Pharmacology · Neuromuscular Blocking Drugs

Clinical Applications and Monitoring

Train-of-four monitoring, disease-altered sensitivity, and reversal decision guide

A TOF ratio below 0.9 defines clinically significant residual block — subjective assessment of twitch strength is unreliable at this margin, and objective monitoring is required before extubation.
Several drug classes potentiate nondepolarizing block by reducing presynaptic acetylcholine release or increasing postsynaptic sensitivity: aminoglycosides, volatile anesthetics, magnesium, and calcium channel blockers all require dose vigilance.
MG destroys postsynaptic nicotinic receptors, producing exquisite sensitivity to nondepolarizing agents and relative resistance to succinylcholine; LEMS impairs presynaptic calcium-dependent release, sensitizing patients to both drug classes.
Neostigmine reversal requires antimuscarinic pretreatment, cannot overcome deep block, and demands at least one to two TOF twitches before administration — premature reversal is a documented cause of postoperative respiratory failure.
Sugammadex encapsulates rocuronium or vecuronium and reverses even zero-TOF block without antimuscarinic pretreatment — the preferred choice when block is deep, reversal is urgent, or neostigmine is inadequate.

	Myasthenia Gravis	Lambert-Eaton Myasthenic Syndrome
Defect site	Postsynaptic — nicotinic receptor loss	Presynaptic — voltage-gated calcium channel antibodies
Nondepolarizing agents	Exquisitely sensitive — use minimal doses	Sensitive — reduced acetylcholine release
Succinylcholine	Relatively resistant — fewer receptors to depolarize	Sensitive — receptors underactivated at baseline
Association	Thymoma	Small cell lung cancer

Suggested References

Author / Organization	Title	Source
Katzung BG (ed)	Basic and Clinical Pharmacology, 15th ed. Chapter 27: Skeletal Muscle Relaxants	McGraw-Hill, 2021
Brunton LL, Knollmann BC (eds)	Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed. Chapter 12: Neuromuscular Blocking Agents	McGraw-Hill, 2023
Murphy GS, Szokol JW, Marymont JH, et al	Residual neuromuscular blockade and critical respiratory events in the postanesthesia care unit	Anesth Analg. 2008;107(1):130–7
Naguib M, Brull SJ, Johnson KB	Conceptual and technical insights into the basis of neuromuscular monitoring	Anaesthesia. 2017;72(Suppl 1):16–37
Drachman DB	Myasthenia gravis	N Engl J Med. 1994;330(25):1797–810
Tim RW, Sanders DB	Lambert-Eaton myasthenic syndrome: clinical diagnosis, immune-mediated mechanisms, and update on therapies	Ann Neurol. 1994;37(Suppl 1):S63–73
Martyn JAJ, Fagerlund MJ, Eriksson LI	Basic principles of neuromuscular transmission	Anaesthesia. 2009;64(Suppl 1):1–9