Anaesthesia

The Neuromuscular Junction

Skeletal muscle contracts when a nerve impulse releases acetylcholine (ACh) from the motor nerve terminal; ACh crosses the synaptic cleft and binds nicotinic ACh receptors on the muscle end-plate, depolarising it and triggering contraction. ACh is then rapidly broken down by acetylcholinesterase. Neuromuscular blocking drugs (muscle relaxants) act at this postsynaptic nicotinic receptor to produce skeletal muscle paralysis, facilitating tracheal intubation, controlled ventilation and surgical access.

Two Classes of Block

Relaxants are either depolarising or non-depolarising. A depolarising agent (suxamethonium) mimics ACh: it binds and depolarises the end-plate, but — not being broken down by acetylcholinesterase — it keeps the receptor occupied, causing a brief fasciculation then flaccid paralysis (a persistently depolarised, unresponsive membrane). A non-depolarising agent is a competitive antagonist that blocks ACh from the receptor without depolarising it.

Uses

Muscle relaxants are used to facilitate tracheal intubation, to provide surgical relaxation (e.g. abdominal surgery), and to allow controlled ventilation. They provide no anaesthesia or analgesia — a paralysed patient must be adequately anaesthetised, or awareness results.

Clinical Consequences of the Two Mechanisms

The mechanistic difference between the two classes explains almost everything that matters clinically. Because a depolarising agent is an agonist that first activates the receptor before paralysing it, it produces the initial fasciculations and their sequelae (muscle pains, potassium release, rises in pressure), it cannot be reversed by an anticholinesterase (more acetylcholine would only add to the depolarisation), and on a nerve stimulator it produces a block without fade. Because a non-depolarising agent is a competitive antagonist, its block can be overcome by increasing acetylcholine (the basis of neostigmine reversal), it produces no fasciculations, and it shows the characteristic fade and post-tetanic facilitation that reveal an incompletely-recovered junction.

• Relaxants act at the postsynaptic nicotinic ACh receptor → skeletal muscle paralysis.
• Depolarising (suxamethonium): agonist → fasciculations then flaccid paralysis; not reversed by neostigmine; no fade.
• Non-depolarising: competitive antagonist → no fasciculations; reversed by neostigmine; fade on TOF.
• Used to aid intubation, provide surgical relaxation, allow controlled ventilation.
• They give NO anaesthesia/analgesia — a paralysed patient must be anaesthetised (awareness risk).

Pharmacology

Suxamethonium (succinylcholine) is the only depolarising relaxant in common use. It is structurally two ACh molecules joined together, and it acts as an agonist at the nicotinic receptor, depolarising the end-plate. Its great advantages are a very rapid onset (~30–60 s) and a very short duration (~3–5 min) — because it is rapidly hydrolysed by plasma (pseudo)cholinesterase. The intubating dose is 1–1.5 mg/kg.

Uses

Its speed and brevity make it the classic relaxant for rapid sequence induction (securing the airway fast in the aspiration-risk patient) and for brief procedures requiring relaxation, and it is invaluable for the rapid relief of laryngospasm.

Complications

Suxamethonium has a long list of adverse effects. Hyperkalaemia — it raises serum potassium (~0.5 mmol/L normally), but causes dangerous, potentially fatal hyperkalaemia in burns, major trauma/crush, spinal cord injury/denervation, and prolonged immobility (from up-regulation of extrajunctional receptors). Muscle pains (myalgia) after fasciculations. Bradycardia (especially with a repeat dose, and in children) — prevented by atropine. It is a trigger for malignant hyperthermia. It raises intra-ocular, intragastric and intracranial pressure. Rarely, anaphylaxis. In those with abnormal cholinesterase it causes prolonged apnoea (suxamethonium apnoea).

Phase II Block

With a large or repeated dose (or an infusion), the character of the block changes from the normal depolarising phase I to a phase II (‘dual’) block that resembles a non-depolarising block (with fade), reflecting desensitisation of the receptor. This prolongs recovery.

Why It Persists Despite Its Hazards

Given its formidable list of complications, it is reasonable to ask why suxamethonium remains in use at all, and the answer lies in the two properties that no non-depolarising agent has traditionally matched: an onset within thirty to sixty seconds and an offset within a few minutes from spontaneous breakdown by plasma cholinesterase. This combination makes it uniquely suited to securing the airway quickly in the aspiration-risk patient and to situations where paralysis may need to be short-lived — and it is the reason the search for a replacement has centred on the rocuronium–sugammadex combination, which can reproduce the rapid onset and now, with sugammadex, a rapid offset.

Fasciculations & Their Consequences

The brief, visible fasciculations that precede paralysis are the outward sign of the initial depolarisation, and several of suxamethonium’s adverse effects flow directly from them. The disorganised muscle contraction releases potassium into the circulation and produces the postoperative muscle pains (myalgia) that patients often notice a day or two later, while the same generalised contraction contributes to transient rises in intra-ocular, intragastric and intracranial pressure. Understanding that these effects share a single origin — the depolarising fasciculation — helps make sense of an otherwise disparate list of complications, and explains why techniques such as a small pre-dose of a non-depolarising agent have been used to attenuate them.

• Suxamethonium: only common depolarising relaxant; agonist; very fast onset, very short (hydrolysed by plasma cholinesterase); 1–1.5 mg/kg.
• Uses: rapid sequence induction, brief relaxation, relief of laryngospasm.
• Complications: dangerous hyperkalaemia (burns/crush/spinal injury/denervation), myalgia, bradycardia (atropine), MH trigger, ↑IOP/ICP/intragastric pressure.
• Suxamethonium apnoea in abnormal cholinesterase; anaphylaxis (rare).
• Large/repeated doses → phase II (dual) block resembling non-depolarising.

Overview

Non-depolarising muscle relaxants are competitive antagonists at the nicotinic receptor, producing paralysis without fasciculations and with a slower onset and longer duration than suxamethonium. They are used for maintenance of relaxation during surgery and are reversible by anticholinesterases (neostigmine) (or, for the aminosteroids, by sugammadex). They are classified as aminosteroids (vecuronium, rocuronium, pancuronium) or benzylisoquinoliniums (atracurium, cisatracurium, mivacurium).

Choosing an Agent

The choice depends on the desired onset, duration and elimination and on patient factors. Rocuronium has a rapid onset and is the main non-depolarising alternative for rapid sequence induction (and is reversible by sugammadex). Atracurium and cisatracurium undergo Hofmann elimination (spontaneous, organ-independent breakdown), making them ideal in renal or hepatic failure. Pancuronium is long-acting and vagolytic (causes tachycardia).

Adverse Effects

Class effects are relatively few. Some (e.g. atracurium, and the older tubocurarine) cause histamine release (hypotension, flushing, bronchospasm); pancuronium causes tachycardia (vagolytic); accumulation occurs if elimination is impaired (except the Hofmann agents). As with all relaxants, residual block at the end of surgery risks hypoventilation and airway compromise — hence monitoring and reversal.

Duration & the Problem of Accumulation

Non-depolarising agents are commonly grouped by duration into short, intermediate and long-acting, and the choice is guided by the length of surgery and the need to avoid residual block at the end. The long-acting agents such as pancuronium provide prolonged relaxation from a single dose but accumulate and are difficult to reverse fully in time for extubation; the intermediate agents (rocuronium, vecuronium, atracurium, cisatracurium) are now favoured for most surgery because they are more easily titrated and reversed. Accumulation is a particular hazard where elimination is impaired — renal failure for the renally-excreted agents, hepatic failure for the hepatically-cleared ones — which is precisely why the organ-independent Hofmann elimination of atracurium and cisatracurium is so valued in these patients.

• Non-depolarising = competitive antagonists; no fasciculations, slower/longer; reversible by neostigmine (sugammadex for aminosteroids).
• Aminosteroids (rocuronium, vecuronium, pancuronium) & benzylisoquinoliniums (atracurium, cisatracurium, mivacurium).
• Rocuronium: rapid onset → RSI alternative, sugammadex-reversible.
• Atracurium/cisatracurium: Hofmann elimination → ideal in renal/hepatic failure.
• Watch for histamine release (atracurium), tachycardia (pancuronium), & residual block — monitor & reverse.

Why Reversal Is Needed

At the end of surgery, any residual non-depolarising block must be reversed (or allowed to wear off fully) before extubation, because residual paralysis causes weakness, hypoventilation, airway obstruction and aspiration. Reversal is confirmed with a nerve stimulator and clinical signs.

Anticholinesterases (Neostigmine)

Neostigmine reverses a non-depolarising block by inhibiting acetylcholinesterase, so ACh accumulates at the junction and outcompetes the relaxant. However, the extra ACh also acts at muscarinic sites, causing bradycardia, salivation, bronchoconstriction and increased gut activity — so neostigmine is always given with an antimuscarinic (atropine or glycopyrrolate) to block these effects. It can only reverse a partial block (some recovery must be present).

Sugammadex

Sugammadex is a modified cyclodextrin that reverses the aminosteroid relaxants (rocuronium, vecuronium) by a completely different mechanism: it encapsulates (chelates) the relaxant molecule in the plasma, removing it from the junction. It provides rapid, complete reversal even of a deep block, does not act on acetylcholinesterase (so needs no antimuscarinic), and has transformed the safety of aminosteroid use (e.g. it can rescue a ‘can’t intubate’ situation after rocuronium).

Confirming Adequate Reversal

Adequate reversal is judged by a train-of-four ratio > 0.9 on a nerve stimulator, supported by clinical signs (a sustained 5-second head-lift, adequate tidal volume, hand grip). Only then is the patient safe to extubate.

Residual Block & Its Prevention

Residual neuromuscular block — an incompletely reversed non-depolarising block at the end of anaesthesia — remains a common and under-recognised problem, and it matters because even a modest degree of residual paralysis impairs the muscles of the pharynx and of breathing, predisposing to airway obstruction, aspiration and hypoventilation in the recovery period. Its prevention rests on three habits: choosing an appropriate agent and dose, monitoring the block objectively with a nerve stimulator rather than relying on clinical impression, and ensuring adequate reversal (a train-of-four ratio above 0.9) before the tracheal tube is removed — with sugammadex offering more reliable reversal of aminosteroid block than neostigmine can provide.

Comparing the Two Reversal Agents

The two means of reversal differ fundamentally and it is worth contrasting them directly. Neostigmine works indirectly, by raising the amount of acetylcholine so that it out-competes the relaxant, which means it can only assist a block that is already partly recovering, it worsens a depolarising block, and its accompanying muscarinic effects oblige the co-administration of an antimuscarinic. Sugammadex works directly, physically capturing the aminosteroid relaxant molecule and removing it from the junction, so it can reverse even a profound block within minutes, needs no antimuscarinic, and produces a more reliable, complete recovery — at the cost of being specific to the aminosteroids and considerably more expensive. The clinician chooses between them according to the relaxant used, the depth of block and the urgency of reversal.

• Residual block is dangerous → reverse (or ensure full recovery) before extubation.
• Neostigmine inhibits acetylcholinesterase → ACh accumulates & outcompetes relaxant; needs an antimuscarinic (atropine/glycopyrrolate).
• Neostigmine reverses only a partial block & worsens phase I (depolarising) block.
• Sugammadex encapsulates rocuronium/vecuronium → rapid, complete reversal of even deep block; no antimuscarinic.
• Confirm reversal: train-of-four ratio > 0.9 + sustained head-lift before extubation.

Why Monitor?

Neuromuscular monitoring — using a peripheral nerve stimulator — objectively assesses the depth of block and the adequacy of recovery/reversal. Clinical signs alone are unreliable, and residual paralysis is a common, dangerous cause of postoperative respiratory problems, so objective monitoring improves safety.

Train-of-Four (TOF)

The commonest test is the train-of-four: four supramaximal stimuli are applied to a peripheral nerve (e.g. ulnar, watching adductor pollicis) and the muscle twitches observed. With a non-depolarising block, the responses show ‘fade’ (each twitch weaker than the last); the TOF ratio (fourth twitch / first twitch) measures the degree of recovery. A depolarising (phase I) block shows reduced but equal twitches (no fade).

Interpreting the TOF

The number of twitches present indicates block depth (fewer twitches = deeper block; no twitches = very deep). The TOF ratio tracks recovery, and a ratio > 0.9 indicates adequate recovery safe for extubation. For very deep blocks (no TOF response) the post-tetanic count is used. Fade and post-tetanic facilitation are hallmarks of a non-depolarising block.

Sites, Patterns & Practical Use

In practice the nerve stimulator is applied over an accessible peripheral nerve — most often the ulnar nerve at the wrist, watching the adductor pollicis — and several stimulation patterns are used for different depths of block. The train-of-four is the everyday pattern for judging moderate block and recovery; when the block is so deep that there is no train-of-four response, the post-tetanic count is used to gauge how profound it is and to predict when the train-of-four will reappear; and double-burst stimulation makes residual fade easier to detect by feel. Choosing the right pattern for the depth of block, and interpreting fade correctly, is what allows the anaesthetist to titrate relaxation during surgery and to confirm genuine recovery at the end.

• Peripheral nerve stimulator objectively measures block depth & recovery (residual block is dangerous).
• Train-of-four: 4 stimuli; non-depolarising → fade; depolarising phase I → reduced but equal (no fade).
• Twitch count = depth; TOF ratio (T4/T1) = recovery; > 0.9 = adequate for extubation.
• Post-tetanic count for very deep block; fade & post-tetanic facilitation = non-depolarising.
• Clinical signs unreliable — confirm recovery objectively.