Residual block is common, invisible, and dangerous

Why it matters

The patient who arrives in the PACU breathing, obeying commands and holding their head up can still have a train-of-four ratio far below the threshold at which the pharyngeal muscles protect the airway. In RECITE-US, patients were judged suitable for extubation by routine clinical judgment and qualitative peripheral nerve stimulation, and 64.7% of them (165 of 255) had a TOF ratio below 0.9 at extubation, with 31.0% of those below 0.6 saager-2019. The examination that was supposed to catch residual block passed two-thirds of the patients who had it.

The consequence of getting this wrong arrives in the first minutes of the PACU stay, as hypoxemia or airway obstruction in a patient nobody thought was at risk.

Mechanism

Non-depolarizing blockers occupy nicotinic receptors at the neuromuscular junction. Recovery is a gradient: the diaphragm and the large muscles that generate a head lift or a hand grip recover before the small muscles of the pharynx and the upper esophagus that keep the airway open and protect it from aspiration. A patient can therefore breathe adequately and pass every bedside test while the muscles that matter in the PACU are still partly blocked. Qualitative nerve stimulation, watching or feeling for fade, cannot resolve the difference between a TOF ratio of 0.5 and 0.9; only a quantitative monitor that measures the ratio can. That gap between what the bedside can see and what the junction is doing is the whole mechanism of the day.

Evidence

Murphy 2008 screened 7459 general anesthetics over one year; 61 patients developed a predefined critical respiratory event in the first 15 min of PACU stay, and 42 were matched to controls murphy-2008. Mean TOF ratio was 0.62 (SD 0.20) in cases and 0.98 (SD 0.07) in controls, a difference of 0.36 (95% CI 0.30 to 0.43, P less than 0.0001); 73.8% of cases had a TOF ratio below 0.70 and no control did murphy-2008. The commonest events were severe hypoxemia in 22 of 42 (52.4%) and upper airway obstruction in 15 of 42 (35.7%) murphy-2008.

RECITE-US supplies the prevalence: in a blinded multicenter cohort across 10 US community and academic hospitals, 64.7% of patients had residual block at extubation despite having been judged suitable for extubation by clinical judgment and qualitative stimulation saager-2019. The Canadian RECITE study is the companion incidence estimate; its primary record has not been reached, so no figure from it appears here fortier-2015.

The 2023 ASA practice guideline states a TOF ratio of 0.9 or more as the recovery threshold, recommends quantitative monitoring over qualitative or clinical assessment, recommends sugammadex for deep, moderate and shallow rocuronium- or vecuronium-induced block, describes neostigmine as a reasonable alternative only from minimal block (TOF ratio 0.4 to less than 0.9), and states that patients with a documented quantitative TOF ratio of 0.9 or more do not require pharmacological antagonism thilen-2023. Its full text was read on 2026-09-06 and the four recommendations that matter here all carry the same grading, strong strength of recommendation on moderate strength of evidence: against clinical assessment alone, because the assessment is insensitive; for quantitative monitoring over qualitative assessment; for confirming a train-of-four ratio of 0.9 or more before extubation; and for using the adductor pollicis rather than the eye muscles thilen-2023.

The size of the effect the guideline is asking for is in its own pooled figures. With sugammadex and a confirmed train-of-four ratio of 0.9 or more before extubation, the pooled incidence of residual blockade was 0.5%, 95% CI 0.0 to 6.0, against 2.2%, 0.5 to 9.0, when the ratio was not confirmed; with neostigmine the same comparison was 5.3%, 2.5 to 10.7, against 44.9%, 29.9 to 60.8 thilen-2023. That last pair is the number to remember. The guideline also states its own limit, verbatim: direct evidence from randomized trials that compare confirming or not confirming train-of-four ratios before extubation are lacking thilen-2023.

POPULAR, a prospective cohort of 22,803 patients from 211 hospitals in 28 European countries, found that use of neuromuscular blocking agents was associated with postoperative pulmonary complications, adjusted OR 1.86 (95% CI 1.53 to 2.26), and reported that neither reversal agents nor neuromuscular monitoring reduced that risk in the cohort kirmeier-2019. Its primary endpoint was a complication occurring or persisting after PACU discharge, so block treated in the PACU is largely excluded from it kirmeier-2019.

An exploratory reanalysis of POPULAR compared a 0.95 with a 0.9 extubation threshold and carries the contested position that 0.9 may be too permissive; that position has been disputed in print blobner-2020.

What this does not show

No randomized trial shows that residual block below 0.9 causes PACU respiratory morbidity, and there could not easily be one murphy-2008. Murphy 2008 is a matched case-control study nested in a single-center cohort; RECITE-US and POPULAR are observational cohorts; the ASA document is a guideline murphy-2008 saager-2019 kirmeier-2019 thilen-2023. The association is strong and consistent. Whether monitoring and reversal prevent the harm is not established here, and POPULAR points the other way for the complications it measured kirmeier-2019. That is why the claim says common, invisible and associated, and not most preventable.

The threshold itself is a convention under challenge. The reanalysis arguing for 0.95 is exploratory and contested, and this page does not adopt it blobner-2020.

The clinical signs fail at a measured point. In awake volunteers, a sustained 5-s head lift was achieved at a mean TOF ratio of 0.60 (range 0.45 to 0.75); diplopia and difficulty tracking a moving object accompanied a ratio of 0.90 or below in every subject; the incisor clench on a tongue depressor did not return until the ratio exceeded 0.85; and grip strength was 59% of control at 0.70, where every subject was symptomatic kopman-1997. A head lift is present well below the threshold the guideline sets.

Neostigmine has a ceiling written on its label: a dose of 0.03 to 0.07 mg/kg will generally achieve a TOF ratio of 90%, the recommended maximum total dose is 0.07 mg/kg or 5 mg, whichever is less, and a first-twitch response of at least 10% of baseline must be present before it is given fda-neostigmine-label-2024. The same label warns that large doses given when blockade is minimal can produce neuromuscular dysfunction, and a volunteer study measured that effect: neostigmine 0.03 mg/kg with glycopyrrolate, given after the TOF ratio had recovered to 1.0, reduced genioglossus activity in response to negative pharyngeal pressure by 37% (SD 29), P=0.005, almost to the level seen at a ratio of 0.5, and increased upper airway collapsibility fda-neostigmine-label-2024 herbstreit-2010. The sources describe weakness from neostigmine at minimal block or after full recovery; none retrieved describes paradoxical weakness from neostigmine given at deep block, and this page does not claim it.

Sugammadex is dosed by depth: 2 mg/kg at the reappearance of the second twitch, 4 mg/kg at one to two post-tetanic counts, and 16 mg/kg about 3 min after 1.2 mg/kg of rocuronium fda-sugammadex-label-2026. Its label records anaphylaxis in 0.3% of 299 healthy volunteers (one case, in the 16 mg/kg group), marked bradycardia, some resulting in cardiac arrest, within minutes of administration, a higher risk of recurrent block after lower-than-recommended doses, an effect on hormonal contraceptives equivalent to a missed dose with a non-hormonal back-up method advised for the next 7 days, and waits before re-dosing a steroidal blocker of 5 min after 1.2 mg/kg rocuronium, 4 h after 0.6 mg/kg rocuronium or 0.1 mg/kg vecuronium, and 24 h after a 16 mg/kg dose or in renal impairment fda-sugammadex-label-2026.

Edrophonium has no marketed finished product in the United States: every FDA application for it, under the brand names Enlon, Enlon-Plus, Reversol and Tensilon and as generics, was listed as discontinued in Drugs@FDA when this was checked on 2026-09-06, and the one DailyMed listing has both of its national drug codes inactivated. No reversal content on this site names it. Two cautions attach to that sentence. It is a dated claim rather than a settled fact, because the FDA approved a labeling supplement for Enlon-Plus in October 2025 and an active sponsor maintaining an application is consistent with an intent to re-market, so it needs re-checking rather than repeating. It now has a ledger row, recording every application and its marketing status, the one inactivated DailyMed listing, and the October 2025 supplement as the reason this is a claim to re-check rather than repeat fda-edrophonium-drugsfda-2026.

How much of this has reached practice is worth knowing, and the answer is not much. Two years after the guideline, a survey of American Society of Anesthesiologists members found quantitative monitors used consistently by 19% of respondents, a majority of 53% never using quantitative monitoring to confirm recovery, and 61% reporting no change in their use of such devices since the guideline appeared; 6% were unaware of it renew-2026. Weigh that against how it was collected. It is a cross-sectional survey with a 3% response rate, and its authors say plainly that conclusions must be tempered by that renew-2026. A 3% response is the kind of number that tells you which way the wind blows and not how hard. Two things this page used to carry as general recommendations have been removed rather than relabeled, because both are factual claims with no source in this ledger. One was that acceleromyographic monitors read high and need normalization to a baseline while electromyographic monitors do not. The other was that sugammadex is not recommended below a creatinine clearance of 30 mL/min: its label covers how long to wait before re-dosing a steroidal relaxant in renal impairment, which is a different statement, and no source here sets that threshold. Both are recorded in the repository’s gap register.

At the bedside

Measure, do not judge. A quantitative TOF ratio is the only bedside finding on this page that distinguishes a recovered junction from a blocked one, and the guideline recommends it over clinical assessment thilen-2023. A head lift or a hand grip tells you about large muscles that recover first.

If the ratio is documented at 0.9 or more, no antagonism is needed; if it is below 0.9, the choice of agent depends on the measured depth, with neostigmine reasonable only from minimal block thilen-2023. Local practice differs. the institutional preference between sugammadex and neostigmine, and the availability of quantitative monitors in the PACU, differ between hospitals.

Treat hypoxemia or airway obstruction in the first minutes of PACU stay as residual block until a monitor says otherwise, because those were the two commonest events in the matched cohort and the cases were, on average, still deeply blocked murphy-2008.

Check yourself

Four items. Every option carries an explanation. Progress is not saved.