Monitor the little finger if you must, but use the non-dominant hand

Design

prospective randomised comparison with simultaneous within-patient electromyographic recording from both muscles

Population

44 patients under general anaesthesia receiving rocuronium and sugammadex

Primary outcome

onset time to train-of-four count 0, recovery to train-of-four ratio 0.9 or more, and time to ratio 1.0

Effect

no overall difference between muscles; on the right hand the abductor digiti minimi read recovery 93 s (25%) earlier for a ratio of 0.9 and 105 s (20%) earlier for 1.0

Guidelines say to record the train-of-four at the adductor pollicis. Many anaesthetists use the abductor digiti minimi instead, because electrodes sit more reliably over the little finger and the trace is cleaner. Forty-four patients under general anaesthesia had simultaneous electromyographic recording from both muscles, with the hand allocated to each muscle randomised and handedness recorded.

Overall there were no significant differences between the two muscles in onset time to a train-of-four count of zero after rocuronium, in recovery to a train-of-four ratio of 0.9 or more after sugammadex, or in time to a ratio of 1.0. That is reassurance for common practice. But the hand mattered. When the abductor digiti minimi was monitored on the right hand, it read recovery as occurring earlier than the adductor pollicis — by a mean of 93 seconds, 25% faster, for a ratio of 0.9, and 105 seconds, 20% faster, for a ratio of 1.0. On the left hand there was no difference.

So the practical rule is not 'use the adductor pollicis' but 'use the non-dominant arm'. A little-finger trace on the dominant hand will tell you the patient is reversed before the thumb agrees, and the direction of that error is the unsafe one: it declares adequate reversal early, which is exactly how residual block reaches recovery.

This is a small study, 44 patients, and the handedness effect is a subgroup finding within it. But it is a costless change — monitor the non-dominant arm, which is usually the more accessible one anyway — and it removes a systematic error in the direction that matters.

• Place neuromuscular monitoring electrodes on the non-dominant arm as the default
• If the dominant hand must be used, record from the adductor pollicis rather than the abductor digiti minimi
• Note the direction of the error: little-finger monitoring on the dominant hand reports recovery about 90 to 105 seconds early
• Keep quantitative monitoring to a train-of-four ratio of 0.9 or more before extubation regardless of site
• Record which muscle and which arm were monitored, so a later concern about residual block can be interpreted

The statistics, in plain English

Ninety-three seconds is a mean difference, not a limit of agreement, so individual patients will differ by more. The overall equivalence between muscles is a negative finding in 44 patients, which cannot exclude a modest difference, and the handedness effect emerged from splitting that small sample by hand — a subgroup analysis. What makes it worth acting on is not statistical strength but asymmetry of consequence: the error runs towards declaring reversal too early, and the correction costs nothing.

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