Local Anesthetics

Mechanism of Sodium Channel Blockade

• Local anesthetics penetrate nerve cell membranes in their lipophilic, non-ionized form (B form) by diffusion through the lipid bilayer
• Once inside the axoplasm, the higher intracellular pH causes protonation to the ionized form (BH+)
• The quaternary ammonium cation (BH+) binds irreversibly to voltage-gated sodium channels from the intracellular side, blocking the channel in its inactive conformation
• This prevents sodium influx during depolarization, raising the threshold for action potential generation and blocking conduction propagation along the nerve fiber
• Blockade is use-dependent: channels must be in the inactive state; resting channels bind anesthetic poorly
• This explains why local anesthetics preferentially block rapidly firing nerves (pain fibers) while sparing motor function initially

Differential Nerve Fiber Blockade

• Type C fibers (pain, temperature, slow postganglionic) block first due to small diameter, high surface-area-to-volume ratio, and lower conduction velocity allowing higher local drug concentrations
• Type A-delta fibers (sharp pain, cold) block second
• Larger myelinated fibers (touch, pressure, motor—types A-alpha, A-beta) require higher concentrations and block last due to their thicker myelin sheath and saltatory conduction reducing drug access to nodes of Ranvier
• This differential blockade explains why patients may lose pain sensation while retaining motor function and touch initially

Pharmacokinetics: Absorption and Metabolism

• Onset depends on pKa (proportion of ionized vs non-ionized drug): lower pKa = faster nerve penetration; lidocaine pKa 7.9 has faster onset than bupivacaine pKa 8.1
• Vascularity of injection site dramatically affects systemic absorption; highly vascular areas (intercostal, IV regional) absorb drug rapidly, while less vascular sites (subcutaneous, epidural) allow slower absorption and longer local duration
• Vasoconstrictor addition (epinephrine 1:200,000 to 1:1,000,000) reduces blood flow, decreasing systemic absorption by up to 3-fold and prolonging local duration while reducing toxicity risk
• Ester local anesthetics (procaine, benzocaine, tetracaine) undergo rapid hydrolysis by plasma pseudocholinesterase (half-life 6-8 hours), making them safer in overdose but requiring more frequent redosing
• Amide local anesthetics (lidocaine, bupivacaine, prilocaine, mepivacaine, articaine) undergo hepatic metabolism via N-dealkylation and oxidation; prilocaine is unique in producing metabolites (o-toluidine) that cause methemoglobinemia
• Hepatic disease prolongs amide metabolism significantly, increasing toxicity risk

Systemic Toxicity Mechanism

• Systemic absorption produces concentration-dependent toxicizing effects on CNS and cardiovascular system
• CNS toxicity occurs at lower plasma concentrations (4-6 μg/mL), manifesting as excitation (circumoral paresthesias, tinnitus, restlessness, seizures) due to selective blockade of inhibitory GABA interneurons
• Higher concentrations cause CNS depression (loss of consciousness, respiratory depression)
• Cardiovascular toxicity emerges at higher concentrations (>5-7 μg/mL for lidocaine, lower for bupivacaine at 1-3 μg/mL), manifesting as myocardial depression, conduction abnormalities, and arrhythmias
• Bupivacaine is significantly more cardiotoxic than lidocaine due to rapid cardiopulmonary collapse and profound bradycardia, particularly with intravenous injection; bupivacaine binds cardiac sodium channels longer with slower dissociation, explaining greater potency and toxicity
• Acidosis, hypoxemia, and hyperkalemia all increase susceptibility to toxicity by altering drug binding and cardiac irritability

Excessive Dosing

• The most common cause of toxicity; maximum safe doses are weight-based: lidocaine without epinephrine 4.5 mg/kg (max 300 mg total), with epinephrine 7 mg/kg (max 500 mg); bupivacaine 2.5 mg/kg (max 175 mg); prilocaine 6 mg/kg due to methemoglobinemia risk
• Clinicians must calculate doses based on concentration (% solution) and volume: 1% = 10 mg/mL, 0.5% = 5 mg/mL
• Repeated injections at close intervals without accounting for residual drug levels

Accidental Intravascular Injection

• Direct injection into blood vessels during regional anesthesia (nerve blocks, epidural, spinal) bypasses the normal absorption gradient, causing rapid systemic toxicity
• Risk factors include block techniques with high vascular density (intercostal, interscalene, paravertebral, inferior alveolar blocks), improper needle positioning, and failure to aspirate before injection
• Intracardiac injection (during central line placement or cardiac interventions) produces immediate, severe cardiovascular collapse

Impaired Drug Metabolism

• Hepatic disease (cirrhosis, acute liver failure): delays amide metabolism by 2-5 fold; ester drugs relatively safer but still at risk from reduced pseudocholinesterase synthesis
• Cardiac failure reduces hepatic blood flow, impeding metabolism of amides
• Pseudocholinesterase deficiency (genetic atypical forms, acquired deficiency from liver disease, malignancy, medications like anticholinesterases, pregnancy) impairs ester hydrolysis
• Enzyme inhibitors: medications that inhibit hepatic metabolism (cimetidine, propranolol, certain anticonvulsants) increase amide accumulation
• Renal failure (though kidneys don't metabolize local anesthetics, they affect clearance of metabolites and electrolyte balance)

Increased Drug Absorption

• High vascularity of injection site (intercostal >caudal >epidural >brachial plexus >subcutaneous) increases systemic absorption even without intravascular injection
• Inflammatory or infected sites: increased blood flow and altered tissue pH enhance absorption
• Omission or inadequate dosing of vasoconstrictors, particularly with long-acting agents like bupivacaine used in highly vascular regions
• Pregnancy: increased cardiac output, decreased pseudocholinesterase levels, and altered volume of distribution increase systemic absorption

Prilocaine-Specific Risk: Methemoglobinemia

• Prilocaine is metabolized to o-toluidine, which oxidizes hemoglobin's iron from Fe²⁺ to Fe³⁺, forming methemoglobin that cannot bind oxygen
• Risk escalates with doses >600 mg in adults or >20 mg/kg; benzocaine also carries this risk, particularly topical benzocaine spray used for upper airway procedures
• Genetic glucose-6-phosphate dehydrogenase (G6PD) deficiency and concurrent oxidant drugs (dapsone, sulfonamides, nitrates) dramatically increase susceptibility
• Neonates have immature methemoglobin reductase activity and are at higher risk

Individual Patient Factors

• Age extremes: elderly patients have reduced hepatic metabolism and cardiac reserve; neonates have immature metabolism and reduced blood-brain barrier protection
• Low body weight: direct impact on maximum safe dosing
• Concurrent cardiopulmonary disease: reduced clearance and increased toxicity susceptibility
• Pregnancy: physiologic changes increase absorption and CNS/cardiac toxicity risk

Early CNS Manifestations (Excitatory Phase)

• Circumoral paresthesias and tongue numbness: earliest and most reliable warning sign of systemic toxicity, resulting from local anesthetic effect on perioral sensory nerves
• Tinnitus and visual disturbances: indicating cortical effects
• Restlessness, agitation, talkativeness: suggesting selective inhibition of inhibitory pathways
• Tremor: particularly of the extremities
• Mild hypertension and tachycardia: from sympathetic activation and catecholamine release

Severe CNS Manifestations (Seizure Phase)

• Generalized tonic-clonic seizures: the hallmark severe CNS toxicity presentation, occurring as concentrations exceed the threshold for GABA blockade effects
• Seizure threshold is lower in acidotic, hypoxic, or hyperkalemic patients and in those taking other CNS-depressant drugs
• Seizures may be brief and self-limited (seconds to 1-2 minutes) or prolonged; recurrent seizures are common if high drug levels persist
• Loss of consciousness may occur without preceding warning signs, particularly with rapid IV injection

Cardiovascular Manifestations

• Mild phase: initial hypertension and tachycardia (from CNS stimulation and catecholamine response)
• Moderate-severe phase: progression to bradycardia, hypotension, conduction abnormalities (widened QRS, prolonged PR interval, peaked T waves)
• Bupivacaine-specific pattern: profound bradycardia and sudden cardiovascular collapse with minimal warning; may present as asystole or severe bradycardia refractory to treatment
• Ventricular arrhythmias: including ventricular fibrillation, particularly if myocardial ischemia or electrolyte abnormalities coexist
• Myocardial depression manifests as decreased cardiac output, hypotension, and shock

Respiratory Effects

• Seizure-associated apnea: if seizures occur, muscle rigidity and loss of airway protection
• Respiratory depression: from CNS depression in later stages
• Aspiration risk: loss of protective airway reflexes with consciousness alterations

Methemoglobinemia (Prilocaine, Benzocaine)

• Cyanosis unresponsive to supplemental oxygen: "chocolate cyanosis" with characteristic brownish discoloration
• Dyspnea, shortness of breath: from impaired oxygen-carrying capacity
• Headache, dizziness, altered mental status: from cerebral hypoxia
• Tachycardia and tachypnea: compensatory response to tissue hypoxia

Allergic Reactions (Ester Anesthetics)

• Urticaria, pruritus, angioedema: typical IgE-mediated type I hypersensitivity
• Anaphylaxis: rare but possible with esters due to para-aminobenzoic acid (PABA) metabolite; amides rarely cause true allergy due to lack of PABA metabolite
• Contact dermatitis: from topical application, particularly with benzocaine

Local Tissue Effects

• Nerve damage (neuropraxia): from direct neurotoxicity or mechanical trauma during injection; manifests as numbness, weakness, or pain in the distribution of the blocked nerve(s)
• Duration varies: temporary (hours to days) with most agents, permanent with high concentrations of certain anesthetics
• Transient neurologic symptoms (TNS): common with spinal lidocaine, manifesting as lower back pain, buttock pain, or thigh pain 24 hours after spinal anesthesia
• Myotoxicity: direct muscle damage from local anesthetics, particularly bupivacaine and doxorubicin combinations
• Necrosis and sloughing: from excessive concentrations, particularly in confined spaces (digital nerve blocks without epinephrine)

Clinical Recognition of Systemic Toxicity

• History and context are paramount: any procedure involving regional anesthesia (nerve block, epidural, spinal, local infiltration) where local anesthetics were recently injected
• Temporal relationship critical: symptoms appearing within minutes (seconds to minutes for IV injection, 5-30 minutes for regional injection) of anesthetic administration strongly suggest toxicity
• Absence of other obvious causes: hypoglycemia, hypoxia, and dysrhythmia should be ruled out rapidly, but their coexistence does not exclude local anesthetic toxicity

Physical Examination Findings

• Circumoral paresthesias on the patient's report (examine light touch sensation around lips and buccal mucosa if safe): most sensitive early warning sign
• Tinnitus on questioning: difficult to objectively confirm but patient-reported tinnitus immediately after injection is highly suspicious
• Vital sign abnormalities: hypertension and tachycardia early; progression to bradycardia, hypotension, arrhythmias in severe cases
• Seizure activity: if present, virtually diagnostic; post-ictal state and respiratory depression follow
• Cyanosis unresolving with oxygen (methemoglobinemia): examine nail beds, lips, mucous membranes; "chocolate" hue is characteristic

Laboratory Confirmation

• Plasma local anesthetic levels: gold-standard confirmation but impractical for acute diagnosis (results take hours, only available at reference labs); useful for forensic or research purposes
• Methemoglobin level: co-oximetry (not standard pulse oximetry) measures methemoglobin; normal <1-2%; symptomatic typically >20%; co-oximetry shows characteristic "unmeasurable SaO₂" or reading of ~85% regardless of oxygen supplementation
• Arterial or venous blood gas: reveals acidosis and hypoxemia commonly present with seizures or severe cardiovascular toxicity; helps assess severity
• Electrolyte panel, glucose: to exclude hypoglycemia, hyperkalemia as alternative diagnoses
• Lactate level: elevated from seizure activity and tissue hypoxia, supports severity assessment
• ECG: shows conduction abnormalities (widened QRS, prolonged PR interval, peaked T waves, bradycardia) with cardiovascular toxicity

Diagnostic Criteria

• Clinical diagnosis is made when any combination of CNS toxicity signs (circumoral paresthesias, restlessness, seizures, loss of consciousness) OR cardiovascular signs (sudden bradycardia, hypotension, arrhythmias) occur temporally related to regional anesthesia or local anesthetic injection
• High-risk scenarios include intercostal or interscalene blocks (highest systemic absorption), use of high-concentration solutions, omission of vasoconstrictors, absence of negative aspiration test, or rapidly escalating doses

Differential Diagnosis Considerations

• Hypoglycemia: presents with confusion, seizures, autonomic signs but no circumoral paresthesias; blood glucose rapidly clarifies
• Anaphylaxis: urticaria, bronchospasm, airway edema prominent; progression slower; better response to epinephrine alone; timing—may occur with ester anesthetics
• Aspiration: hypoxemia, bronchospasm, respiratory distress; history of coughing during procedure; chest infiltrate on imaging
• Venous air embolism: sudden cardiovascular collapse, "mill wheel" murmur; occurs during procedures where veins are opened (central line placement, epidural); transesophageal echo can visualize air
• Sepsis/infection: fever, prolonged timeline (hours); elevated lactate and WBC; no temporal relationship to immediate anesthetic injection
• Intracranial hemorrhage/stroke: loss of consciousness without seizures; focal neurologic deficits; imaging abnormalities

Immediate Management of Systemic Toxicity

First-line: Airway, Breathing, Circulation

• Immediately stop injection of local anesthetic and notify all team members
• Call for help and request lipid emulsion and resuscitation equipment
• Establish airway control: if seizures are present, secure airway with endotracheal intubation by those trained (anesthesia, ICU); succinylcholine is acceptable for rapid sequence intubation despite depolarization from the local anesthetic, as seizure prevention takes priority
• **100

Local anesthetic systemic toxicity (LAST)

• CNS excitation then depression: preferential blockade of cortical inhibitory interneurons unmasks excitatory pathways, producing circumoral numbness, tinnitus, and metallic taste, then seizures, then global depression, coma, and apnea as concentrations climb.
• Cardiovascular collapse: sodium channel blockade in Purkinje tissue and myocardium slows phase 0 upstroke (QRS widening, PR prolongation) and depresses contractility. Bupivacaine dissociates slowly from cardiac sodium channels ("fast-in, slow-out"), so arrest may precede any CNS warning; ropivacaine and levobupivacaine (S-enantiomers) are less cardiotoxic.
• Antidote: 20% intravenous lipid emulsion (lipid rescue), per the ASRA Practice Advisory on LAST — roughly 1.5 mL/kg bolus followed by infusion, repeated for persistent instability. Propofol is not an acceptable substitute (lipid content too low, myocardial depressant).
• Modifications to ACLS emphasized by ASRA: reduce epinephrine to small boluses (on the order of ≤1 mcg/kg), and avoid vasopressin, calcium channel blockers, beta blockers, and any additional sodium channel blocker (lidocaine, procainamide). Amiodarone is the preferred antiarrhythmic. Hypoxia and acidosis worsen channel binding, so ventilation is therapeutic.
• Monitoring: continued observation after apparent recovery — ASRA advises a period of hours of monitoring after cardiovascular instability and a shorter interval after an isolated CNS event, since redistribution can cause relapse.

Methemoglobinemia

• Benzocaine (topical/airway spray, subject of an FDA safety warning) and prilocaine (via o-toluidine) oxidize heme iron to Fe³⁺. Look for chocolate-brown blood, cyanosis refractory to oxygen, and a saturation gap (normal PaO₂, low pulse-ox); confirm by co-oximetry.
• Antidote: methylene blue 1–2 mg/kg IV, which NADPH-methemoglobin reductase uses to regenerate hemoglobin. Contraindicated in G6PD deficiency (ineffective and hemolytic); use ascorbic acid or exchange transfusion instead.

Other

• Ester allergy: PABA metabolite drives true IgE reactions; amides are the safe alternative, though methylparaben preservative can rarely sensitize.
• Neurotoxicity: transient neurologic symptoms and cauda equina syndrome with concentrated intrathecal lidocaine.
• Epinephrine-containing solutions: caution with uncontrolled hypertension, unstable coronary disease, and MAOI/TCA use.

• Esters have one "i"; amides have two: pro*c*aine, tetracaine, benzocaine, chloroprocaine (plasma pseudocholinesterase) versus lidocaine, bupivacaine, prilocaine, mepivacaine (hepatic). The mnemonic tells you which patient (liver disease vs. pseudocholinesterase deficiency) is at risk and which class causes PABA-mediated allergy.
• Infected or inflamed tissue is the classic "the block didn't work" stem: acidic pH traps the drug in its ionized form extracellularly, so less non-ionized base crosses the axolemma. Answer is a regional/field block outside the inflamed area or drainage first — not simply more anesthetic.
• Circumoral paresthesia, tinnitus, and metallic taste after a nerve block = earliest LAST. Single best next step: stop the injection, give 100% oxygen, and call for 20% lipid emulsion — do not wait for the seizure.
• Bupivacaine is the cardiotoxicity answer; lidocaine is the seizure answer. Bupivacaine is never used for IV regional (Bier) blocks for this reason. The classic distractor is treating the resulting wide-complex arrhythmia with lidocaine or procainamide — both add sodium channel blockade and are wrong; amiodarone plus lipid emulsion is correct per ASRA.
• Cyanosis unresponsive to oxygen after topical benzocaine airway spray (or a teething gel in an infant) = methemoglobinemia: chocolate-brown blood, saturation gap, treat with methylene blue — except in G6PD deficiency, where it triggers hemolysis.
• Order of blockade: small unmyelinated C fibers (pain, temperature, autonomic) first, then A-delta, then touch/pressure, with motor last — hence the laboring patient who feels pressure but not pain.
• Epinephrine coadministration prolongs duration, reduces bleeding, and raises the maximum safe lidocaine dose by limiting systemic absorption; the traditional prohibition against epinephrine in digits, nose, ears, and penis is longstanding teaching but has been challenged by modern case series.
• Blockade is use-dependent and from the intracellular side — rapidly firing pain fibers are hit hardest, which is also why lidocaine works as a class IB antiarrhythmic in ischemic, depolarized myocardium.