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)

		Peripheral	Epidural	Spinal	Infiltration	IVRA
Amides
Bupivacaine	Concentration	0.25–0.5%	0.25–0.75%	0.5–0.75%	n/a	Contraindicated
	Dose	10–40 mL	5–20 mL	1–3 mL	n/a	Contraindicated
	Duration	4–12 h	60–120 min	1–3 h	n/a	Contraindicated
	Onset	Slow	Intermediate	Rapid	n/a	Contraindicated
Lidocaine	Concentration	1–1.5%	1.5–2%	1.5–5%	0.5–1%	0.25–0.5%
	Dose	10–40 mL	20 mL
	Duration	1–3 h	1.5–2 h	0.5–1.5 h	2–8 h	0.5–1 h
	Onset	Rapid	Rapid	Rapid	Rapid	Rapid
Mepivacaine	Concentration	1–1.5%	1.5–2%	2–4%	n/a	n/a
	Dose
	Duration	2–4 h	1–2 h	1–2 h	n/a	n/a
	Onset	Rapid	Rapid	Rapid	n/a	n/a
Ropivacaine	Concentration	0.5–1%	0.5–1%	0.5–0.75%	0.2–0.5%	n/a
	Dose	10–20 mL
	Duration	5–8 h	2–3 h	1–3 h	2–5 h	n/a
	Onset	Slow	Intermediate	n/a	Rapid	n/a
Esters
Benzocaine	Concentration	n/a	n/a	n/a	Topical: maximum 20%
	Dose
	Duration	n/a	n/a	n/a	0.5–1 h
	Onset	n/a	n/a	n/a	Rapid
Chloroprocaine	Concentration	2%	2–3%	2–3%	1%
	Dose
	Duration	0.5–1 h	0.5–1 h	1–2 h	0.5–1 h
	Onset	Rapid	Rapid	Rapid	Rapid
Cocaine	Concentration	n/a	n/a	n/a	Topical: 4–10%
	Dose	n/a	n/a	2–5 mL
	Duration	n/a	n/a	n/a	0.5–1 h
	Onset	n/a	n/a	n/a	Rapid
Procaine	Concentration	n/a	n/a	2%	n/a
	Dose	n/a	n/a		n/a
	Duration	n/a	n/a	0.5–1 h	n/a
	Onset	n/a	n/a	Rapid	n/a
Tetracaine	Concentration	n/a	n/a	0.5%	Topical: 2%
	Dose	n/a	n/a	15 mg
	Duration	n/a	n/a	2–4 h	0.5–1 h
	Onset	n/a	n/a	Rapid	Rapid
Prilocaine	Concentration	4%	n/a	2%	4%	0.5%
	Dose	10–15 mL	n/a	80 mg		50 mL
	Duration	n/a	n/a	2–4 h	0.5–1 h	45–90 min
	Onset	Rapid	n/a	Rapid	Rapid	Rapid

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Maximum Recommended Doses of Local Anesthetics...

	Maximum recommended dose		Maximum recommended dose with addition of epinephrine
Medication	mg (adult)	mg/kg	mg (adult)	mg/kg
Lidocaine	300	3–4.5	500	6–7
Mepivacaine	400	4.5	550	7
Bupivacaine	175	2–3	225

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