor Hallucis Brevis Muscles: A Prospective, Comparative Analysis. Anesth Analg. 2024 Jul 1;139(1):36-43. doi: 10.1213/ANE.0000000000006926. Epub 2024 Jun 17. PMID: 38885397.
“Quantitative measurement of neuromuscular blockade allows for objective criteria including not only the number of twitches in the train of four (TOF) sequence (TOF count, or TOFC) but also the calculation of the TOF ration (TOFR) by measuring the individual response of the fourth and first twitch heights and calculating their ratio (T4/T1 ratio or TOFR).”3, 6, 7 Quantitative technology for TOF monitoring includes mechanomyography, acceleromyography, or electromyography (EMG). Unfortunately, quantitative monitoring has not yet taken off in infants and children.4 “Cited reasons for failure to monitor neuromuscular block in infants and children have included a lack of reliable monitors for use in this population, the potential impact on clinical productivity due to the time required to apply the devices, and challenges with calibration of existing acceleromyographic monitors.”3 Acceleromyography requires a limb that can move freely for accuracy, which is often impossible in pediatrics because the arms tucked along the patients side, or the hand is being used for IV access or other monitors. Electromyography may not have these problems .because it does not require muscle movement and instead measures the amplitude of the individual EMG responses. Nevertheless, access to the upper extremity is often limited. “The current study evaluated the feasibility of recording cAMPs from the lower extremity and compared paired responses from the foot and the hand in pediatric patients undergoing surgical procedures requiring the administration of an NMBA.”3
In this single center study (Nationwide Children’s Hospital, Columbus OH), 50 patients were simultaneously monitored in the hand (ulnar nerve, adductor pollicis muscle) and foot (posterior tibial nerve, flexor hallucis brevis muscle) as seen in the photo using the Tetragraph monitor (Senzime AB).
Ok, what did they find? “The onset of neuromuscular block and recovery after antagonism with sugammadex were delayed at the foot compared to the hand.” Thus, when using the foot as an alternative site “delays at the foot in neuromuscular block onset and recovery need to be considered when timing laryngoscopy and tracheal extubation, respectively”3 The authors concluded: “Quantitative EMG-based TOF monitoring at the foot of infants and children can help clinical care align with the recently introduced practice guidelines for monitoring of neuromuscular block in adult patients if the hand is not available for neuromuscular monitoring. Although the ulnar nerve and adductor pollicis muscle remain the recommended site, alternative sites may be required due to patient positioning, surgical draping, surgical procedure, or the use of the upper extremity for other invasive and noninvasive monitoring devices. The EMG-based quantitative monitor functioned effectively on the lower extremity, but there were potentially clinically relevant differences when compared to results from the hand, including slower onset and recovery times. These differences need to be considered when interpreting the information from the monitor. Additional studies correlating the neuromuscular response at the foot with conditions for tracheal intubation and extubation are needed to develop recommendations on the timing of these events.”3
Why these responses at the hand and foot are different is unclear, but not unheard-of. This phenomenon has been noted in previous adult studies, though methodological differences have made meaningful comparisons between studies and to the current study difficult.8, 9 Proposed mechanisms for this difference have included effects of regional blood flow, muscle fiber composition (fast vs slow-twitch), fiber size in individual muscles, or even the type of NMB agent used and variable sites of action of each at the neuromuscular junction. For children, the changes in physiologic make-up and response must also be factored into these potentially complex interactions. Tobias et al. speculate that “longer circulation time to the foot than to the hand (and thus, delayed delivery of NMBA to the foot) and to different foot muscle sensitivity to NMBA effects compared with the hand muscles.”3 We feel that more likely than circulation time, greater or lesser regional blood flow to a given muscle and differential sensitivities of those muscles may be at play (such as the diaphragm versus obicullaris oculi vs pharyngeal muscle sensitivities).
Ultimately, one would like to understand the depth of block or extent of recovery in the muscles of clinical interest. For intubation, that may be when jaw and vocal cords relaxation gives good intubating conditions. For intraoperative conditions, that will depend on the surgical procedure – do you need the diaphragm to be quiet for an intrathoracic case, or peripheral musculature for surgery on the extremities? For extubation, return of pharyngeal muscle tone to maintain airway patency is desired. Unfortunately, it is impractical to monitor at all of these sites, but understanding how monitoring at particular muscles, relate to others can give us useful information to guide these practices. Tobais et al. provide important new contributions to this literature and our practice and portends to even more discovery in the future.
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. Jan 1 2023;138(1):13-41. 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. Feb 1 2023;40(2):82-94. doi:10.1097/eja.0000000000001769
3. Tobias JD, Epstein RH, Rice-Weimer J, Yemele Kitio SA, Brull SJ, Kalsotra S. Pediatric Intraoperative Electromyographic Responses at the Adductor Pollicis and Flexor Hallucis Brevis Muscles: A Prospective, Comparative Analysis. Anesthesia and analgesia. Jul 1 2024;139(1):36-43. doi:10.1213/ane.0000000000006926
4. Faulk DJ, Austin TM, Thomas JJ, Strupp K, Macrae AW, Yaster M. A Survey of the Society for Pediatric Anesthesia on the Use, Monitoring, and Antagonism of Neuromuscular Blockade. Anesthesia and analgesia. Jun 1 2021;132(6):1518-1526. doi:10.1213/ane.0000000000005386
5. Faulk DJ, Karlik JB, Strupp KM, et al. The Incidence of Residual Neuromuscular Block in Pediatrics: A Prospective, Pragmatic, Multi-institutional Cohort Study. Cureus. Mar 2024;16(3):e56408. doi:10.7759/cureus.56408
6. Murphy GS, Brull SJ. Quantitative Neuromuscular Monitoring and Postoperative Outcomes: A Narrative Review. Anesthesiology. Feb 1 2022;136(2):345-361. doi:10.1097/aln.0000000000004044
7. Brull SJ, Kopman AF. Current Status of Neuromuscular Reversal and Monitoring: Challenges and Opportunities. Anesthesiology. Jan 2017;126(1):173-190. doi:10.1097/aln.0000000000001409
8. Heier T, Hetland S. A comparison of train-of-four monitoring: mechanomyography at the thumb vs acceleromyography at the big toe. Acta anaesthesiologica Scandinavica. May 1999;43(5):550-5. doi:10.1034/j.1399-6576.1999.430511.x
9. Betz M, Aguirre J, Schubert M, et al. Hand or foot train-of-four tests and surgical site muscle relaxation assessed with multiple motor evoked potentials: A prospective observational study. European journal of anaesthesiology. Aug 1 2021;38(8):872-879. doi:10.1097/eja.0000000000001398