Muscle Relaxants and Their Reversal
CA-1 · draft
CA-1 Bootcamp day 9. Authored from cited abstracts; every claim carries a PMID.
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until you approve it. Read every point and its source first — they were drafted
from search results, not from clinical judgement.
Before induction
Succinylcholine 1 mg/kg and rocuronium 0.6 mg/kg produced intubating conditions at 60 seconds that a blinded assessor could not tell apart, but succinylcholine both arrived and left faster: twitch depression to 5% of control took 0.8 minutes versus 1.2 minutes, and recovery to 90% of control twitch took 11.3 minutes after succinylcholine against 36.1 minutes after rocuronium.
The Cochrane review of 50 trials and 4,151 patients found succinylcholine superior to rocuronium for achieving excellent intubating conditions in rapid sequence induction (risk ratio 0.86), with no statistical difference against rocuronium 1.2 mg/kg — where succinylcholine remained clinically superior only because of its shorter duration of action.
The dose of rocuronium buys speed and pays for it in time: in patients over 80, 0.9 mg/kg reached a train-of-four count of zero in 108 seconds and lasted 118 minutes, while 0.3 mg/kg took 228 seconds and lasted 46 minutes — and two-thirds of the low-dose patients never reached a count of zero at all.
Neostigmine antagonises block only where spontaneous recovery has already begun — the standing advice is to wait until at least the second twitch of the train-of-four is detectable, its full effect still takes up to 10 minutes, and because it works by flooding every muscarinic receptor in the body it must be given with an antimuscarinic, a pairing that is itself dose-dependently associated with postoperative tachycardia.
Sugammadex encapsulates rocuronium rather than out-competing it at the junction, so its dose is chosen from the depth of block — about 2 mg/kg once the second twitch has returned, 4 to 8 mg/kg at a post-tetanic count of 1 to 2, and 16 mg/kg for immediate reversal — and in a Cochrane meta-analysis of 41 trials it reached a train-of-four ratio above 0.9 in 2.0 minutes from the second twitch against 12.9 minutes for neostigmine, and in 2.9 minutes from a post-tetanic count of 1 to 5 against 48.8 minutes.
Choosing the better reversal drug helps but does not absolve you — a matched cohort of 45,712 American surgical patients found sugammadex associated with a 30% lower risk of major pulmonary complications than neostigmine, while a 22,803-patient European prospective study found that using a neuromuscular blocking agent at all was associated with more pulmonary complications, and neither monitoring, nor giving a reversal agent, nor extubating at a train-of-four ratio of 0.9 or more removed that association.
Questions in the room
If you cannot intubate and cannot ventilate this patient, which relaxant do you wish you had given, and how long until it does not matter?
How deep is the block right now? Tell me what the monitor says, not what the clock says.
You have neostigmine and sugammadex on the cart. What decides which one — and what decides the dose?
He is lifting his head off the pillow. Is he reversed? How would you know if he were not?
Oral boards stem
A 45-year-old with a small bowel obstruction, vomiting since yesterday, needs a laparotomy tonight. The examiner asks which relaxant you would give at induction and why, and what your plan is if the first look is a grade 3 view. Then he jumps to the end of the case: rocuronium was redosed 20 minutes ago because the surgeon wanted more room, the fascia is closed, and he asks what you will give to reverse, how you will choose the dose, and what you need to have seen before the tube comes out.
Sources
Pühringer et al., Anesth Analg 1992
PMID 1616158
Tran et al., Cochrane Database Syst Rev 2015
PMID 26512948
Vested et al., Acta Anaesthesiol Scand 2022
PMID 35675032
Srivastava et al., Br J Anaesth 2009
PMID 19468024
Shay et al., Anesth Analg 2020
PMID 30896593
Hristovska et al., Anaesthesia 2018
PMID 29280475
Jones et al., Anesthesiology 2008
PMID 18946293
Kheterpal et al., Anesthesiology 2020
PMID 32282427
Kirmeier et al., Lancet Respir Med 2019
PMID 30224322
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