What is the fastest acting local anesthetic?

Fastest Acting Local Anesthetics

Chloroprocaine is the fastest acting local anesthetic, with onset of action within 30-60 seconds, making it the optimal choice when rapid anesthesia is required.

Comparison of Local Anesthetic Onset Times

Local anesthetics can be classified based on their onset of action, with significant variations that impact clinical utility:

• Fastest onset (seconds to 1-2 minutes):

  • Chloroprocaine (30-60 seconds)
  • Lidocaine (90 seconds - 2 minutes)
  • Mepivacaine (30-120 seconds in upper jaw; 1-4 minutes in lower jaw) 1
  • Articaine (30-120 seconds) 2, 3

• Intermediate onset (2-5 minutes):

  • Prilocaine
  • Etidocaine

• Slowest onset (5-15 minutes):

  • Bupivacaine
  • Ropivacaine
  • Tetracaine

Pharmacological Basis for Rapid Onset

The onset of action for local anesthetics is primarily determined by:

1. pKa value: Agents with pKa closer to physiologic pH (7.4) have more molecules in the un-ionized form, allowing faster diffusion across nerve membranes 4

2. Lipid solubility: More lipid-soluble agents can penetrate nerve membranes more readily, though extremely high lipid solubility may actually slow onset by causing the drug to be "trapped" in the myelin sheath

3. Concentration gradient: Higher concentrations create steeper diffusion gradients

Clinical Applications Based on Onset Speed

When selecting a local anesthetic based on speed of onset:

• Emergency procedures: Choose chloroprocaine or lidocaine when immediate pain control is needed
• Dental procedures: Articaine and mepivacaine are preferred due to their rapid onset in oral tissues 1, 2
• Infiltration anesthesia: Chloroprocaine, lidocaine, mepivacaine, and articaine all provide rapid onset 4
• Nerve blocks: While onset is slightly delayed compared to infiltration, the same agents maintain their relative speed advantage

Safety Considerations with Rapid-Onset Agents

Faster-acting agents typically have shorter duration of action and different safety profiles:

• Chloroprocaine: Shortest duration (30-60 minutes) but lowest systemic toxicity due to rapid hydrolysis by plasma esterases
• Lidocaine: Moderate duration (90-200 minutes) with well-established safety profile when used at appropriate doses
• Articaine: Unique among amide anesthetics for its additional ester group, allowing rapid metabolism (half-life ~20 minutes) 5

Dosing and Maximum Limits

When using rapid-onset local anesthetics, adhere to maximum safe doses to prevent toxicity:

• Chloroprocaine: 20 mg/kg with epinephrine; 12 mg/kg without epinephrine 6
• Lidocaine: 7 mg/kg with epinephrine; 4.4 mg/kg without epinephrine 6
• Mepivacaine: 7 mg/kg with epinephrine; 4.4 mg/kg without epinephrine 6
• Articaine: 7 mg/kg (not recommended for children under 4 years) 6

Important Clinical Considerations

• Always aspirate before injection to avoid intravascular administration
• Calculate maximum allowable dose before administration, especially in pediatric patients
• Have 20% lipid emulsion available when using amide local anesthetics (particularly long-acting ones) for potential toxicity management 6
• Onset of action may be delayed in infected or inflamed tissues due to altered tissue pH

Conclusion

When rapid onset of anesthesia is the primary consideration, chloroprocaine offers the fastest action, followed closely by lidocaine, mepivacaine, and articaine. The choice should be balanced with consideration of required duration, anatomical location, and patient factors.