Infiltrative Anesthetics

Two groups:

  • Amides (2 I’s in name): Prilocaine, Etidocaine, Lidocaine, Mepivacaine, Bupivacaine
  • Esters (only 1 I in name): Cocaine, Procaine, Tetracaine, Benzocaine

Note:

Lidocaine Pregnancy Category B, but with epinephrine Pregnancy Category C since it may decrease uterine blood flow

Lidocaine (most commonly used)

  • Concentrations: 1% (10 mg/ mL), 2% (20 mg/mL), 0.1% tumescent (1 mg/mL)
  • Maximum dose without epinephrine: 4.5–5 mg/kg  (Pediatric: 1.2–2 mg/kg)
  • Maximum dose with epinephrine: 7 mg/kg (Pediatric: 3–4.5 mg/kg)
  • Maximum dose Tumescent anesthesia: 55 mg/kg 
    (10-fold dilution of standard 1% lidocaine with 1: 100,000 epinephrine (= 0.1% lidocaine with 1: 1,000,000 epinephrine)

  • Block nerve conduction via reversible inhibition of sodium ion influx preventing depolarization
  • Smaller diameter nerves are most susceptible (C & Aδ fibers – pain & temperature) whereas large nerves are more resistant (Aβ fibers -pressure)
  • Most resistant nerves are proprioception & motor nerves

Note: 

  • All anesthetics are vasodilators except Cocaine

Absorption

  • Influenced by local anesthetic properties, presence of epinephrine, site of injection, quantity & technique

Protein binding

  • Affects duration of action (Bupivacaine longest acting & Procaine shortest)

Lipid solubility

  • Affects potency (more lipophilic = more potent)

Metabolism

  • Amides: metabolized by CYP3A4 (Fastest to slowest: Prilocaine, Etidocaine, Lidocaine, Mepivacaine, Bupivacaine)
  • Esters: Hydrolyzed by plasma pseudo-cholinesterase 

Excretion

  • Via kidneys

FDA

  • Infiltrative anesthesia
  • Regional nerve blocks
  • Topical anesthesia

Off Label

  • Tumescent anesthesia
  • Postherpetic neuralgia
  • Pruritus

Contraindications to local anesthesias:

Absolute

  • Hypersensitivity to local anesthetic or preservatives (paraben, sulfites)

Relative

  • Allergy to para-aminobenzoic acid or derivatives 
  • Pregnancy
  • Severely labile blood pressure
  • Myasthenia gravis
  • Significant hepatic impairment (amide)
  • Significant cardiac impairment (amide)
  • Pseudo-cholinesterase deficiency (ester)
  • Renal insufficiency (ester)

Contraindications to epinephrine:

Absolute

  • Sensitivity to sodium metabisulphite
  • Pheochromocytoma

Relative

  • Pregnancy
  • Uncontrolled cardiac disease
  • Uncontrolled hyperthyroidism 
  • Poorly controlled diabetes
  • Severe Peripheral vascular disease
  • Angle-closure glaucoma
  • Cocaine abuse

Lidocaine Toxicity

  • Blood levels 1-5 ug/mL: Increased anxiety, talkativeness, tinnitus, circumoral tingling/paresthesia, nausea, vomiting, metallic taste, double vision
  • Blood levels 5-8 ug/mL: Nystagmus, muscle twitching, tremor
  • Blood levels 8-12 ug/mL:  Seizures, cardiopulmonary depression
  • Blood levels 20–25 μg/mL: Cardiopulmonary arrest & Coma (If toxicity occurs, discontinue anesthetic; do ABC’s +/- give lorazepam)

Allergic Reactions

  • To local anesthetic or preservatives (Esters > amides)

Epinephrine Effects

  • Anxiety, headache, tremor, restlessness, palpitations

Related to injection procedure

  • Vasovagal reactions, hematoma, ecchymosis, nerve laceration, infection

Note: 

  • Risk of cardiac toxicity with Bupivacaine

CYP3A4 inducers decrease level of anesthetic, inhibitors increase duration

Inducers

  • Quinidine, benzos, Griseofulvin, carbamazepine, phenytoin, rifampin, alcohol

 

Inhibitors

  • Macrolides, fluoroquinolones, cephalosporins, azoles, HIV protease inhibitors, calcium channel blockers, cimetidine, methylprednisolone, thiazides, grapefruit, SSRIs, propranolol

Measures to reduce pain of infiltrative local anesthetic

  • Application of ice 
  • Warming anesthesia (40C)
  • Buffering with bicarbonate
  • Pinching skin before & during infiltration
  • Mechanical counter-stimulation/verbal distraction 
  • Needle related: smaller size/ if multiple punctures needed to anesthetize then go through with needle in previously anesthetized areas
  • Slower rate of infusion

Purposes of epinephrine

  • Hemostasis
  • Prolong local anesthetic effect 
  • Decrease systemic absorption

 Alternatives to esters & amides

  • Cryoanesthesia 
  • Diphenhydramine hydrochloride (blockage of sodium channels, side effects: somnolence, skin irritancy, digital necrosis)
  • Intradermal normal saline