Chapter 15: Local Anesthetics — Module 4: Toxicity, Adverse Effects, and Special Populations Tier: Clinical Vignette (11 questions)
1. A 58-year-old man is receiving an ultrasound-guided interscalene block with 0.5% ropivacaine for shoulder surgery. Midway through injection he reports a metallic taste, perioral numbness, and ringing in his ears, then becomes agitated. What is the most appropriate immediate action?
A) Continue the injection quickly to finish before symptoms worsen
B) Administer a large 1 mg bolus of epinephrine and prepare to defibrillate
C) Stop the injection immediately, call for help and lipid emulsion, and manage the airway and any seizure with a benzodiazepine while monitoring for cardiovascular deterioration
D) Give an antihistamine and corticosteroid for a presumed allergic reaction
E) Reassure the patient that these are normal block sensations and proceed
ANSWER: C
Rationale:
The metallic taste, perioral numbness, tinnitus, and agitation are the classic early CNS prodrome of evolving LAST. The correct immediate action is to stop injecting at once, summon help and lipid emulsion, secure the airway, and treat any seizure with a benzodiazepine while watching for cardiovascular signs. Early recognition during the prodromal phase is precisely the window in which stopping the drug can prevent progression to seizures and cardiovascular collapse.
Option A: Option A is incorrect because continuing to inject delivers more drug and accelerates progression to severe toxicity.
Option B: Option B is incorrect because a standard 1 mg epinephrine bolus is specifically discouraged in LAST, where reduced doses are used; defibrillation is premature in a patient who has not arrested.
Option D: Option D is incorrect because the prodrome is dose-dependent toxicity, not an allergic reaction.
Option E: Option E is incorrect because these are not normal block sensations but warning signs that demand immediate cessation.
2. A 34-year-old woman undergoing a lumbar plexus block with 0.5% bupivacaine suddenly loses consciousness and develops a wide-complex ventricular dysrhythmia followed by pulseless arrest. Two standard cycles of ACLS with 1 mg epinephrine and defibrillation have not restored a perfusing rhythm. What management is most appropriate now?
A) Begin intravenous lipid emulsion (1.5 mL/kg bolus then infusion), switch to reduced epinephrine doses, avoid amiodarone and vasopressin, and continue prolonged high-quality CPR while the drug redistributes
B) Continue escalating to 2 mg epinephrine boluses every cycle as the primary strategy
C) Administer amiodarone for the ventricular dysrhythmia
D) Give a calcium channel blocker to counteract the sodium channel block
E) Stop resuscitation, since bupivacaine arrest is uniformly fatal
ANSWER: A
Rationale:
This is bupivacaine cardiotoxicity refractory to standard ACLS — the expected pattern given bupivacaine's slow dissociation from cardiac sodium channels. The correct management is intravenous lipid emulsion (1.5 mL/kg bolus followed by infusion), a switch to reduced epinephrine doses, avoidance of amiodarone and vasopressin, and prolonged high-quality CPR to maintain perfusion until the drug redistributes away from the myocardium.
Option B: Option B is incorrect because escalating standard or high-dose epinephrine can worsen arrhythmias in the poisoned myocardium and is specifically discouraged.
Option C: Option C is incorrect because amiodarone adds further sodium and potassium channel blockade to already-impaired conduction and should be avoided.
Option D: Option D is incorrect because routine calcium channel blockade is not part of LAST management and would not reverse the toxicity.
Option E: Option E is incorrect because bupivacaine arrest is potentially survivable with lipid emulsion and prolonged resuscitation, and abandoning effort is inappropriate.
3. A 45-year-old woman underwent outpatient tumescent liposuction with dilute lidocaine and epinephrine and was discharged home 3 hours after the procedure. About 8 hours after the infiltration she develops perioral numbness, agitation, and then a brief seizure at home. Which statement best explains this presentation?
A) This timing rules out local anesthetic toxicity, since LAST always occurs within minutes of injection
B) The seizure must be unrelated to the procedure because she had already been discharged
C) The presentation indicates an allergic reaction to lidocaine developing hours later
D) Tumescent infiltration produces extremely slow absorption, so peak lidocaine levels can occur many hours later; LAST can therefore present in delayed fashion after discharge, which is why extended post-procedure monitoring is recommended
E) The delayed onset proves the dose was within safe limits and no intervention is needed
ANSWER: D
Rationale:
The defining pharmacokinetic feature of tumescent anesthesia is extraordinarily slow systemic absorption from dilute, epinephrine-containing subcutaneous infiltrate, so peak plasma lidocaine may not be reached for many hours after the procedure. This is exactly why LAST from the tumescent technique can present late — in recovery or even after discharge — and why facilities are advised to extend monitoring and ensure lipid emulsion availability. Her delayed prodrome and seizure are consistent with peak absorption occurring hours post-procedure.
Option A: Option A is incorrect because delayed presentation is characteristic of the tumescent technique, not a reason to exclude LAST.
Option B: Option B is incorrect because discharge does not preclude evolving toxicity when the peak is delayed.
Option C: Option C is incorrect because the CNS prodrome and seizure reflect dose-dependent toxicity, not allergy.
Option E: Option E is incorrect because an actual seizure requires immediate management and does not indicate a safe dose.
4. A 3-week-old neonate is scheduled for a single-shot caudal epidural block to supplement general anesthesia for an inguinal hernia repair. The anesthesiologist is selecting the agent and planning monitoring. Which approach best reflects sound neonatal local anesthetic management?
A) Use the adult per-kilogram maximum dose, since dosing scales linearly with weight at all ages
B) Use bupivacaine 0.25% at 1 mL/kg (or ropivacaine 0.2% at 1 mL/kg) as a single-shot caudal, recognizing that immature hepatic metabolism and low protein binding prolong clearance, so repeated or continuous dosing must be reduced and monitoring extended
C) Place a continuous infusion at standard adult rates because neonates clear amides quickly
D) Avoid all regional anesthesia in neonates because local anesthetics are contraindicated under 1 year
E) Choose benzocaine infiltration to minimize systemic toxicity in this age group
ANSWER: B
Rationale:
Caudal epidural anesthesia is the most common pediatric regional technique, and the standard single-shot formulation is bupivacaine 0.25% at 1 mL/kg (ropivacaine 0.2% at 1 mL/kg is increasingly preferred for less motor block and a marginally better cardiac profile). Neonatal pharmacology is distinctive: immature CYP3A4 and CYP1A2 prolong amide half-life and reduce clearance, and low alpha-1-acid glycoprotein raises the free fraction, so per-kilogram maxima are lower, continuous infusion rates must be reduced, and monitoring should be extended.
Option A: Option A is incorrect because neonatal maxima are lower than adult per-kilogram values, not equal.
Option C: Option C is incorrect because neonates clear amides more slowly, making standard adult infusion rates dangerous.
Option D: Option D is incorrect because caudal blockade is a mainstay of pediatric anesthesia, not contraindicated.
Option E: Option E is incorrect because benzocaine risks methemoglobinemia, to which neonates are especially susceptible, and is not appropriate for caudal anesthesia.
5. A 62-year-old man receives a topical benzocaine spray to the oropharynx before a transesophageal echocardiogram. Within minutes he becomes dusky and cyanotic. His pulse oximeter reads 85% and does not improve despite 100% oxygen by non-rebreather mask, and he appears more cyanotic than his vital signs would suggest. What is the most appropriate next step?
A) Increase oxygen flow and observe, since the saturation will normalize with time
B) Treat empirically for an acute coronary syndrome with aspirin and nitroglycerin
C) Administer naloxone for presumed opioid-induced hypoventilation
D) Intubate for presumed anaphylaxis and give epinephrine
E) Suspect methemoglobinemia from benzocaine; obtain co-oximetry to measure the methemoglobin fraction, and treat significant or symptomatic methemoglobinemia with intravenous methylene blue
ANSWER: E
Rationale:
Benzocaine is a classic cause of acquired methemoglobinemia. The hallmark presentation is central cyanosis that does not improve with oxygen, a pulse oximeter that plateaus near 85% regardless of the true methemoglobin level, and cyanosis out of proportion to the apparent vitals. The correct step is to suspect methemoglobinemia, confirm with co-oximetry (which directly measures the methemoglobin fraction), and treat significant or symptomatic cases with intravenous methylene blue.
Option A: Option A is incorrect because the saturation will not normalize with more oxygen, since the defect is the hemoglobin's inability to carry oxygen.
Option B: Option B is incorrect because the oxygen-resistant cyanosis with a fixed 85% reading points to methemoglobinemia, not coronary ischemia.
Option C: Option C is incorrect because there is no opioid exposure and naloxone does not affect methemoglobin.
Option D: Option D is incorrect because the picture is not the urticaria, bronchospasm, and hypotension of anaphylaxis but rather oxygen-resistant cyanosis.
6. A 40-year-old man with known glucose-6-phosphate dehydrogenase (G6PD) deficiency develops symptomatic methemoglobinemia (measured fraction 32%) after prilocaine administration. He is dyspneic and visibly cyanotic. What is the most appropriate treatment?
A) Administer standard intravenous methylene blue 1 to 2 mg/kg as first-line therapy
B) Double the methylene blue dose to overcome the enzyme deficiency
C) Avoid methylene blue because his G6PD deficiency limits the NADPH needed to activate it (and methylene blue may worsen oxidant hemolysis); treat with ascorbic acid and, if severe or refractory, exchange transfusion, while supporting oxygenation
D) Withhold treatment, since a 32% methemoglobin fraction is harmless
E) Treat with high-flow oxygen alone, which will reduce the methemoglobin
ANSWER: C
Rationale:
Methylene blue requires NADPH generated by the hexose monophosphate shunt to be reduced to its active form; G6PD-deficient patients cannot supply adequate NADPH, so methylene blue is ineffective and, being an oxidant itself, may precipitate or worsen hemolysis. In this symptomatic patient the correct approach is to avoid methylene blue and use ascorbic acid, reserving exchange transfusion for severe or refractory cases, while supporting oxygenation.
Option A: Option A is incorrect because methylene blue is contraindicated in G6PD deficiency.
Option B: Option B is incorrect because the problem is lack of NADPH-dependent activation, which a higher dose cannot remedy and which increases oxidant risk.
Option D: Option D is incorrect because a symptomatic 32% methemoglobin fraction is dangerous and requires treatment.
Option E: Option E is incorrect because supplemental oxygen alone does not reduce methemoglobin back to functional hemoglobin.
7. A 29-year-old woman scheduled for laceration repair states she is "allergic to lidocaine." On detailed history, her only prior reaction was a racing, pounding heartbeat, sweating, and anxiety lasting a few minutes immediately after a dental injection, with no rash, swelling, or breathing difficulty. How should the clinician proceed?
A) Recognize that her symptoms are most consistent with systemic effects of the epinephrine in the dental anesthetic (or an anxiety/vasovagal response) rather than true allergy, since amide allergy is very rare, and proceed with an appropriate local anesthetic while monitoring
B) Document a confirmed lidocaine anaphylaxis and avoid all local anesthetics for life
C) Switch to general anesthesia for the laceration repair to avoid any local anesthetic exposure
D) Give a prophylactic epinephrine autoinjector before infiltration
E) Conclude she is allergic to all amide and ester agents and perform the repair without anesthesia
ANSWER: A
Rationale:
A brief episode of palpitations, sweating, and anxiety immediately after a dental injection — without urticaria, angioedema, or bronchospasm — is the classic picture of systemically absorbed epinephrine (commonly included in dental cartridges) or an anxiety/vasovagal response, not an IgE-mediated allergy. True amide allergy is very rare. The reassuring history allows the clinician to proceed with an appropriate local anesthetic, monitoring as usual; formal allergy evaluation is reserved for genuinely ambiguous histories.
Option B: Option B is incorrect because the symptoms are sympathomimetic, not anaphylactic, and lifelong avoidance is unwarranted.
Option C: Option C is incorrect because escalating to general anesthesia is unnecessary for a benign sympathomimetic reaction.
Option D: Option D is incorrect because a prophylactic epinephrine autoinjector is inappropriate and would reproduce the very sympathomimetic symptoms in question.
Option E: Option E is incorrect because there is no basis for assuming cross-class allergy, and withholding anesthesia is not justified.
8. A 30-year-old woman at 39 weeks gestation requests labor epidural analgesia. The anesthesiologist is selecting an initial dose. Compared with a non-pregnant adult, how should the dosing approach be modified, and why?
A) Increase the dose, because pregnancy expands the epidural space and requires more drug to achieve spread
B) Keep the dose identical to a non-pregnant adult, since pregnancy does not alter local anesthetic pharmacology
C) Increase protein-bound drug delivery, since albumin rises in pregnancy and binds more anesthetic
D) Reduce the dose, because the engorged epidural venous plexus reduces neuraxial space volume and increases cephalad spread, and reduced protein binding raises the free fraction, together heightening the risk of a high block and systemic toxicity
E) Use a benzocaine-based epidural solution to reduce the risk of systemic toxicity
ANSWER: D
Rationale:
At term, aortocaval compression engorges the epidural venous plexus and reduces the volume of the epidural and subarachnoid spaces, so a given dose spreads more cephalad and a high block can result; concurrently, dilutional hypoproteinemia reduces protein binding and raises the free fraction, increasing systemic toxicity risk. The rational modification is therefore careful dose reduction and titration.
Option A: Option A is incorrect because the neuraxial space is reduced, not expanded, so less drug is needed for a given spread.
Option B: Option B is incorrect because pregnancy substantially alters both distribution and binding.
Option C: Option C is incorrect because albumin falls in pregnancy, lowering binding and raising free drug.
Option E: Option E is incorrect because benzocaine is a topical agent associated with methemoglobinemia and is not used for epidural analgesia.
9. A 64-year-old man with Child-Pugh class C cirrhosis is to receive a continuous perineural catheter infusion of an amide local anesthetic for several days of postoperative analgesia after lower-extremity surgery. What is the most appropriate plan?
A) Use standard amide infusion rates, since hepatic disease does not affect local anesthetic clearance
B) Reduce the amide dose and infusion rate (commonly by 25 to 50%), monitor for early toxicity, and recognize that reduced hepatic clearance plus lower protein binding raise accumulation and free-drug risk; an ester agent for any infiltration component avoids the hepatic-metabolism concern entirely
C) Increase the amide infusion rate to compensate for faster clearance in cirrhosis
D) Avoid all regional anesthesia because local anesthetics are absolutely contraindicated in cirrhosis
E) Choose racemic bupivacaine at maximal dosing because it is the safest agent in liver disease
ANSWER: B
Rationale:
Amide local anesthetics depend on hepatic metabolism, and advanced cirrhosis reduces clearance through both impaired enzyme capacity and reduced hepatic blood flow, prolonging half-life and promoting accumulation during continuous infusion; reduced albumin and alpha-1-acid glycoprotein further raise the free fraction. The appropriate plan is to reduce the amide dose and infusion rate (commonly 25 to 50%), monitor for early toxicity, and, for any infiltration component, consider an ester agent, which is hydrolyzed by plasma esterases and bypasses the hepatic concern.
Option A: Option A is incorrect because hepatic disease clearly reduces amide clearance.
Option C: Option C is incorrect because clearance is reduced, not increased, so a higher rate would worsen accumulation.
Option D: Option D is incorrect because regional anesthesia is not absolutely contraindicated; it requires dose adjustment and monitoring.
Option E: Option E is incorrect because racemic bupivacaine is the most cardiotoxic amide and maximal dosing in cirrhosis is unsafe.
10. A 38-year-old man received hyperbaric lidocaine 5% spinal anesthesia in the lithotomy position for a brief urologic procedure. About 14 hours later he develops aching pain in both buttocks radiating to the posterior thighs. Neurologic examination is normal, with intact strength, sensation, and bowel and bladder function. What is the most likely diagnosis and appropriate management?
A) Cauda equina syndrome; obtain urgent MRI and neurosurgical consultation for decompression
B) Epidural hematoma; reverse anticoagulation and image emergently
C) Spinal abscess; start empiric antibiotics and obtain imaging
D) Anterior spinal artery syndrome; begin blood pressure augmentation
E) Transient neurologic symptoms (TNS); manage with NSAIDs and reassurance, expecting spontaneous resolution within about 72 hours without permanent deficit, while remaining alert for any true neurologic deficit
ANSWER: E
Rationale:
Bilateral buttock and posterior-thigh pain beginning within 6 to 24 hours of spinal anesthesia, with an entirely normal neurologic examination, is the classic presentation of transient neurologic symptoms (TNS) — most common after intrathecal hyperbaric lidocaine 5%, especially in the lithotomy position. TNS is self-limited, resolving within about 72 hours without permanent deficit, and is managed with NSAIDs and reassurance (low-dose opioids if pain is severe).
Option A: Option A is incorrect because cauda equina syndrome produces objective deficits such as saddle anesthesia and bowel or bladder dysfunction, which are absent here.
Option B: Option B is incorrect because an epidural hematoma typically causes progressive neurologic deficits, not isolated pain with a normal exam.
Option C: Option C is incorrect because a spinal abscess usually presents with fever, back pain, and evolving deficits over days.
Option D: Option D is incorrect because anterior spinal artery syndrome causes motor and sensory deficits, not isolated self-limited pain with intact function.
11. A 70-year-old man with atrial fibrillation maintained on flecainide (a Class I antiarrhythmic that blocks cardiac sodium channels) and a baseline widened QRS-complex (the ventricular depolarization waveform, QRS) on his electrocardiogram requires a large-volume lower-extremity nerve block. Which plan best reduces his cardiac risk?
A) Use a large volume of racemic bupivacaine, since it is the safest long-acting agent for this patient
B) Proceed with standard dosing, because flecainide protects the heart against local anesthetic toxicity
C) Recognize that the local anesthetic and flecainide both block cardiac sodium channels with additive effects, lowering his threshold for conduction toxicity; choose ropivacaine over racemic bupivacaine, use the lowest effective dose, fractionate the injection, and ensure lipid emulsion and resuscitation resources are available
D) Choose bupivacaine specifically because it dissociates from sodium channels faster than ropivacaine
E) Disregard the widened QRS, since baseline conduction has no bearing on local anesthetic selection
ANSWER: C
Rationale:
Local anesthetics produce cardiac toxicity by blocking the Nav channel, and flecainide blocks the same channel; their effects are additive, so a patient with baseline conduction slowing (widened QRS) has a reduced threshold for further conduction toxicity from a large anesthetic load. The risk-minimizing plan selects the less cardiotoxic agent (ropivacaine rather than racemic bupivacaine), uses the lowest effective dose with fractionated injection, and ensures lipid emulsion and resuscitation resources are ready.
Option A: Option A is incorrect because racemic bupivacaine is the most cardiotoxic option and a large volume compounds the additive risk.
Option B: Option B is incorrect because flecainide adds to, rather than protects against, sodium-channel toxicity.
Option D: Option D is incorrect because bupivacaine dissociates slowly, not faster, and is more cardiotoxic than ropivacaine.
Option E: Option E is incorrect because a baseline widened QRS signals heightened conduction risk that should directly inform agent and dose selection.
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