Anaes · Neuromuscular blockade & reversal

Neuromuscular monitoring and the train-of-four

Also known as Train-of-four (TOF) monitoring · Peripheral nerve stimulation · Quantitative neuromuscular monitoring · Acceleromyography · Post-tetanic count

Neuromuscular monitoring is the objective assessment of the depth and recovery of a neuromuscular block by delivering a supramaximal electrical stimulus to a peripheral motor nerve and observing the evoked muscle response. The train-of-four — four stimuli at 2 Hz, 0.5 seconds apart — is the workhorse pattern: it yields a TOF count (the number of detectable twitches, 0 to 4) and a TOF ratio (T4 divided by T1). A non-depolarising block shows fade (ratio below 1.0, with sequential loss of T4 then T3 then T2 then T1) whereas a depolarising phase I block shows no fade (all four twitches reduced proportionally, ratio near 1.0). Residual neuromuscular blockade, defined as a TOF ratio below 0.9 at extubation, causes postoperative airway obstruction, aspiration and hypoxia; the 2023 ASA practice guideline recommends quantitative monitoring whenever a neuromuscular blocker is administered, with a confirmed TOF ratio of at least 0.9 before extubation.