Neuromuscular blocking agents (NMBs) are used in general anesthesia to relax skeletal muscles, facilitate tracheal intubation, and improve surgical conditions by reducing patient movement. However, the safe and effective use of these agents requires accurate monitoring to ensure that the desired level of muscle relaxation has been achieved and that full respiratory recovery occurs prior to extubation. Inadequate monitoring can result in complications such as intraoperative patient movement or postoperative residual muscle weakness going undetected. There are several methods to test the success of neuromuscular blockade, including electrical nerve stimulation and twitch monitoring.
Peripheral nerve stimulation is commonly used to test the success of neuromuscular blockade. This involves delivering small electrical pulses to a peripheral motor nerve and observing the corresponding muscle contractions. One commonly tested site is the ulnar nerve at the wrist—its stimulation elicits a visible twitch of the thumb via the adductor pollicis muscle. Another commonly tested site is the facial nerve. By comparing muscle responses before and after drug administration, clinicians can gauge the onset, depth, and duration of a neuromuscular block 1,2.
Train-of-four monitoring is the gold standard for accurately assessing the depth of neuromuscular blockade. It involves delivering four electrical impulses in rapid succession. In the absence of a neuromuscular blockade, all four muscle twitches will appear with equal strength. As the degree of blockade increases, the number and amplitude of twitches decrease. A train-of-four count of zero indicates a deep blockade, while the presence of one to four twitches reflects varying degrees of partial block. Clinicians also measure the train-of-four ratio: the strength of the fourth twitch compared to the first. A ratio of 0.9 or greater is generally accepted as reflecting adequate recovery post-anesthesia 3–5.
When no twitches are observed during train-of-four stimulation, a useful tool can be the post-tetanic count. This method applies a high-frequency tetanic stimulus followed by single pulses to assess the extent of a profound neuromuscular block. The number of visible twitches that follow the tetanus reflects how deeply the blockade has been established and can help estimate the time to the return of train-of-four responses 4,6,7.
Double-burst stimulation offers an alternative to train-of-four for detecting residual blockade. The clinician delivers two brief bursts of electrical stimuli and compares the muscle responses to each burst. Fade, or a reduction in strength from the first burst to the second, signals the presence of residual blockade. This technique can improve the sensitivity of a manual or visual assessment, particularly when neuromuscular monitoring equipment is limited 8,9.
While less reliable than quantitative methods, clinical signs are often used to complement neuromuscular monitoring. These may include the patient’s ability to lift their head or hand for several seconds, generate a strong grip, open their eyes fully, or produce an effective cough. While these indicators offer practical reassurance, they can be misleading if used alone 10,11.
The use of accurate and reliable tests of neuromuscular blockade success is a cornerstone of safe anesthetic practice. Combining technology with clinical judgment ensures optimal muscle relaxation during surgery and promotes full neuromuscular recovery in the postoperative period.
References
1. Monitoring neuromuscular blockade – UpToDate. https://www.uptodate.com/contents/monitoring-neuromuscular-blockade.
2. Rodney, G., Raju, P. & Brull, S. J. Neuromuscular block management: evidence-based principles and practice. BJA Education 24, 13–22 (2024). DOI: 10.1016/j.bjae.2023.10.005
3. Duţu, M. et al. Neuromuscular monitoring: an update. Rom J Anaesth Intensive Care 25, 55–60 (2018). DOI: 10.21454/rjaic.7518.251.nrm
4. openanesthesia. Monitoring Depth of Neuromuscular Blockade. OpenAnesthesia https://www.openanesthesia.org/keywords/monitoring-depth-of-neuromuscular-blockade/.
5. Peripheral Nerve Stimulator – Train of Four Monitoring: Overview, Periprocedural Care, Technique. (2023).
6. Hakim, D., Drolet, P., Donati, F. & Fortier, L.-P. Performing post-tetanic count during rocuronium blockade has limited impact on subsequent twitch height or train-of-four responses. Can J Anesth/J Can Anesth 63, 828–833 (2016). DOI: 10.1007/s12630-016-0619-9
7. Baykara, N., Solak, M. & Toker, K. Predicting recovery from deep neuromuscular block by rocuronium in the elderly. J Clin Anesth 15, 328–333 (2003). DOI: 10.1016/s0952-8180(03)00062-x
8. Drenck, N. E. et al. Manual evaluation of residual curarization using double burst stimulation: a comparison with train-of-four. Anesthesiology 70, 578–581 (1989). DOI: 10.1097/00000542-198904000-00003
9. Engbaek, J., Ostergaard, D. & Viby-Mogensen, J. Double burst stimulation (DBS): a new pattern of nerve stimulation to identify residual neuromuscular block. Br J Anaesth 62, 274–278 (1989). DOI: 10.1093/bja/62.3.274
10. Price, S. W. & Ved, S. Monitoring Neuromuscular Function. in Anesthesiology Core Review: Part One Basic Exam (eds. Freeman, B. S. & Berger, J. S.) (McGraw-Hill Education, New York, NY, 2014).
11. Monitoring of Neuromuscular Blockade: What Would You Expect If You Were the Patient? Anesthesia Patient Safety Foundation https://www.apsf.org/article/monitoring-of-neuromuscular-blockade-what-would-you-expect-if-you-were-the-patient/.