Basic Principles of Analgesia

Review of Local Anesthetics

• 1.

  The two most commonly used local anesthetics are lidocaine (Xylocaine) and bupivacaine (Marcaine, Sensorcaine) ( Table 1.1 ).

  • a.

    Recent studies have suggested that bupivacaine may be toxic to chondrocytes.

  • b.

    Ropivacaine (Naropin) has been found to be significantly less toxic to chondrocytes.

  • c.

    Whenever possible, ropivacaine should be used instead of bupivacaine when administering an intraarticular injection. Unfortunately, ropivacaine is not routinely available in hospital emergency departments.

  Table 1.1

  Local Anesthetics

  	Maximum Dosage Concentration	Maximum Dosage Amount	Time Until Effective	Analgesic Action Time	Suggested Use	Comments
  Lidocaine	5 mg/kg without epinephrine 7 mg/kg with epinephrine	1% concentration without epinephrine 30-kg child: 15 mL 70-kg adult: 35 mL	2–5 min	45 min to 2 h	Short-acting analgesia for a 10–20-min procedure	Can cause vasodilation if not used with epinephrine
  Bupivacaine	1.5 mg/kg without epinephrine 3 mg/kg with epinephrine	0.5% concentration without epinephrine 30-kg child: 9 mL 70-kg adult: 21 mL	10–30 min	3–6 h	Procedure >20 min	Avoid use in intraarticular injection if possible
  Ropivacaine	3 mg/kg without epinephrine Not used with epinephrine	0.75% concentration 30-kg child: 12 mL 70-kg adult: 28 mL	7–20 min	2–5 h	Procedure >20 min safer for use in pediatric population than bupivacaine	Less chondrotoxic than bupivacaine; strong intrinsic vasoconstrictive properties

• 2.

  Epinephrine is a vasoconstrictor and is often added to the local anesthetic.

  • a.

    Epinephrine improves the onset of action, decreases drug uptake, and prolongs action.

  • b.

    An epinephrine concentration of 1:200,000 is typically used.

  • c.

    Because it is a vasoconstrictor, epinephrine should be used with caution in the distal extremities. The following well-known mnemonic is often used to recall the areas where epinephrine should not be used: nose, hose (penis), digits, and toes.