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
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.
The most important effect of inadvertent intravenous administration of a large dose of lidocaine is
Of the effects listed, the most important in local anesthetic overdose (of both amide and ester types) concern the CNS. Such effects can include sedation or restlessness, nystagmus, coma, respiratory depression, and seizures. Intravenous diazepam is commonly used for seizures caused by local anesthetics. Methemoglobinemia is caused by a prilocaine metabolite. The answer is D.
All of the following factors influence the action of local anesthetics except
(A) Acetylcholinesterase activity in the region of the injection site
(B) Blood flow through the tissue in which the injection is made
(C) Dose of local anesthetic injected
(D) The use of vasoconstrictors
Local anesthetics are poor substrates for acetylcholinesterase, and the activity of this enzyme does not play a part in terminating the actions of local anesthetics. Ester-type local anesthetics are hydrolyzed by plasma (and tissue) pseudocholinesterases. Persons with genetically based defects in pseudocholinesterase activity are unusually sensitive to procaine and other esters. The answer is A.
You have a vial containing 10 mL of a 2% solution of lidocaine. How much lidocaine is present in 1 mL?
The fact that you have 10 mL of the solution of lidocaine is irrelevant. A 2% solution of any drug contains 2 g/100 mL. The amount of lidocaine in 1 mL of a 2% solution is thus 0.02 g, or 20 mg. The answer is D.
Which statement about the toxicity of local anesthetics is correct?
(A) Bupivacaine is the safest local anesthetic to use in patients at risk for cardiac arrhythmias
(B) In overdosage, hyperventilation (with oxygen) is helpful to correct acidosis and lower extracellular potassium
(C) Intravenous injection of local anesthetics may stimulate ectopic cardiac pacemaker activity
(D) Most local anesthetics cause vasoconstriction
(E) Serious cardiovascular reactions are more likely to occur with tetracaine than with bupivacaine
Acidosis resulting from tissue hypoxia favors local anesthetic toxicity because these drugs bind more avidly (or dissociate more slowly) from the sodium channel binding site when they are in the charged state. (Note that onset of therapeutic effect may be slower because charged local anesthetics penetrate the membrane less rapidly; see text.) Hyperkalemia depolarizes the membrane, which also favors local anesthetic binding. Oxygenation reduces both acidosis and hyperkalemia. Bupivacaine may cause severe cardiotoxicity including arrhythmias. Rapid administration of a lipid emulsion may improve recovery from bupivacaine and levobupivacaine. The answer is B.
A vasoconstrictor added to a solution of lidocaine for a peripheral nerve block will
(A) Decrease the risk of a seizure
(B) Increase the duration of anesthetic action of the local anesthetic
Epinephrine increases the duration of a nerve block when it is administered with short- and medium-duration local anesthetics. As a result of the vasoconstriction that prolongs the duration of this block, less local anesthetic is required, so the risk of toxicity (eg, a seizure) is reduced. The answer is C.
A child requires multiple minor surgical procedures involving the nasopharynx. Which drug has high surface local anesthetic activity and intrinsic vasoconstrictor actions that reduce bleeding in mucous membranes?
Most local anesthetics cause vasodilation. Cocaine is the only local anesthetic with intrinsic vasoconstrictor activity owing to its action to block the reuptake of norepinephrine released from sympathetic nerve endings (Chapter 9). Cocaine also has significant surface local anesthetic activity and is favored for head, neck, and pharyngeal surgery. The answer is B.
Prilocaine is relatively contraindicated in patients with cardiovascular or pulmonary disease because the drug
(A) Acts as an antagonist at β adrenoceptors in the heart and the lung
(B) Causes decompensation through formation of methemoglobin
(C) Inhibits cyclooxygenase in cardiac and pulmonary cells
(D) Is a potent bronchoconstrictor
Large doses of prilocaine may cause accumulation of o-toluidine, a metabolite that converts hemoglobin to methemoglobin. Patients may become cyanotic with blood “chocolate colored.” High blood levels of methemoglobin have resulted in decompensation in patients who have cardiac or pulmonary diseases. The answer is B.