Neuromuscular blockade is a reliable source of NCE questions because it combines pharmacology, physiology, equipment and a calculation, all in one stem. If you can read a twitch pattern and match it to a reversal dose, you will pick up points that other candidates leave behind.
Depolarizing versus nondepolarizing patterns
The single most tested idea is the difference between the two kinds of block on a nerve stimulator.
| Pattern | Phase I (succinylcholine) | Nondepolarizing | Phase II (prolonged succinylcholine) |
|---|---|---|---|
| Train-of-four fade | No | Yes | Yes |
| Tetanic fade | No | Yes | Yes |
| Posttetanic potentiation | No | Yes | Yes |
| Effect of anticholinesterase | Augments the block | Reverses the block | May reverse |
A phase I block is equal across all four twitches. Fade means the prejunctional receptors are blocked, which is the signature of a nondepolarizing drug or a phase II block.
Reading the train-of-four
Four stimuli at 2 Hz. The ratio of the fourth twitch to the first (T4/T1) is the train-of-four ratio.
- 0 twitches: profound block. Use the posttetanic count to see how deep.
- 1 twitch: about 90 percent or more of receptors blocked.
- 2 twitches: about 80 to 90 percent.
- 3 twitches: about 75 to 80 percent.
- 4 twitches: under about 75 percent, but the ratio still matters.
The modern standard for safe extubation is a TOF ratio of 0.9 or more, measured quantitatively. Human eyes and fingers cannot reliably detect fade once the ratio is above about 0.4, which is why “no visible fade” does not mean full recovery.
Which muscle you watch matters
Muscles differ in sensitivity:
- The diaphragm is the most resistant and recovers first.
- The adductor pollicis (ulnar nerve, thumb) is sensitive. Recovery there suggests the airway and diaphragm have recovered too, which is why it is the preferred site for judging reversal.
- The orbicularis oculi can overestimate recovery. The corrugator supercilii better reflects the laryngeal muscles and diaphragm.
The trap: a patient monitored at the eye may look recovered while the pharyngeal muscles are still weak.
Sugammadex dosing
Sugammadex is a modified gamma cyclodextrin that encapsulates rocuronium (and vecuronium, less tightly). It does not work on succinylcholine or the benzylisoquinoliniums like cisatracurium.
| Depth of block | Dose |
|---|---|
| Moderate: T2 has reappeared | 2 mg/kg |
| Deep: 1 to 2 posttetanic counts, no TOF response | 4 mg/kg |
| Immediate reversal after rocuronium 1.2 mg/kg | 16 mg/kg |
A common calculation: a 70 kg patient with 2 twitches needs 2 mg/kg, which is 140 mg.
Points the exam likes:
- Hormonal contraceptives: sugammadex can bind progestogen. Counsel the patient to treat it like a missed pill and use a backup method for 7 days.
- Bradycardia and anaphylaxis are rare but listed adverse effects.
- Re-paralysis with rocuronium after sugammadex requires waiting or a higher dose, so plan the airway.
Neostigmine and glycopyrrolate
Anticholinesterases raise acetylcholine at the junction so it can outcompete a nondepolarizing drug. They have a ceiling effect, and they cannot reverse a deep block.
- Neostigmine: 0.03 to 0.07 mg/kg, maximum about 5 mg.
- Glycopyrrolate: about 0.2 mg for every 1 mg of neostigmine, given with or before it to block the muscarinic effects (bradycardia, secretions, bronchospasm).
- Give neostigmine only when there are at least 2 to 4 twitches, ideally 4.
- Atropine pairs better with edrophonium because their onsets match.
Things that prolong a nondepolarizing block
- Volatile anesthetics
- Aminoglycoside antibiotics
- Magnesium
- Hypothermia
- Acidosis and hypokalemia
- Local anesthetics and lithium
Quick hits for test day
- Fade means nondepolarizing or phase II.
- TOF ratio of 0.9 or more before extubation.
- Adductor pollicis to judge recovery. The eye can fool you.
- Sugammadex: 2, 4, 16 mg/kg by depth.
- Neostigmine maximum about 5 mg, glycopyrrolate 0.2 mg per mg.
The neuromuscular blockade and reversal lesson is one of the five free lessons on CRNA Dojo, so you can work through it with a free account.
Sources
- Barash, Clinical Anesthesia
- Nagelhout, Nurse Anesthesia
- Bridion (sugammadex) prescribing information
