A 63-year-old man hospitalized for chronic obstructive pulmonary disease exacerbation develops hypercapnic respiratory failure. He is transferred to the intensive care unit, where he undergoes intubation after premedication with muscle relaxant X and an appropriate sedative. The patient is then started on mechanical ventilation; however, he remains apneic longer than expected. Anesthesiology is consulted, and the patient's neuromuscular blockade is assessed by electrically stimulating a peripheral nerve 4 times in quick succession and observing the muscular response. The procedure is repeated 30 minutes later and the results are shown below. Muscle relaxant X is most likely to be which of the following medications?

Question

A 63-year-old man hospitalized for chronic obstructive pulmonary disease exacerbation develops hypercapnic respiratory failure.  He is transferred to the intensive care unit, where he undergoes intubation after premedication with muscle relaxant X and an appropriate sedative.  The patient is then started on mechanical ventilation; however, he remains apneic longer than expected.  Anesthesiology is consulted, and the patient's neuromuscular blockade is assessed by electrically stimulating a peripheral nerve 4 times in quick succession and observing the muscular response.  The procedure is repeated 30 minutes later and the results are shown below.  Muscle relaxant X is most likely to be which of the following medications?

Quizplus · Accepted Answer

11ec160b_3c62_29b4_9249_f71fd2c0eabb_MD0001_00 Train-of-four (TOF) stimulation is used during anesthesia to assess the degree of paralysis induced by neuromuscular junction (NMJ)-blocking agents. A peripheral nerve is stimulated 4 times in quick succession and the muscular response is recorded. The height of each bar represents the strength of each twitch; higher bars indicate the activation of increasing numbers of individual muscle fibers (myocytes). When a nondepolarizing NMJ blocker (eg, vecuronium) is administered, competitive inhibition of postsynaptic acetylcholine receptors at the motor endplate prevents some of these fibers from activating, decreasing the strength of the twitch. TOF stimulation shows a progressive reduction in each of the 4 responses (fading pattern) as a result of less acetylcholine being released with each subsequent impulse (due to the additional effect of presynaptic acetylcholine receptor blockade). In contrast, depolarizing blockers (eg, succinylcholine) initially function by preventing repolarization of the motor endplate and show equal reduction of all 4 twitches during TOF stimulation (phase I blockade). The responses remain equal because the presynaptic acetylcholine receptor stimulation helps to mobilize presynaptic acetylcholine vesicles for release. Persistent exposure to succinylcholine results in eventual transition to phase II blockade as the acetylcholine receptors become desensitized and inactivated (ie, functionally similar to nondepolarizing blockade). Succinylcholine is commonly administered for rapid-sequence intubation due to its rapid onset (<1 minute). The duration of action is determined by its metabolism by plasma cholinesterase and is typically <10 minutes. However, some patients are homozygous for an atypical plasma cholinesterase, which breaks down succinylcholine more slowly. In these patients, the paralysis can last for hours and they must be maintained on mechanical ventilation until spontaneous respirations resume. (Choice A) Dantrolene relaxes skeletal muscle by reducing the release of Ca²⁺ from the sarcoplasmic reticulum. Dantrolene is used to treat malignant hyperthermia and neuroleptic malignant syndrome. It is not routinely used as a neuromuscular paralytic agent. (Choice B) Benzodiazepines are effective for sedation, but they have no direct action at the NMJ and do not provide sufficient muscle paralysis to facilitate intubation. (Choices C and E) Pancuronium and tubocurarine are nondepolarizing NMJ blockers. Unlike depolarizing NMJ drugs, these agents do not function in distinct phases and their TOF responses always display a fading pattern. Neostigmine administration reverses nondepolarizing NMJ blockade. Educational objective: Succinylcholine is a fast-acting, depolarizing neuromuscular blocking agent used for rapid-sequence intubation that causes equal reduction of all 4 twitches during train-of-four stimulation (phase I blockade). Prolonged administration of succinylcholine or use in patients with abnormal plasma cholinesterase activity causes transition to a phase II (nondepolarizing) block, seen as a progressive reduction in each of the 4 twitches.

A 63-Year-Old Man Hospitalized for Chronic Obstructive Pulmonary Disease Exacerbation