Mechanism of action
Lidocaine blocks voltage-gated sodium channels by binding to the intracellular portion of the channel in its inactivated state. This use-dependent blockade is more pronounced in rapidly firing or depolarized tissues, explaining its selectivity for ectopic cardiac foci and sensory neurons transmitting pain.
Local anesthetic action
Lidocaine prevents the generation and conduction of nerve impulses by blocking sodium influx during depolarization. Small-diameter, myelinated fibers (Aδ pain and temperature) and unmyelinated C fibers (pain) are blocked before larger motor fibers, producing differential blockade. The drug must cross the lipid nerve sheath in its uncharged form, then becomes ionized in the axoplasm to bind the sodium channel from the intracellular side.
Antiarrhythmic action (Class Ib)
Lidocaine shortens the action potential duration and effective refractory period in ventricular tissue, particularly in ischemic or depolarized myocardium. It suppresses automaticity in Purkinje fibers and ventricular muscle while having minimal effect on atrial tissue or AV nodal conduction at therapeutic concentrations. This selectivity makes it useful for ventricular arrhythmias but ineffective for atrial arrhythmias.
- Lidocaine is an amide local anesthetic with class Ib antiarrhythmic properties due to voltage-gated sodium channel blockade
- CNS toxicity precedes cardiovascular toxicity: perioral numbness → seizures → cardiac depression and arrest
- Adding epinephrine prolongs duration, reduces systemic absorption, and increases maximum safe dose to ~7 mg/kg (vs 4.5 mg/kg alone)
- Effective for ventricular arrhythmias but NOT atrial arrhythmias due to minimal effect on atrial tissue
- Metabolized by liver CYP3A4/CYP1A2; toxicity risk increased with hepatic dysfunction, heart failure, or enzyme inhibitors (cimetidine, propranolol)
- Lipid emulsion (Intralipid 20%) is antidote for severe local anesthetic systemic toxicity (LAST)
- Amide local anesthetics (lidocaine, bupivacaine) rarely cause allergic reactions; no cross-reactivity with ester anesthetics
- Never use epinephrine-containing preparations in areas with end-artery circulation (digits, nose, ears, penis)
Indications
Lidocaine has diverse clinical applications across multiple specialties:
Local and regional anesthesia
- Infiltration anesthesia for minor surgical and dental procedures
- Peripheral nerve blocks
- Epidural and spinal anesthesia (preservative-free formulations only)
- Topical anesthesia of mucous membranes (oral, nasal, urogenital)
Cardiac indications
- Ventricular tachycardia and ventricular fibrillation (particularly in acute myocardial infarction settings, though less commonly used now)
- Prophylaxis of ventricular arrhythmias during cardiac procedures
- Ventricular ectopy refractory to other measures
Other uses
- Neuropathic pain (intravenous or topical formulations)
- Cough suppression in procedures requiring airway instrumentation
- Topical treatment of post-herpetic neuralgia (patch formulation)
Note that current ACLS guidelines have de-emphasized prophylactic lidocaine use post-MI due to lack of mortality benefit, though it remains useful for acute ventricular arrhythmias.
Pharmacokinetics
Absorption and distribution
Systemic absorption after local administration depends on dose, site of injection, vascularity of tissue, and presence of vasoconstrictors. Addition of epinephrine (1:100,000 or 1:200,000) decreases systemic absorption, prolongs duration, and reduces toxicity risk. Lidocaine is highly lipophilic and rapidly distributes into well-perfused tissues including the brain and heart.
Metabolism and elimination
Lidocaine undergoes extensive first-pass hepatic metabolism (approximately 70%), making oral bioavailability poor. It is metabolized by hepatic cytochrome P450 enzymes (primarily CYP3A4 and CYP1A2) to active metabolites monoethylglycinexylidide (MEGX) and glycinexylidide (GX), which retain some anesthetic and toxic properties. The elimination half-life is 1.5-2 hours but may be prolonged in patients with hepatic dysfunction, heart failure, or reduced hepatic blood flow.
Volume of distribution and protein binding
Lidocaine is approximately 60-80% protein bound, primarily to alpha-1-acid glycoprotein. Conditions that increase this acute-phase reactant (MI, trauma, cancer, surgery) increase protein binding and may reduce free drug concentrations. The drug crosses the placenta and appears in breast milk but is considered relatively safe in pregnancy when used appropriately.
Adverse effects
Toxicity follows a predictable sequence as plasma concentrations rise, affecting the CNS before the cardiovascular system.
CNS toxicity (most common)
Early signs include:
- Perioral numbness and tongue paresthesias
- Metallic taste
- Tinnitus
- Lightheadedness and dizziness
- Visual disturbances
Progression leads to:
- Muscle twitching and tremor
- Confusion and disorientation
- Seizures (generalized tonic-clonic)
- Respiratory depression and apnea
- Coma
Cardiovascular toxicity
Occurs at higher concentrations than CNS effects:
- Hypotension (vasodilation, negative inotropy)
- Bradycardia and conduction blocks
- Ventricular arrhythmias
- Cardiovascular collapse and cardiac arrest
Allergic reactions
True allergy is rare with amide local anesthetics (in contrast to ester anesthetics). Reactions are more commonly vasovagal responses or anxiety-related. Cross-reactivity between amide and ester local anesthetics does not occur.
Methemoglobinemia
Rare but can occur with large doses or in patients with glucose-6-phosphate dehydrogenase deficiency. Presents with cyanosis unresponsive to oxygen, chocolate-brown blood, and low oxygen saturation on pulse oximetry despite adequate PaO2.
Local tissue effects
Transient neurologic symptoms (TNS) can occur after spinal anesthesia, presenting as back pain and dysesthesias radiating to the legs. Myotoxicity is possible with repeated intramuscular injections.
Contraindications and warnings
Absolute contraindications
- Hypersensitivity to amide local anesthetics
- Complete heart block (for antiarrhythmic use)
- Severe sino-atrial or intraventricular conduction disturbances without a pacemaker
Relative contraindications and cautions
Hepatic impairment: Reduce dose and monitor for toxicity as clearance is significantly decreased.
Heart failure and shock: Reduced hepatic blood flow decreases clearance; lower doses required.
Elderly patients: Age-related reduction in clearance necessitates dose reduction.
Seizure disorders: Use with extreme caution as lidocaine can precipitate seizures at toxic levels.
Porphyria: Considered possibly unsafe; may precipitate acute attacks.
Pregnancy and lactation: FDA pregnancy category B. Generally considered safe for obstetric anesthesia. Crosses placenta but no evidence of fetal harm at therapeutic doses. Small amounts enter breast milk but compatible with breastfeeding.
Drug-specific preparations: Lidocaine preparations containing preservatives (methylparaben) must not be used for spinal or epidural anesthesia. Preparations with epinephrine should not be used for nerve blocks in areas supplied by end arteries (fingers, toes, nose, ears, penis).
Maximum safe doses: Without epinephrine, approximately 4.5 mg/kg; with epinephrine, approximately 7 mg/kg. However, dosing should follow institutional protocols and product labeling.
Drug interactions
CYP450 interactions
CYP3A4 and CYP1A2 inhibitors increase lidocaine levels and toxicity risk:
- Cimetidine (decreases clearance by up to 50%)
- Beta-blockers (especially propranolol)
- Azole antifungals
- Macrolide antibiotics (erythromycin, clarithromycin)
- Grapefruit juice
- Fluvoxamine and ciprofloxacin (CYP1A2 inhibitors)
CYP inducers may decrease lidocaine effectiveness:
- Phenytoin, phenobarbital, rifampin
Additive cardiac effects
Other antiarrhythmic agents: Additive cardiac depression and increased risk of arrhythmias. Use caution when combining class I antiarrhythmics.
Beta-blockers: Decrease hepatic blood flow and reduce lidocaine clearance; also increase risk of bradycardia and heart block.
Cardiac depressants: Volatile anesthetics, calcium channel blockers, and other negative inotropes increase risk of cardiovascular toxicity.
Additive CNS depression
Sedatives, hypnotics, and anticonvulsants may have additive CNS depressant effects.
Neuromuscular blockers
Lidocaine may prolong the action of succinylcholine by inhibiting plasma cholinesterase. It may also enhance neuromuscular blockade of non-depolarizing agents.
Other local anesthetics
Combined use increases total systemic load and toxicity risk; additive effects must be considered when calculating maximum safe dose.
Clinical pearls
Lidocaine with epinephrine: The addition of epinephrine causes vasoconstriction, which slows systemic absorption, prolongs local anesthetic duration, and increases the maximum safe dose. Never use in areas with end-artery circulation (digits, nose, ears, penis) due to risk of ischemic necrosis.
Alkalinization: Adding sodium bicarbonate to lidocaine solutions (1 mEq per 10 mL of lidocaine) raises pH, increases the uncharged fraction, speeds onset, and reduces injection pain. The solution should be used promptly as alkalinization may decrease stability.
LAST protocol: Local Anesthetic Systemic Toxicity is treated with lipid emulsion therapy (Intralipid 20%). The bolus is 1.5 mL/kg lean body weight over 1 minute, followed by infusion. Lipid rescue should be initiated at first signs of severe toxicity.
Differential diagnosis of "allergic" reactions: Most reactions attributed to local anesthetic allergy are actually vasovagal syncope, anxiety, or reactions to epinephrine (tachycardia, palpitations). True IgE-mediated allergy to amide local anesthetics is extremely rare. Patients with documented ester allergy (procaine, benzocaine, tetracaine) can safely receive amides.
Cardiac vs anesthetic formulations: Intravenous lidocaine for arrhythmias must be preservative-free and epinephrine-free. Accidental IV injection of lidocaine with epinephrine can cause severe hypertension and arrhythmias.
Test dose technique: When performing epidural or major nerve blocks, administer a small test dose (3-5 mL) containing epinephrine first. Intravascular injection will cause tachycardia within 30-60 seconds; intrathecal injection will cause rapid onset of spinal anesthesia.
Class Ib vs Ia vs Ic: Lidocaine (Ib) shortens repolarization and has minimal effect on conduction velocity in normal tissue, unlike class Ia drugs (quinidine, procainamide) which prolong repolarization, and class Ic drugs (flecainide, propafenone) which markedly slow conduction.
Ventricular vs atrial selectivity: Lidocaine is effective for ventricular arrhythmias but not atrial arrhythmias because it has minimal effect on atrial action potentials and does not slow AV nodal conduction significantly at therapeutic doses.
Practice questions
1. A 55-year-old man undergoes dental extraction under local anesthesia with lidocaine. Five minutes after injection, he reports perioral numbness, metallic taste, and ringing in his ears. His speech becomes slurred and he appears confused. Which of the following is the most appropriate immediate action?
2. A 68-year-old woman with a history of congestive heart failure is started on intravenous lidocaine for recurrent ventricular tachycardia. She is also taking propranolol for rate control. Two hours later, she develops confusion and muscle twitching. Which of the following best explains her symptoms?
3. A 42-year-old man presents to the emergency department with sustained monomorphic ventricular tachycardia after an acute inferior MI. He is hemodynamically stable. After electrical cardioversion, which medication would be most appropriate for suppressing recurrent ventricular ectopy?
Frequently asked questions
How does lidocaine work?
Lidocaine blocks voltage-gated sodium channels, preventing nerve depolarization and impulse conduction. It preferentially binds to channels in their inactivated state, making it more effective in rapidly firing or depolarized tissues like pain fibers and ectopic cardiac foci. This use-dependent blockade explains its dual role as both a local anesthetic and an antiarrhythmic agent for ventricular arrhythmias.
What are the side effects of lidocaine?
CNS effects appear first and include perioral numbness, metallic taste, tinnitus, dizziness, and confusion, progressing to seizures at higher levels. Cardiovascular toxicity occurs at even higher concentrations, causing hypotension, bradycardia, conduction blocks, and potentially cardiac arrest. Allergic reactions to amide anesthetics like lidocaine are extremely rare. Methemoglobinemia can occur with large doses.
Is lidocaine a controlled substance?
No, lidocaine is not a controlled substance in the United States or most other countries. It is widely available in various formulations including injectable solutions for anesthesia, topical creams and gels, patches, and intravenous preparations for cardiac use. Many over-the-counter products contain low concentrations of lidocaine for temporary pain relief.
Lidocaine vs bupivacaine — what is the difference?
Both are amide local anesthetics, but bupivacaine is more potent and longer-acting (2-4 hours vs 30-120 minutes for lidocaine). Bupivacaine has greater cardiotoxicity and is more difficult to resuscitate if toxicity occurs, while lidocaine toxicity is generally more manageable. Lidocaine has a faster onset and is also used systemically as an antiarrhythmic, whereas bupivacaine is used only for local/regional anesthesia.
Can you use lidocaine with epinephrine on fingers?
No, lidocaine with epinephrine should never be used in areas supplied by end arteries such as fingers, toes, nose, ears, or penis. Epinephrine causes vasoconstriction, and in these areas without collateral circulation, prolonged vasoconstriction can lead to ischemic necrosis and tissue loss. Plain lidocaine without epinephrine is safe for digital nerve blocks.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: C01BB01WHO ATC/DDD index: C05AD01
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.