01How they work & how you monitor
Neuromuscular blockers (NMBs, "paralytics") relax skeletal muscle at the neuromuscular junction — so the surgeon can operate and the patient doesn't fight the ventilator. Two types:
- Depolarizing (succinylcholine) — briefly depolarizes the muscle, so you see fasciculations when it's given.
- Non-depolarizing ("-onium" / "-urium" agents) — competitively block the receptor; no movement on administration.
Monitoring: a peripheral nerve stimulator delivers a train-of-four (TOF) — four twitches — and you watch how many return and whether tetany is sustained. Reversal is given only after at least one twitch returns.
Muscle relaxants are the most common cause of anaphylaxis in the OR.
02Succinylcholine
The fastest-on, shortest-acting paralytic — the classic RSI drug. - Dose 1–1.5 mg/kg (≈180 mg); onset ~1 min, duration ~5–10 min. - An MH trigger. - Contraindicated where up-regulated/extrajunctional ACh receptors cause lethal hyperkalemia: burns (after ~24 h), denervation injury (stroke, spinal cord injury, prolonged paralysis), muscular dystrophy/myopathy. - Other effects: myalgia (worse at higher doses), bradycardia (especially repeat doses and in children — pretreat peds with atropine), ↑intragastric/intraocular/intracranial pressure.
- Giving sux to a burn, denervation, or dystrophy patient — the potassium spike can cause cardiac arrest.
03Non-depolarizing agents
| Agent | Intubating dose | Onset | Duration | Notes |
|---|---|---|---|---|
| Rocuronium | 0.6 mg/kg (1.2 for RSI) | 1–2 min (RSI) | 30–60 min | Comes ready in solution; sux alternative; can be reversed by sugammadex |
| Vecuronium | 0.08–0.1 mg/kg | 3–5 min | 25–40 min | Cheap, reliable; comes as a powder to reconstitute |
| Cisatracurium | 0.15–0.2 mg/kg | 2–3 min | 30–60 min | Hofmann elimination — organ-independent; best in renal/hepatic failure |
When sux is contraindicated and you need fast intubating conditions, rocuronium 1.2 mg/kg is the go-to.
04Reversal
Neostigmine — an anticholinesterase that raises acetylcholine to out-compete the non-depolarizer. - 0.04–0.07 mg/kg, max ~5 mg; requires at least one twitch back (it can't reverse a deep block). - Side effects: bradycardia and SLUDGE (Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis). - Don't confuse "Neo" (neostigmine) with "Neo" (neosynephrine = phenylephrine).
Glycopyrrolate — given with neostigmine to block the bradycardia/SLUDGE. - ~0.2 mg per 1 mg of neostigmine (max ~1 mg). Handy trick: draw the same volume of glycopyrrolate as neostigmine (both are common concentrations that work out close).
Sugammadex — a newer reversal that encapsulates rocuronium/vecuronium and can reverse even a deep block (it does not reverse cisatracurium). Many programs now use it preferentially.
- Giving too much reversal can itself cause weakness — discuss the dose with your attending based on the TOF.