Excerpt from Local Anesthetic Agents, Infiltrative Administration

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Mechanism of action

Local anesthetics reversibly block nerve conduction near their site of administration, thereby producing temporary loss of sensation in a limited area. Nerve impulse conduction is blocked by a decrease in nerve cell membrane permeability to sodium ions, possibly by competing with calcium-binding sites that control sodium permeability. This change in permeability results in decreased depolarization and an increased excitability threshold that ultimately prevents the nerve action potential from forming.^{1, 2, 3}

Indications

Parenteral local anesthetics are used for infiltration and nerve block anesthesia. Because of variation in systemic absorption and toxicity, the ideal choice of local anesthetic and concentration depends on the intended procedure. Infiltration anesthesia is often used for minor surgical and dental procedures. Nerve block anesthesia is used for surgical, dental, and diagnostic procedures and for pain management.^{4, 5}
 
Dosage and administration guidelines

• Lower concentrations of local anesthetics are typically used for infiltration anesthesia.
• Variation in local anesthetic dose depends on the procedure, the degree of anesthesia required, and the individual patient's circumstances.
• Reduced dosage is indicated in patients who are debilitated or acutely ill, in patients who are very young or very old, and in patients with liver disease, arteriosclerosis, or occlusive arterial disease.

Administrative techniques

• Infiltration anesthesia is accomplished with administration of the local anesthetic solution intradermally (ID), subcutaneously (SC), or submucosally across the nerve path that supplies the area of the body that requires anesthesia.^{4, 3}
• A common administrative technique is to inject the local anesthetic subcutaneously in a circular pattern around the operative site; this administration is often referred to as field block technique.^{1, 3}

• Concentrations: Drug concentration is expressed as a percentage (eg, bupivacaine 0.25%, lidocaine 1%).
• • Percentage is measured in grams per 100 mL (ie, 1% is 1 g/100 mL [1000 mg/100 mL], or 10 mg/mL).
  • Calculate the mg/mL concentration quickly from the percentage by moving the decimal point 1 place to the right, as follows:
  • • Bupivacaine 0.25% = 2.5 mg/mL
    • Tetracaine 0.5% = 5 mg/mL
    • Lidocaine 1% = 10 mg/mL
    • Viscous lidocaine 2% = 20 mg/mL
    • Benzocaine 20% = 200 mg/mL
• Dilutions: When epinephrine is combined in an anesthetic solution, the result is expressed as a dilution (eg, 1:100,000).
• • 1:1,000 means 1 mg per 1 mL (ie, 0.1% )
  • 1:10,000 means 1 mg per 10 mL (ie, 0.01%)
  • 1:2,000 means 1 mg per 2 mL (ie, 0.05%)
  • 1:20,000 means 1 mg per 20 mL (ie, 0.005%)
  • 0.1 mL of 1:1000 epinephrine added to 10 mL of anesthetic solution = 1:100,000 dilution, or 0.01 mg/mL
• Epinephrine content examples: From the information provided in the table below, 50 mL of 1% lidocaine with epinephrine 1:100,000 contains 500 mg of lidocaine and 0.5 mg of epinephrine.
  
  

  Solution Volume	1:100,000 (1 mg/100 mL)	1:200,000 (1 mg/200 mL)
  1 mL	0.01 mg	0.005 mg
  5 mL	0.05 mg	0.025 mg
  10 mL	0.1 mg	0.05 mg
  20 mL	0.2 mg	0.1 mg

Adverse effects

Adverse effects are usually caused by high plasma concentrations of a local anesthetic drug that result from inadvertent intravascular injection, excessive dose or rate of injection, delayed drug clearance, or administration into vascular tissue.^{1, 2} Possible adverse effects include the following:

• CNS: High plasma concentration initially produces CNS stimulation (including seizures), followed by CNS depression (including respiratory arrest). The CNS stimulatory effect may be absent in some patients, particularly when amides are administered. Solutions that contain epinephrine may add to the CNS stimulatory effect.^{1, 4, 5}
• Cardiovascular: High plasma levels typically depress the heart and may result in bradycardia, arrhythmias, hypotension, cardiovascular collapse, and cardiac arrest. Local anesthetics that contain epinephrine may cause opposite effects, including hypertension, tachycardia, and angina.^{1, 2, 4}
• Other body systems can also experience adverse effects.^{4, 5}
• • Transient burning sensation
  • Skin discoloration
  • Swelling
  • Neuritis
  • Tissue necrosis and sloughing
  • Methemoglobinemia (with prilocaine)

• Onset of action
• • pKa level is the primary factor that determines onset of action.
  • A lower pKa level increases tissue penetration and shortens onset of action; this is because of the more lipid-soluble nonionized particles.
  • A pKa level that is lower than the pH level optimizes penetration.
  • Inflammation in the extracellular space may decrease the pH level and may slow onset of action.
  • The site of administration influences onset (ie, onset is prolonged in areas with increased tissue or nerve sheath size).
• Potency
• • Anesthetics with high partition coefficients that increase lipophilic properties pass easily into the lipid nerve membrane.
  • The higher degree of vasodilation promotes vascular absorption, thereby reducing the locally available drug and decreasing potency.
  • The addition of epinephrine or sodium bicarbonate increases the pH level, thereby increasing nonionized particles, which are more lipid-soluble.
  • Generally, anesthetic solutions that contain premixed epinephrine contain preservatives; these solutions have the pH level adjusted lower to maintain stability of the epinephrine and antioxidants.
• Duration of action
• • The addition of epinephrine to some local anesthetic solutions prolongs duration of action by causing vasoconstriction and decreasing systemic absorption.
  • The degree of protein binding primarily determines the duration of action; high protein binding increases the duration of action.
  • Increasing pH level (by addition of epinephrine or sodium bicarbonate) prolongs duration of action.

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