What is Lidocaine?
Lidocaine (also known as lignocaine in many countries) is an amide-type local anesthetic and Class Ib antiarrhythmic agent. First synthesized in 1943 by Swedish chemist Nils Löfgren, it rapidly became the most widely used local anesthetic in the world and remains so today. Lidocaine is available in numerous formulations including injectable solutions, topical creams, patches, gels, and sprays.
As a local anesthetic, lidocaine provides reversible loss of sensation in a specific area of the body without causing loss of consciousness. It has a rapid onset of action (typically 1–3 minutes for infiltration), an intermediate duration (30–120 minutes depending on the technique and whether epinephrine is used), and an excellent safety profile when used within recommended dosing limits.
Lidocaine is listed on the World Health Organization's List of Essential Medicines and is one of the most commonly administered drugs in emergency departments, dental offices, dermatology clinics, and surgical suites worldwide. Its versatility, predictable pharmacology, and favorable safety profile make it the benchmark against which other local anesthetics are compared.
Mechanism of Action
Lidocaine works by blocking voltage-gated sodium channels in neuronal cell membranes. Under normal conditions, sodium channels open in response to membrane depolarization, allowing sodium ions to flow into the cell and propagate the action potential along the nerve fiber. Lidocaine binds to the intracellular portion of these sodium channels in their open and inactivated states, preventing sodium influx and thereby blocking nerve impulse conduction.
The drug preferentially blocks smaller, unmyelinated nerve fibers (C fibers carrying pain and temperature) before larger, myelinated fibers (A-alpha fibers carrying motor and proprioceptive signals). This differential blockade explains why patients typically lose pain sensation before losing motor function, and why pain relief persists after motor function returns.
Lidocaine is metabolized primarily in the liver by CYP1A2 and CYP3A4 enzymes to two active metabolites: monoethylglycinexylidide (MEGX) and glycinexylidide (GX). Patients with hepatic dysfunction or reduced hepatic blood flow (such as those in heart failure) metabolize lidocaine more slowly, increasing the risk of toxicity. The elimination half-life is approximately 1.5–2 hours in healthy adults but may extend to 3 hours or more in patients with liver disease.
Clinical Indications
Lidocaine has a remarkably broad range of clinical applications:
- Local infiltration: Wound repair, skin biopsies, minor surgical procedures, IV cannulation
- Dental anesthesia: Inferior alveolar nerve blocks, infiltration for dental procedures
- Regional anesthesia: Peripheral nerve blocks, epidural anesthesia, spinal anesthesia
- Topical anesthesia: Mucosal surfaces (oropharynx, urethra), skin (EMLA cream contains lidocaine + prilocaine)
- Antiarrhythmic: IV lidocaine for ventricular tachycardia and ventricular fibrillation refractory to defibrillation
- Pain management: Lidocaine patches for postherpetic neuralgia, IV lidocaine infusions for chronic pain
- Airway management: Topical application to suppress gag reflex and attenuate hemodynamic response to intubation
Dosing: With vs Without Epinephrine
The maximum safe dose of lidocaine depends critically on whether epinephrine is co-administered:
Epinephrine (typically at a concentration of 1:100,000 or 1:200,000) is added to local anesthetic solutions for several important reasons. As a potent vasoconstrictor, it reduces blood flow to the injection site, which slows systemic absorption of lidocaine, thereby reducing peak plasma levels and the risk of toxicity. This slower absorption also prolongs the duration of anesthesia from approximately 30–60 minutes to 60–120 minutes and reduces bleeding at the surgical site.
The higher maximum dose with epinephrine reflects the reduced rate of systemic absorption. However, epinephrine should be used with caution in patients with uncontrolled hypertension, severe cardiac arrhythmias, or thyrotoxicosis. The traditional teaching to avoid epinephrine in end-arterial areas (fingers, toes, nose, ears, penis) has been largely debunked by modern evidence, though low concentrations (1:200,000) are recommended in these locations.
To calculate the maximum injectable volume for a given concentration:
Local Anesthetic Systemic Toxicity (LAST)
Local Anesthetic Systemic Toxicity (LAST) is a potentially life-threatening complication that occurs when plasma levels of local anesthetic exceed the threshold for systemic toxicity. Symptoms progress in a predictable pattern as plasma levels rise:
| Stage | Plasma Level | Symptoms |
|---|---|---|
| Early CNS | 5–10 mcg/mL | Perioral numbness, metallic taste, tinnitus, lightheadedness, visual disturbances |
| CNS excitation | 10–15 mcg/mL | Tremors, muscle twitching, shivering, agitation |
| CNS depression | 15–25 mcg/mL | Seizures (tonic-clonic), loss of consciousness, respiratory arrest |
| Cardiovascular | >25 mcg/mL | Hypotension, bradycardia, conduction blocks, ventricular arrhythmias, asystole, cardiac arrest |
It is essential to recognize that CNS symptoms typically precede cardiovascular collapse, providing a window for intervention. However, in cases of rapid intravascular injection, cardiovascular toxicity may occur without preceding CNS warning signs.
The definitive treatment for severe LAST is 20% Intralipid (lipid emulsion therapy). The current ASRA (American Society of Regional Anesthesia) protocol recommends an initial bolus of 1.5 mL/kg of 20% lipid emulsion, followed by an infusion of 0.25 mL/kg/min. Lipid emulsion acts as a “lipid sink,” sequestering lipophilic local anesthetic molecules away from cardiac and neural tissue. Every facility where local anesthetics are administered should have lipid emulsion readily available.
Concentration Reference Table
| Percentage | mg/mL | Common Uses |
|---|---|---|
| 0.5% | 5 mg/mL | Large-volume infiltration, tumescent liposuction |
| 1% | 10 mg/mL | Standard infiltration, nerve blocks, wound repair |
| 1.5% | 15 mg/mL | Epidural anesthesia, peripheral nerve blocks |
| 2% | 20 mg/mL | Dental blocks, nerve blocks requiring dense anesthesia, spinal anesthesia |
| 4% | 40 mg/mL | Topical only (oropharyngeal spray) |
| 5% | 50 mg/mL | Hyperbaric spinal anesthesia (with dextrose) |
Higher concentrations provide denser and faster-onset anesthesia but allow less volume to be injected before reaching the maximum dose. For procedures requiring large-volume infiltration, lower concentrations (0.5% or 1%) are preferred.
Contraindications
Absolute and relative contraindications to lidocaine use include:
- Absolute: Known allergy to amide-type local anesthetics (extremely rare, estimated at less than 1% of reported “allergies”), complete heart block (for systemic use)
- Relative: Severe hepatic dysfunction (reduced metabolism), severe heart failure (reduced hepatic blood flow), hypovolemia, concurrent use of other local anesthetics (additive toxicity), myasthenia gravis
- Epinephrine-specific: Severe uncontrolled hypertension, unstable cardiac arrhythmias, thyrotoxicosis, use with MAO inhibitors or tricyclic antidepressants (potentiated response)
True allergy to amide local anesthetics is exceedingly rare. Most reported “allergies” are actually vasovagal reactions, anxiety responses, or reactions to preservatives (methylparaben) or epinephrine. Patients with a reported allergy should be evaluated with skin testing before being labeled as allergic.
Frequently Asked Questions
Why is the maximum dose different with and without epinephrine?
Epinephrine causes local vasoconstriction, which slows the rate at which lidocaine is absorbed into the bloodstream. This reduces peak plasma levels by approximately 30–50%, effectively allowing a larger total dose to be administered safely. The tissue still receives the full anesthetic dose; it simply enters the systemic circulation more gradually.
What should I do if I suspect lidocaine toxicity?
Stop the injection immediately. Call for help and prepare for advanced life support. For mild CNS symptoms (tinnitus, perioral numbness), monitor closely and provide supplemental oxygen. For seizures, administer a benzodiazepine (midazolam 2–4 mg IV). For cardiovascular collapse, initiate CPR and administer 20% lipid emulsion (Intralipid) per ASRA guidelines. Avoid vasopressin and beta-blockers.
Can lidocaine be used in pregnant patients?
Yes. Lidocaine is FDA Category B and is one of the safest local anesthetics for use during pregnancy. It crosses the placenta, but at therapeutic doses, fetal effects are minimal. Lidocaine with epinephrine is routinely used for dental procedures and epidural anesthesia during labor and delivery.
How do I convert between percentage and mg/mL?
A 1% solution contains 1 g per 100 mL, which equals 10 mg/mL. Simply multiply the percentage by 10 to get mg/mL. For example, 2% = 20 mg/mL, 0.5% = 5 mg/mL.
Should the dose be reduced in obese patients?
This is debated. Some experts recommend dosing based on ideal body weight (IBW) rather than total body weight to avoid potential overdosing, as adipose tissue has lower blood flow and different drug distribution characteristics. A conservative approach is to use lean body weight or IBW for maximum dose calculations in significantly obese patients.
Can multiple local anesthetics be used together?
Yes, but their toxicities are additive. When combining local anesthetics (e.g., lidocaine for quick onset plus bupivacaine for long duration), the total dose of each agent should be expressed as a fraction of its maximum, and the sum of the fractions should not exceed 1. For example, using 50% of the maximum lidocaine dose means you should use no more than 50% of the maximum bupivacaine dose.