Select the true statement about the interaction of nondepolarizing neuromuscular blocking drugs when durations are dissimilar.

Select the true statement about the interaction of nondepolarizing neuromuscular blocking drugs when durations are dissimilar.

• A. If a long-acting drug is administered after an intermediate-acting drug, the duration of the long-acting drug will be longer than normal
• B. If a long-acting drug is administered after an intermediate-acting drug, the duration of the long-acting drug will be about the same as expected
• C. If an intermediate-acting drug is administered after a long-acting drug, the duration of the intermediate-acting drug will be about the same as expected
• D. If an intermediate-acting drug is administered after a long-acting drug, the duration of action of the intermediate-acting drug will be shorter than expected

Answer: (D)

Nondepolarizing neuromuscular blockers are competitive antagonists at the nicotinic acetylcholine receptor at the motor end plate. Their duration depends on how long they occupy receptors and how quickly they dissociate and are cleared. When a long-acting blocker is already occupying many receptors, adding a second blocker with a shorter duration has fewer available receptors to bind to. That means the incremental blockade produced by the shorter-acting drug is limited, so its apparent duration ends up shorter than when it’s used alone. The overall blockade may persist because the long-acting drug remains, but the shorter-acting drug itself cannot manifest its full duration in the presence of the long-acting blocker. That’s why the intermediate-acting drug’s duration appears shorter than expected when given after a long-acting one. If the order is reversed, the long-acting drug will still provide ongoing blockade according to its own pharmacokinetics, and the presence of the shorter-acting blocker won’t substantially shorten its duration. The other statements imply that the durations are unchanged or extended, which isn’t supported by this competitive-receptor interaction.

Nondepolarizing neuromuscular blockers are competitive antagonists at the nicotinic acetylcholine receptor at the motor end plate. Their duration depends on how long they occupy receptors and how quickly they dissociate and are cleared. When a long-acting blocker is already occupying many receptors, adding a second blocker with a shorter duration has fewer available receptors to bind to. That means the incremental blockade produced by the shorter-acting drug is limited, so its apparent duration ends up shorter than when it’s used alone. The overall blockade may persist because the long-acting drug remains, but the shorter-acting drug itself cannot manifest its full duration in the presence of the long-acting blocker. That’s why the intermediate-acting drug’s duration appears shorter than expected when given after a long-acting one.

If the order is reversed, the long-acting drug will still provide ongoing blockade according to its own pharmacokinetics, and the presence of the shorter-acting blocker won’t substantially shorten its duration. The other statements imply that the durations are unchanged or extended, which isn’t supported by this competitive-receptor interaction.