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Katzung & Trevor's Pharmacology: Examination & Board Review, 12e

Chapter 27: Skeletal Muscle Relaxants

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Characteristics of phase I depolarizing neuromuscular blockade due to succinylcholine include

(A) Easy reversibility with nicotinic receptor antagonists

(B) Marked muscarinic blockade

(C) Muscle fasciculations only in the later stages of block

(D) Reversibility by acetylcholinesterase (AChE) inhibitors

(E) Sustained tension during a period of tetanic stimulation

Phase I depolarizing blockade caused by succinylcholine is not associated with antagonism at muscarinic receptors, nor is it reversible with cholinesterase inhibitors. Muscle fasciculations occur at the start of the action of succinylcholine. The answer is E.

A patient underwent a surgical procedure of 2 h. Anesthesia was provided by isoflurane, supplemented by intravenous midazolam and a nondepolarizing muscle relaxant. At the end of the procedure, a low dose of atropine was administered followed by pyridostigmine.

The main reason for administering atropine was to

(A) Block cardiac muscarinic receptors

(B) Enhance the action of pyridostigmine

(C) Prevent spasm of gastrointestinal smooth muscle

(D) Provide postoperative analgesia

(E) Reverse the effects of the muscle relaxant

Acetylcholinesterase inhibitors used for reversing the effects of nondepolarizing muscle relaxants cause increases in ACh at all sites where it acts as a neurotransmitter. To offset the resulting side effects, including bradycardia, a muscarinic blocking agent is used concomitantly. Although atropine is effective, glycopyrrolate is usually preferred because it lacks CNS effects. The answer is A.

A muscarinic receptor antagonist would probably not be needed when a cholinesterase inhibitor was given for reversal of the skeletal muscle relaxant actions of a nondepolarizing drug if the NM blocking agent used was

(A) Cisatracurium

(B) Mivacurium

(C) Pancuronium

(D) Tubocurarine

(E) Vecuronium

One of the distinctive characteristics of pancuronium is that it can block muscarinic receptors, especially those in the heart. It has sometimes caused tachycardia and hypertension and may cause dysrhythmias in predisposed individuals. The answer is C.

Which of the following drugs is the most effective in the emergency management of malignant hyperthermia?

(A) Atropine

(B) Dantrolene

(C) Haloperidol

(D) Succinylcholine

(E) Vecuronium

Prompt treatment is essential in malignant hyperthermia to control body temperature, correct acidosis, and prevent calcium release. Dantrolene interacts with the RyR1 channel to block the release of activator calcium from the sarcoplasmic reticulum, which prevents the tension-generating interaction of actin with myosin. The answer is B.

The clinical use of succinylcholine, especially in patients with diabetes, is associated with

(A) Antagonism by pyridostigmine during the early phase of blockade

(B) Aspiration of gastric contents

(C) Decreased intragastric pressure

(D) Histamine release in a genetically determined population

(E) Metabolism at the neuromuscular junction by acetylcholinesterase

Fasciculations associated with succinylcholine may increase intragastric pressure with possible complications of regurgitation and aspiration of gastric contents. The complication is more likely in patients with delayed gastric emptying such as those with esophageal dysfunction or diabetes. Histamine release resulting from succinylcholine is not genetically determined. The answer is B.

Which drug is most often associated with hypotension and is related to clonidine?

(A) Baclofen

(B) Pancuronium

(C) Succinylcholine

(D) Tizanidine

(E) Vecuronium

Tizanidine causes hypotension via α₂-adrenoceptor activation, like its congener clonidine. Hypotension may occur with tubocurarine (not listed) due partly to histamine release and to ganglionic blockade. The answer is D.

Regarding the spasmolytic drugs, which of the following statements is least accurate?

(A) Baclofen acts on GABA receptors in the spinal cord to increase chloride ion conductance

(B) Cyclobenzaprine decreases both oropharyngeal secretions and gut motility

(C) Dantrolene has no significant effect on the release of calcium from sarcoplasmic reticulum in cardiac muscle

(D) Diazepam causes slight sedation at doses commonly used to reduce muscle spasms

(E) Intrathecal use of baclofen is effective in some refractory cases of muscle spasticity

Baclofen activates GABA_B receptors in the spinal cord. However, these receptors are coupled to K⁺ channels (see Chapter 22). GABA_A receptors in the CNS modulate chloride ion channels, an action facilitated by diazepam and other benzodiazepines. The answer is A.

Which drug is most likely to cause hyperkalemia leading to cardiac arrest in patients with spinal cord injuries?

(A) Baclofen

(B) Dantrolene

(C) Pancuronium

(D) Succinylcholine

(E) Vecuronium

Skeletal muscle depolarization by succinylcholine releases potassium from the cells, and the ensuing hyperkalemia can be life-threatening in terms of cardiac arrest. Patients most susceptible include those with extensive burns, spinal cord injuries, neurologic dysfunction, or intra-abdominal infection. The answer is D.

Which drug has spasmolytic activity and could also be used in the management of seizures caused by overdose of a local anesthetic?

(A) Baclofen

(B) Cyclobenzaprine

(C) Diazepam

(D) Gabapentin

(E) Tizanidine

Diazepam is both an effective antiseizure drug and a spasmolytic. The spasmolytic action of diazepam is thought to be exerted partly in the spinal cord because it reduces spasm of skeletal muscle in patients with cord transection. Cyclobenzaprine is used for acute local spasm and has no antiseizure activity. The answer is C.

Myalgias are a common postoperative complaint of patients who receive large doses of succinylcholine, possibly the result of muscle fasciculations caused by depolarization. Which drug administered in the operating room can be used to prevent postoperative pain caused by succinylcholine?

(A) Atracurium

(B) Baclofen

(C) Dantrolene

(D) Diazepam

(E) Lidocaine

The depolarizing action of succinylcholine at the skeletal muscle end plate can be antagonized by small doses of nondepolarizing blockers. To prevent skeletal muscle fasciculations and the resulting postoperative pain caused by succinylcholine, a small nonparalyzing dose of a nondepolarizing drug (eg, atracurium) is sometimes given immediately before succinylcholine. The answer is A.

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Chapter 27: Skeletal Muscle Relaxants

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