Lidocaine (Lignocaine)

Drug Name & Class

• Generic Name: Lidocaine (also known as Lignocaine outside the US)
• Brand Name: Xylocaine
• Drug Class: Amide local anesthetic; Class Ib antiarrhythmic

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Mechanism of Action (MoA)

Lidocaine's mechanism is dose- and site-dependent, centered on blocking voltage-gated sodium channels.

• Local Anesthetic Effect: It binds to the intracellular portion of the sodium channel on neuronal axons, preventing the influx of sodium ions. This halts the initiation and propagation of action potentials, resulting in a reversible block of nerve conduction. Small, unmyelinated C-fibers (pain) are blocked first, followed by larger myelinated fibers (temperature, touch, motor).
• Systemic (IV) Effect:
  • Cardiac: In cardiac myocytes, it shortens the action potential duration and refractory period, suppresses automaticity in Purkinje fibers, and is effective against ventricular arrhythmias.
  • Analgesic/Anti-inflammatory: It modulates central sensitization and reduces the inflammatory response, providing analgesic and anti-hyperalgesic effects when given intravenously.

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Clinical Uses & Indications

1. Local & Regional Anesthesia: Infiltration, peripheral nerve blocks, field blocks, topical anesthesia, and central neuraxial blocks (spinal, epidural, caudal).
2. Treatment of Ventricular Arrhythmias: First-line drug for hemodynamically stable ventricular tachycardia (VT) and for preventing ventricular ectopy after a myocardial infarction.
3. Intravenous Analgesia/Anti-hyperalgesia: As an adjunct during general anesthesia to reduce the requirement for other anesthetic agents, blunt the hemodynamic response to intubation and surgical incision, and provide postoperative analgesia.
4. Management of Chronic Neuropathic Pain: Used in specialized pain clinics for systemic infusion or nerve blocks.

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Dosage and Administration

• Route: Subcutaneous, intradermal, intravenous, epidural, intrathecal, topical.
• Maximum Recommended Dose (MRD) for Infiltration (Adult):
  • Without Epinephrine: 4.5 mg/kg (not to exceed 300 mg)
  • With Epinephrine (1:200,000): 7 mg/kg (not to exceed 500 mg)
• IV Bolus for Arrhythmia: 1-1.5 mg/kg. May repeat once if necessary.
• IV Infusion for Analgesia: Loading dose of 1-1.5 mg/kg over 10 minutes, followed by an infusion of 1-2 mg/kg/min.

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Contraindications

• Known hypersensitivity to lidocaine or other amide anesthetics.
• Stokes-Adams syndrome, Wolff-Parkinson-White syndrome, or severe degrees of SA or AV block (without a pacemaker).
• Avoid intravenous regional anesthesia (Bier block) in patients with severe peripheral vascular disease or uncontrolled hypertension.

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Cautions & Warnings

• Risk of Toxicity: Has a narrow therapeutic index. Toxicity can be life-threatening.
• Hepatic Impairment: Reduce dose significantly as clearance is markedly reduced. Accumulation can lead to toxicity.
• Renal Failure: While lidocaine is metabolized by the liver, active metabolites can accumulate.
• Elderly, Debilitated, and Acutely Ill Patients: More susceptible to toxicity; use reduced doses.
• Seizure Disorders: Can lower the seizure threshold.
• Pregnancy: Crosses the placenta; use caution, especially for paracervical block.

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Adverse Effects & Side Effects

These are primarily signs and symptoms of systemic toxicity.

• CNS (Early Signs): Circumoral numbness, metallic taste, tinnitus, dizziness, anxiety.
• CNS (Late Signs): Muscle twitching, tremors, progressing to generalized tonic-clonic seizures, coma, and respiratory arrest.
• Cardiovascular: Bradycardia, hypotension, atrioventricular block, QRS complex widening, ventricular arrhythmias, cardiovascular collapse.
• Allergic: Rare with amides; more common with ester-type anesthetics.

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Key Drug Interactions

• CYP3A4 Inhibitors: (e.g., erythromycin, ketoconazole, cimetidine) can decrease lidocaine metabolism, increasing plasma levels and toxicity risk.
• Propranolol: Reduces hepatic blood flow, decreasing lidocaine clearance.
• Other Local Anesthetics or Class I Antiarrhythmics: Additive cardiotoxicity.
• Neuromuscular Blocking Agents: May potentiate the neuromuscular block.

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Monitoring Parameters

Monitoring is crucial to prevent and detect toxicity early.

• For Local/Regional Anesthesia:
  • Patient Communication: Continuously ask the patient about early symptoms of toxicity (tinnitus, metallic taste, dizziness).
  • ECG: Monitor for QRS widening and arrhythmias.
  • Hemodynamics: Monitor blood pressure and heart rate.
  • Level of Consciousness: Observe for signs of agitation or drowsiness.
• For IV Infusion/Bolus:
  • Continuous ECG: Mandatory. Watch for changes in QRS duration and any new arrhythmias.
  • Frequent Blood Pressure Monitoring: Every 1-2 minutes for bolus, continuously for high-risk infusions (arterial line recommended).
  • Serum Lidocaine Levels: For prolonged infusions (>24 hours) or in high-risk patients (e.g., liver failure), aim for therapeutic levels of 3-5 mcg/mL.

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Clinical Pearls & Key Takeaways

• "Aspirate Before You Inject": Always aspirate the syringe before injecting a local anesthetic to rule out intravascular placement.
• CNS Toxicity Precedes Cardiac Toxicity: The first signs of trouble are usually neurological. Heed them as a warning to stop administration.
• Epinephrine is Your Best Friend: Adding epinephrine (1:200,000) doubles the maximum safe dose, prolongs the duration of action, and provides a test dose (if injected IV, a transient increase in heart rate is a warning sign).
• The Liver is the Gatekeeper: Always assess liver function. In severe liver disease, avoid large doses or use with extreme caution.
• IV Lidocaine is a Multitool: It's more than just a local anesthetic; it's a powerful adjunct for analgesia, blunting hemodynamic responses, and treating arrhythmias.