Local Anesthetics Summary

Common ED Agents

  • Lidocaine: Fast onset, 1-2h duration
  • Bupivacaine: Slow onset, 3-12h duration
  • Prilocaine: Bier's block specialist

Mechanism of Action

  • Block voltage-gated sodium channels
  • Prevent action potential propagation
  • Reversible inhibition

Common Indications

  • Wound repair and closure
  • Peripheral nerve blocks
  • Topical anesthesia
  • IV regional anesthesia

Topical Agents

  • EMLA: Prilocaine/lidocaine (1h application)
  • Ametop: Tetracaine (vasodilator)
  • LAT gel: For open wounds

Adrenaline Use

  • Prolongs duration
  • Reduces systemic absorption
  • Contraindicated in digits
  • Avoid in cardiac patients

Maximum Doses

  • Lidocaine: 3mg/kg (7mg/kg with adrenaline)
  • Bupivacaine: 2mg/kg
  • Prilocaine: 6mg/kg (9mg/kg with adrenaline)

Critical Contraindications

  • Adrenaline in digits/appendages
  • Adrenaline in severe hypertension
  • Adrenaline in unstable arrhythmias
  • EMLA/Ametop on open wounds

Local Anesthetics (LAs)

Comprehensive guide to mechanism, indications, and clinical use in emergency settings

Overview

Local anesthetics (LAs) are used to block transmission of impulses in nerve fibers, to reduce or eliminate sensation. They are essential tools in emergency medicine for various procedures.

Key Principle: LAs provide reversible anesthesia for procedures while maintaining patient consciousness and cooperation.

Mechanism of Action

Molecular Action

  • LAs reversibly inhibit neurotransmission by binding voltage-gated sodium channels (Nav) in the nerve plasma membrane
  • Nav channels are integral membrane proteins anchored in the plasma membrane
  • When LAs bind the sodium channel, they render it impermeable to Na⁺
  • This prevents action potential initiation and propagation

Clinical Effect

  • Blocks nerve conduction in a reversible manner
  • Effect is concentration-dependent
  • Small diameter fibers blocked before large diameter fibers
  • Sequence of blockade: autonomic → sensory → motor

Indications for Local Anesthesia

Topical Anesthesia

  • Prior to venepuncture or venous cannulation
  • Arterial puncture or cannulation
  • Minor superficial procedures

Infiltration Anesthesia

  • Local subcutaneous and tissue infiltration
  • Cleaning, exploration and closure of wounds
  • Minor surgical procedures

Regional Anesthesia

  • Peripheral nerve blocks: e.g., femoral nerve block for femoral shaft fracture
  • Intravenous regional blocks: e.g., Bier's block for distal forearm fractures
  • Neuraxial anesthesia: spinal or epidural anesthesia

Special Applications

  • Dental procedures
  • Ophthalmic surgery
  • Obstetric procedures
  • Chronic pain management

Choice of Agent in Emergency Department

The most commonly used local anesthetics in ED are lidocaine, bupivacaine and prilocaine.

Comparison of Common Agents

Drug Primary Use Onset Duration Max Dose
Lidocaine Widely used for local infiltration, peripheral nerve blocks and topically 5-10 mins 1-2 hours (without adrenaline)
2-4 hours (with adrenaline)		 3 mg/kg (without adrenaline)
7 mg/kg (with adrenaline)
Prilocaine Bier's block specialist 5-10 mins 1-2 hours (without adrenaline)
2-4 hours (with adrenaline)		 6 mg/kg (without adrenaline)
9 mg/kg (with adrenaline)
Bupivacaine Fascia iliaca block, long procedures 10-15 mins 3-12 hours 2 mg/kg

Important: Always calculate maximum safe dose based on patient weight and check for allergies before administration.

Topical Anesthetics

EMLA Cream

  • Composition: 50/50 mixture of 2.5% prilocaine and 2.5% lidocaine
  • Application: Must be applied for at least an hour before venepuncture
  • Limitation: Long application time limits ED use
  • Contraindication: Should not be applied to open wounds

Ametop Gel

  • Composition: Tetracaine (amethocaine)
  • Advantage: Acts more quickly than EMLA
  • Benefit: Causes vasodilation aiding venous cannulation
  • Contraindication: Should not be applied to open wounds

LAT Gel

  • Composition: Lidocaine 4%, adrenaline 0.1% and tetracaine 0.5%
  • Advantage: Can be applied to open wounds requiring cleaning and suturing
  • Pediatric Use: Reduces need for injection/sedation in children
  • Application: Apply for 30 mins prior, complete procedure within 15 minutes of removal
  • Safety: Vasoconstrictor reduces systemic absorption and adverse effects

Adrenaline and Local Anesthetics

Local anesthetics cause dilatation of blood vessels. The addition of a vasoconstrictor such as adrenaline/epinephrine to the local anesthetic preparation diminishes local blood flow, slowing the rate of absorption and thereby prolonging the anesthetic effect and reducing the risk of systemic toxicity.

Benefits of Adrenaline Combination

Pharmacological Advantages

  • Prolongs duration of anesthesia
  • Reduces systemic absorption
  • Lowers risk of toxicity
  • Allows higher maximum doses

Clinical Advantages

  • Reduces bleeding at the site
  • Improves surgical field visibility
  • Enhances anesthetic effectiveness

Important Precautions

Critical Contraindications

  • Digital blocks: Avoid adrenaline in digits or appendages served by end-arteries (risk of ischemic necrosis)
  • Cardiac patients: Avoid in severe hypertension or unstable cardiac rhythm
  • IV administration: Great care to avoid inadvertent intravenous administration

Safe Practice Guidelines

  • Use low concentration of adrenaline (typically 1:200,000)
  • Always calculate safe maximum dose of local anesthetic
  • Aspirate before injection to avoid intravascular administration
  • Use adrenaline-free solutions for digital blocks and in high-risk cardiac patients

Clinical Pearls and Safety

Toxicity Management

  • CNS effects: Timitus, perioral numbness, seizures
  • Cardiac effects: Myocardial depression, arrhythmias
  • Treatment: Lipid emulsion therapy for severe toxicity
  • Prevention: Careful dose calculation, slow injection

Special Populations

  • Elderly: Reduced maximum doses
  • Children: Weight-based dosing critical
  • Liver disease: Reduced metabolism of amide LAs
  • Pregnancy: Use lowest effective dose

Practical Administration Tips

Optimal Injection Technique
  1. Use the smallest gauge needle appropriate
  2. Warm the solution to body temperature if possible
  3. Inject slowly to minimize pain
  4. Move needle while injecting to distribute anesthetic
  5. Aspirate frequently to check for blood return
  6. Wait adequate time for full effect before proceeding