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Chapter 17: Muscle Relaxants

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How does succinylcholine cause neuromuscular blockade at the neuromuscular junction?

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(A) competitive inhibition of acetylcholine

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(B) sensitization of the nicotinic acetylcholine receptor (nAChR)

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(C) inactivation of voltage gated sodium channels

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(D) decreased potassium permeability

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(E) decreased depolarization of the end-plate

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The answer is C. Succinylcholine's mechanism of action remains incompletely understood. Succinylcholine binds to the nAChR and causes prolonged depolarization of the end-plate. This leads to:

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  • inactivation of voltage gated sodium channels

  • desensitization of the nAChR

  • increased potassium permeability

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Nondepolarizing muscle relaxants are competitive inhibitors of acetylcholine.

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Ref: Miller RD. Miller's Anesthesia. 8th ed. Philadelphia, PA: Elsevier; 2015.

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A nondepolarizing muscle relaxant (NDMR) binds to which subunit of the nicotinic acetylcholine receptor (nAChR)?

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(A) alpha

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(B) beta

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(C) delta

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(D) epsilon

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(E) gamma

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The answer is A. The nAChR is made up of 2 alpha-subunits, 1 beta-subunit, 1 delta-subunit, and either 1 gamma-subunit or 1 epsilon-subunit. A NDMR binds to the alpha-subunit of the nAChR. The adult nAChR contains the gamma-subunit and the fetal nAChR contains the epsilon-subunit.

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Ref: Miller RD. Miller's Anesthesia. 8th ed. Philadelphia, PA: Elsevier; 2015.

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Concurrent disease states can alter the response to nondepolarizing muscle relaxants (NDMRs). Which disease state results in a hypersensitive response to NDMRs?

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(A) burn injury

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(B) cerebral palsy

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(C) peripheral nerve injury

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(D) hemiplegia

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(E) myasthenia gravis

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The answer is E. Myasthenia gravis results in a hypersensitive response to nondepolarizing muscle relaxants (NDMRs). Burn injury, cerebral palsy, peripheral nerve injury, and hemiplegia cause resistance to NDMRs.

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Ref: Butterworth JF IV, Mackey DC, Wasnick JD. Morgan & Mikhail's Clinical Anesthesiology. 5th ed. New York, NY: McGraw Hill; 2013.

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Liver failure will have the most significant effect on the duration of action of which nondepolarizing muscle relaxant (NDMR)?

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(A) vecuronium

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(B) cisatracurium

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(C) rocuronium

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