Local Anesthesia

Block conduction in nerves by impairing propagation of action potential (sodium channel mediated)

• a.

  Only uncharged forms enter peripheral nerves; thus local anesthetics perform better in alkaline environments.

• b.

  Infected and inflamed fields can be treated more effectively by alkalinizing the solution with bicarbonate (HCO ₃ ) (see later for preparation).

• c.

  After the agent reaches the cytoplasm, the ionized form is able to block sodium channels.

Differential blockade of nerve fibers

• a.

  Myelinated and smaller nerves are most affected.

• b.

  Clinical sequence of sensations anesthetized based on order of nerve fibers blocked is autonomic/vasodilation > pain > temperature > pressure > motor function.

Amides

• a.

  Metabolism is hepatic.

• b.

  Elimination half-life is 2–3 hours.

• c.

  True allergy is rare.

• d.

  These include a second letter i in their name (i.e., lidocaine).

• e.

  Examples are lidocaine, bupivacaine, prilocaine, and ropivacaine.

Esters

• a.

  Metabolism is by pseudocholinesterases, which create a short plasma half-life.

• b.

  These do not contain a second letter i in their name.

• c.

  Examples are procaine, chloroprocaine, cocaine, and tetracaine.

Commonly available local anesthetics

• a.

  Table 7.1

  TABLE 7.1

  Properties of Commonly Available Local Anesthetics

  Name	Class	Onset (min)	Duration (h) (Without/With Epinephrine)	Max Dose (mg/kg) (Without/With Epinephrine)
  Procaine(Novocaine)	Ester	2–5	0.25–1	7
  Chloroprocaine(Nesacaine)	Ester	<1	0.5	11
  Lidocaine(Xylocaine)	Amide	2–5	1–2/2–4	4.5/7
  Bupivacaine(Marcaine)	Amide	5–15	3–4/4–8	2.5/3
  Liposomal bupivacaine(Exparel)	Amide (slow release)	5–15	Up to 72 h	266 mg total (amount in 20-mL vial)
  Ropivacaine(Naropin)	Amide	10–15	2–6	2.5

Commonly used solutions

• a.

  Lidocaine (quick onset, shorter duration) and bupivacaine (slightly longer onset, longer duration) are the most common local anesthetics used.

• b.

  Quickly calculating maximum dosage

  • (1)

    Convert concentration of local from percentage to mg/cc by multiplying by 10.

    • (a)

      Example: 1% lidocaine = 1 g/100 cc = 10 mg/1 cc (or 0.1 cc/mg)

• c.

  Multiply patient’s weight (kg) by listed dose maximum (mg/kg), and then divide by local anesthetic concentration (mg/cc).

  • (1)

    Example: Using 1% lidocaine with epinephrine (7 mg/kg max dose) on a 70-kg patient: [(70 kg × 7 mg/kg)] / [10 mg/cc] = 49 cc until toxic

• d.

  Standard solutions:

  • (1)

    For the majority of adult emergency room procedures, a solution of 1% lidocaine with epinephrine works quickly, lasts for a long enough period of time to comfortably perform a procedure, and can be given in enough quantity to avoid problems with toxicity.

  • (2)

    In multitrauma patients or in children, in whom the amount of needed local anesthetic increases and/or the weight of the patient decreases, the need for less concentrated formulations of lidocaine (such as 0.5% and 0.25%) becomes more important to avoid toxicity.

  • (3)

    For combination short- and long-term pain control, prepare a syringe with a 1:1 mixture of 1% lidocaine and 0.5% bupivacaine (may also include bicarbonate as previously discussed to reduce pain of injection).

Central nervous system toxicity

• a.

  Prodromal symptoms: light-headedness, dizziness, metallic taste in mouth, circumoral numbness, and tinnitus

• b.

  Severe toxicity: seizures, loss of consciousness

Cardiovascular toxicity

• a.

  Symptoms: tachyarrhythmia or bradyarrhythmia, ventricular fibrillation, hypotension

Prevention

• a.

  Avoid excessive doses (calculate maximum dose based on patient’s weight).

• b.

  Avoid intravascular injection (aspirate before injecting to confirm needle tip is not intravascular).

Management

• a.

  Give no more local anesthetic.

• b.

  Supportive management with airway protection, supplemental oxygen, benzodiazepines for seizure treatment, intravenous (IV) fluid administration for cardiovascular support, arrhythmia treatment according to advanced cardiac life support (ACLS) guidelines

• c.

  Lipid emulsion therapy

  • (1)

    IV infusion of a 20% lipid emulsion (e.g., Intralipid 20%) has become an accepted part of treatment for systemic toxicity from local anesthetics and particularly for cardiac arrest that is unresponsive to standard therapy.

  • (2)

    American Society of Regional Anesthesia and Pain Medicine (ASRA) guidelines recommend considering the use of lipid emulsion therapy at the first signs of systemic toxicity from local anesthetics, after airway management.

    • (a)

      Give bolus of 1.5 mL/kg over 1 minute.

    • (b)

      Convert to an infusion at a rate of 0.25 mL/kg per minute for 20–60 minutes, or until hemodynamic stability is restored.

Utility

• a.

  Used in conjunction with local anesthetic to provide hemostasis

• b.

  Delays absorption of the local anesthetic into the systemic circulation

• c.

  Allows for a greater concentration of local anesthetic to be used before reaching toxic levels (see Table 7.1 )

Safety

• a.

  Subcutaneous injection of epinephrine in combination with local anesthetics to distal areas (such as fingers and toes) has been shown to be safe without evidence of necrosis over multiple studies.

• b.

  Current recommendations include low-dose use of epinephrine, typically injected at a concentration of 1:100,000–400,000.

• c.

  Avoid circumferential injection if possible.

Optimal timing between injection and incision

• a.

  The time until maximal cutaneous vasoconstriction after injection of lidocaine with epinephrine is historically reported as 7–10 minutes.

• b.

  If optimal visualization is desired, waiting longer to make an incision after injection with epinephrine must be considered and weighed against delaying the start of the procedure.