Neuromuscular Blockers and Reversal
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Direct answer
Succinylcholine (1-1.5 mg/kg) remains the fastest route to a paralysed, intubatable airway — onset in 45-60 seconds, offset in 6-10 minutes — but its depolarising mechanism makes potassium leak from muscle, so burns beyond 72 hours, crush injury, denervation, prolonged immobilisation and muscle dystrophies contraindicate it through dangerous hyperkalaemia. Rocuronium 0.6-1.2 mg/kg matches it for rapid sequence induction when reversal matters less, and sugammadex — 2 mg/kg at two twitches on train-of-four, up to 16 mg/kg for immediate reversal — encapsulates rocuronium or vecuronium specifically, reversing them cleanly with no cholinergic storm. Neostigmine 0.05 mg/kg with glycopyrrolate reverses any competitive block but demands some spontaneous recovery first; it does nothing for succinylcholine, which must fade on its own or, in malignant hyperthermia, be fought with dantrolene.
What you must remember
- Succinylcholine numbers: 1-1.5 mg/kg intravenously, intubating conditions in 45-60 seconds, apnoea for 6-10 minutes; metabolised by plasma cholinesterase — prolonged apnoea in genetically deficient patients.
- The hyperkalaemia list: burns (beyond 24-72 hours), crush injury, denervation or stroke with immobility, muscular dystrophies — up-regulated extrajunctional nicotinic receptors open to potassium efflux.
- Malignant hyperthermia: succinylcholine plus volatile agents; rigidity, hypercarbia, hyperthermia, rhabdomyolysis — dantrolene 2.5 mg/kg intravenously, repeated as needed, with active cooling and stopping the trigger.
- Rocuronium: 0.6 mg/kg for routine intubation, 1.0-1.2 mg/kg for rapid sequence; intermediate duration; no histamine release; the sugammadex partner.
- Sugammadex selectivity: a cyclodextrin that encapsulates rocuronium and vecuronium only — useless for succinylcholine, atracurium or cisatracurium; doses 2 mg/kg at second-twitch recovery, 4 mg/kg in deep block, 16 mg/kg for immediate reversal three minutes after rocuronium.
- Neostigmine plus glycopyrrolate: 0.05 mg/kg with 0.01 mg/kg respectively; requires visible recovery (at least two to four twitches), and under-dosing in obesity leaves residual block — a cause of postoperative hypoxia.
- Cisatracurium: Hofmann elimination independent of liver and kidney — the organ-failure choice; its precursor atracurium releases histamine.
- Monitoring craft: train-of-four ratio above 0.9 before extubation; qualitative observation of four equal twitches is not sensitive enough for deep residual blockade.
A worked airway decision
A 30-year-old with 40% burns from a house fire three days ago needs emergency laparotomy. The burns are beyond 72 hours old — the up-regulation window — so succinylcholine is off the table entirely, whatever the airway urgency. Rapid sequence induction proceeds with rocuronium 1.2 mg/kg and cricoid pressure; the intubating conditions arrive nearly as fast, the price being an hour of paralysis rather than ten minutes. At closure, train-of-four shows two twitches: sugammadex 2 mg/kg restores the ratio above 0.9 within minutes, without the bradycardia, secretions or bowel cramping of an anticholinesterase — and with no dependence on the patient's cholinesterase genotype or kidney function being perfect.
The contrast case is the theatre emergency every trainee must know cold: masseter spasm, rising end-tidal carbon dioxide despite constant ventilation, temperature climbing, urine turning dark. Malignant hyperthermia: volatile agent off, hundred percent oxygen, dantrolene drawn and given at 2.5 mg/kg with repeat dosing until rigidity melts, active cooling, and treatment of hyperkalaemia — the pharmacology of two drugs colliding, and the reason succinylcholine's speed never earns it an unconditional pass.
Where examiners probe
Three probes recur. First, the hyperkalaemia vignette: a stem with "burned three days ago" or "crush injury" exists solely to exclude succinylcholine — the timing qualifier (not fresh burns, but burns older than a day or two) is the discriminator candidates miss. Second, sugammadex specificity: "which block can sugammadex not reverse?" — succinylcholine and the benzylisoquinoliniums; and the mechanism word "encapsulation" earns more than "antagonism". Third, residual block: why neostigmine fails when given too early or dosed on ideal rather than actual body weight, and why train-of-four above 0.9 — not "four twitches present" — is the extubation standard. Indian context: sugammadex's cost keeps neostigmine-glycopyrrolate the default in most government theatres, so knowing both regimens — and the monitoring that makes the cheap one safe — is the practical exam answer, not a footnote.
Frequently asked questions
Why is succinylcholine avoided in patients with burns or crush injury?
Extrajunctional receptor proliferation makes the depolarising block release large amounts of potassium, risking fatal arrhythmia from about 24-72 hours after injury onward.
Which neuromuscular blockers can sugammadex reverse?
Only rocuronium and vecuronium — it encapsulates these aminosteroids and has no effect on succinylcholine or cisatracurium.
What dose of sugammadex reverses rocuronium at two twitches?
2 mg/kg, with 4 mg/kg in deep block and 16 mg/kg for immediate reversal given three minutes after intubating-dose rocuronium.
Why must neostigmine wait for spontaneous recovery?
It inhibits acetylcholinesterase to tilt the competition at the junction; with no acetylcholine transmission yet present, it merely produces bradycardia and secretions without reversal.
What is the malignant hyperthermia regimen?
Stop volatile agents and succinylcholine, give dantrolene 2.5 mg/kg intravenously with repeats, hundred percent oxygen, active cooling and hyperkalaemia treatment.