Why can a patient breathe with a train-of-four count of two?
Pharmacology — Non-Depolarising Agents: Onset, Duration and Elimination
The question
At the end of a laparotomy a patient who received rocuronium is breathing spontaneously with tidal volumes of 450 mL and a regular respiratory pattern. Quantitative train-of-four stimulation of the ulnar nerve, recorded at the adductor pollicis, shows two twitches out of four. Which explanation best accounts for these apparently contradictory findings?
- The stimulating electrodes have been placed over the median rather than the ulnar nerve, giving a falsely low count
- Adequate spontaneous ventilation confirms that neuromuscular transmission has recovered and the patient may be extubated
- The diaphragm is relatively resistant to non-depolarising blockade and recovers earlier than the adductor pollicis
- The diaphragm is more sensitive to non-depolarising agents than peripheral muscle, so it is the last muscle to recover
- Pharyngeal and laryngeal muscles recover before the diaphragm, which is what permits spontaneous ventilation
The answer
The diaphragm is relatively resistant to non-depolarising blockade and recovers earlier than the adductor pollicis
The stimulating electrodes have been placed over the median rather than the ulnar nerve, giving a falsely low count — Electrode misplacement is a genuine source of error. Stimulating the median nerve at the wrist produces a thenar twitch that is easily mistaken for an adductor pollicis response, and it can yield reproducible readings. However, this does not account for the combination presented in the stem: a genuinely adequate tidal volume alongside a partial count requires a real difference in sensitivity between muscle groups, not a monitoring artefact.
Adequate spontaneous ventilation confirms that neuromuscular transmission has recovered and the patient may be extubated — Adequate ventilation confirms that the diaphragm is functioning. This is not equivalent to full neuromuscular recovery. The pharyngeal and upper airway muscles are more sensitive to non-depolarising blockade than the adductor pollicis, so a patient who is ventilating adequately may still be unable to protect the airway or swallow safely.
The diaphragm is relatively resistant to non-depolarising blockade and recovers earlier than the adductor pollicis — Correct. The diaphragm requires approximately 1.5 to 2 times the dose needed to paralyse the adductor pollicis for the same depth of block. It also takes up drug and recovers more rapidly owing to its rich blood supply. Adequate spontaneous ventilation in the presence of a partial train-of-four count at the hand is therefore an expected and well-described finding.
The diaphragm is more sensitive to non-depolarising agents than peripheral muscle, so it is the last muscle to recover — This reverses the true relationship. If the diaphragm were more sensitive than peripheral muscle, a patient showing two twitches at the adductor pollicis would be apnoeic. This is not what is observed clinically.
Pharyngeal and laryngeal muscles recover before the diaphragm, which is what permits spontaneous ventilation — This contains two errors. The pharyngeal muscles are among the most sensitive to non-depolarising blockade and recover late, not early. Furthermore, it is diaphragmatic contraction that generates a tidal volume, not pharyngeal function.
Explanation
Different muscle groups recover from non-depolarising block at different rates, and the differences are large enough to be clinically significant. The diaphragm and the laryngeal adductors are the most resistant: they require approximately 1.5 to 2 times the dose needed at the adductor pollicis for the same depth of block. They also exhibit faster onset (their rich blood supply delivers drug sooner) and faster recovery. The abdominal muscles occupy an intermediate position, being somewhat more resistant than the adductor pollicis. The pharyngeal constrictors, geniohyoid, and genioglossus (the muscles responsible for maintaining airway patency and safe swallowing) are the most sensitive of all.
The basis for this differential sensitivity lies in the neuromuscular junction rather than in the drug. Muscles vary in receptor density, fibre type composition, junctional acetylcholinesterase activity, and the margin of safety of neuromuscular transmission: the surplus of released acetylcholine over the quantity actually required to generate an action potential. A muscle with a larger safety margin tolerates a greater proportion of occupied receptors before transmission fails. The diaphragm has a particularly generous margin of safety.
This hierarchy of sensitivity accounts for two observations that appear contradictory until recognised as expressions of the same physiology. First, a patient can generate adequate tidal volumes while the adductor pollicis still shows a partial block. That is the scenario presented in this question. Second, and of greater clinical importance, a train-of-four ratio at the adductor pollicis that appears reassuring does not guarantee recovery of the pharyngeal musculature. This is why the accepted threshold for adequate recovery is a ratio of 0.9 or above measured objectively, and why aspiration risk persists below that value.
The same principle informs the choice of monitoring site. Monitoring at the adductor pollicis is deliberately conservative for airway safety because the thumb lags behind the diaphragm. Monitoring at the face introduces additional complexity: the corrugator supercilii behaves like the diaphragm and laryngeal muscles, and therefore overestimates recovery at peripheral sites. The orbicularis oculi is closer in sensitivity to the adductor pollicis. A count taken at the brow and interpreted as though it came from the hand would make a partially paralysed patient appear ready for extubation.
Tips for the candidate
When a stem presents a clinical observation alongside a monitored value, the answer almost always lies in the fact that different muscles are being assessed. A useful habit is to identify which muscle each half of the stem refers to. A tidal volume reflects diaphragmatic function. A train-of-four count reflects whichever muscle the electrodes overlie. A head lift or grip tests peripheral skeletal muscle. A swallow tests the pharynx.
The sensitivity order is worth committing to memory as a single sequence, since several question types depend on it: pharynx and upper airway (most sensitive) > peripheral muscle including adductor pollicis > abdominal wall > larynx and diaphragm (most resistant). Resistant muscles are blocked later and recover sooner. This is one principle expressed in two directions, not two separate facts.
Monitoring site is a common discriminator in examination questions. If the stem specifies that the response was recorded at the corrugator supercilii or over the eyebrow, the answer is likely to involve overestimated recovery, because that muscle behaves like the diaphragm. If it specifies the adductor pollicis, the monitoring site is appropriate and the question usually concerns the interpretation of the numerical value itself.
Any option stating that adequate spontaneous ventilation confirms full reversal should be treated with suspicion. Spontaneous ventilation is compatible with substantial residual block, and examiners include that option because it reflects the error that causes patient harm. The threshold to cite is a train-of-four ratio of 0.9 or above, and bedside tests such as head lift and hand squeeze are not adequate substitutes for objective measurement.