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Chapter 26: Local Anesthetics

Characteristic properties of local anesthetics include all of the following except

(A) An increase in membrane refractory period

(B) Blockade of voltage-dependent sodium channels

(C) Effects on vascular tone

(D) Preferential binding to resting channels

(E) Slowing of axonal impulse conduction

Local anesthetics bind preferentially to sodium channels in the open and inactivated states. Recovery from drug-induced block is 10–1000 times slower than recovery of channels from normal inactivation. Resting channels have a lower affinity for local anesthetics. The answer is D.

The pKa of lidocaine is 7.7. In infected tissue, which can be acidic, for example, at pH 6.7, the percentage of the drug in the nonionized form will be

(A) 1%

(B) 10%

(C) 50%

(D) 90%

(E) 99%

Because the drug is a weak base, it is more ionized (protonated) at pH values lower than its pKa. Because the pH given is 1 log unit lower (more acid) than the pKa, the ratio of ionized to nonionized drug will be approximately 90:10. The answer is B. (Recall from Chapter 1 that at a pH equal to pKa, the ratio is 1:1; at 1 log unit difference, the ratio is approximately 90:10; at 2 log units difference, 99:1; and so on.)

Which statement about the speed of onset of nerve blockade with local anesthetics is correct?

(A) Faster in hypercalcemia

(B) Faster in myelinated fibers

(C) Faster in tissues that are infected

(D) Slower in hyperkalemia

(E) Slower in the periphery of a nerve bundle than in the center of a bundle

Myelinated nerve fibers are blocked by local anesthetics more readily than unmyelinated ones. See the Skill Keeper answer for an explanation of the effects of hypocalcemia and hyperkalemia on nerve blockade by local anesthetics. The answer is B.

A patient experiences a strong toxic effect after the administration of a large intravenous dose of lidocaine. He is then quickly treated with intravenous diazepam. Which toxicity is this patient most likely experiencing?


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