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Chapter 120. Local Anesthetics and Adjuvants

Robert N. Keddis, MD

  • Weak bases, hydrophilic, tertiary amines that block voltage-gated Na+ channels, preventing depolarization
  • Sensitivity of nerve fibers to blockade:
    • More sensitive: small diameter and lack of myelin
    • Spinal and peripheral nerves: autonomic > sensory > motor
  • Onset/duration of action:
    • Unionized (uncharged) form: lipid-soluble, crosses membranes
    • Increased lipid solubility = decreased onset time, increased duration, increased potency
    • Increased concentration and total dose =faster onset
    • Site of injection: more vascularized means shorter duration due to systemic uptake
    • Note: Epinephrine is frequently added to increase duration of blockade; however, it is unstable in alkaline conditions:
      • Bottles containing epinephrine have an acidic pH = slower onset than when epinephrine added extemporaneously
  • Systemic vascular absorption (greatest to least):
    • Intercostal nerve block > caudal > epidural > brachial plexus > sciatic–femoral > subcutaneous
  • Divided into two groups:
    • Amides (mnemonic: have two I's in the name):
      • Metabolized in the liver; liver disease can increase duration and toxicity
    • Esters:
      • Mostly metabolized by pseudocholinesterase
      • Pseudocholinesterase deficiency severely prolongs duration; avoid in these patients

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Dosage of Local Anesthetics (Also See Chapters 122 and 123)
PeripheralEpiduralSpinalInfiltrationIVRA
Amides
BupivacaineConcentration0.25–0.5%0.25–0.75%0.5–0.75%n/aContraindicated
Dose10–40 mL5–20 mL1–3 mLn/aContraindicated
Duration4–12 h60–120 min1–3 hn/aContraindicated
OnsetSlowIntermediateRapidn/aContraindicated
LidocaineConcentration1–1.5%1.5–2%1.5–5%0.5–1%0.25–0.5%
Dose10–40 mL20 mL
Duration1–3 h1.5–2 h0.5–1.5 h2–8 h0.5–1 h
OnsetRapidRapidRapidRapidRapid
MepivacaineConcentration1–1.5%1.5–2%2–4%n/an/a
Dose
Duration2–4 h1–2 h1–2 hn/an/a
OnsetRapidRapidRapidn/an/a
RopivacaineConcentration0.5–1%0.5–1%0.5–0.75%0.2–0.5%n/a
Dose10–20 mL
Duration5–8 h2–3 h1–3 h2–5 hn/a
OnsetSlowIntermediaten/aRapidn/a
Esters
BenzocaineConcentrationn/an/an/aTopical: maximum 20%
Dose
Durationn/an/an/a0.5–1 h
Onsetn/an/an/aRapid
ChloroprocaineConcentration2%2–3%2–3%1%
Dose
Duration0.5–1 h0.5–1 h1–2 h0.5–1 h
OnsetRapidRapidRapidRapid
CocaineConcentrationn/an/an/aTopical: 4–10%
Dosen/an/a2–5 mL
Durationn/an/an/a0.5–1 h
Onsetn/an/an/aRapid
ProcaineConcentrationn/an/a2%n/a
Dosen/an/an/a
Durationn/an/a0.5–1 hn/a
Onsetn/an/aRapidn/a
TetracaineConcentrationn/an/a0.5%Topical: 2%
Dosen/an/a15 mg
Durationn/an/a2–4 h0.5–1 h
Onsetn/an/aRapidRapid
PrilocaineConcentration4%n/a2%4%0.5%
Dose10–15 mLn/a80 mg50 mL
Durationn/an/a2–4 h0.5–1 h45–90 min
OnsetRapidn/aRapidRapidRapid
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Maximum Recommended Doses of Local Anesthetics...
Maximum recommended doseMaximum recommended dose with addition of epinephrine
Medicationmg (adult)mg/kgmg (adult)mg/kg
Lidocaine3003–4.55006–7
Mepivacaine4004.55507
Bupivacaine1752–32252
Levobupivacaine1502

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