Local Anesthesia

   
 Loss of sensation occurs in following order: temperature and pain (C-type fibers), touch, pressure, vibration, proprioception, motor function 
   
Anesthetics (Tables 6-12 , 6-13 , 6-14)
  • Mechanism of action: reversible nerve conduction blockage (interferes with influx of sodium ions into cell resulting in inability for depolarization); two types: amides and esters
  • Three major components: connecting chain (ester or amide), amine portion, aromatic end
  • Amides
       
     Amides with two i’s in the name 
       
    • Metabolized by microsomal liver enzymes (cyt p450 3A4); excreted by kidneys
    • Risk factors for toxicity: severe liver disease, drugs that  half-life (i.e., propranolol)
    • Cross-reactivity: amides do not have any derivates of PABA (para-aminobenzoic acid), so ↓ cross-reactivity and sensitization compared to esters (but methylparabens may be used as preservative, which is metabolized to PABA by-product)
  • Esters: cocaine, procaine, tetracaine, chloroprocaine
    • Cleared via hydrolysis by plasma pseudocholinesterase; excretion by kidneys
    • Do not use if pseudocholinesterase deficiency or hypersensitivity to PABA or derivates
    • Allergic reaction typically due to PABA, an ester intermediate metabolite (may cross-react with paraphenylenediamine (PPD), para-aminosalicylic acid, sulfonamides, azo dyes, other ester anesthetics, benzocaine (topical), and PABA sunscreens)
  • Tumescent anesthesia: large volumes of dilute lidocaine (0.05–0.1%) and epinephrine (1:1,000,000) to produce complete anesthesia and hemostasis; infuse over 90–120 min; peak plasma dose 4–14 h after infusion, lasts up to 24 h; max safe dose is 55 mg/kg
  • Digital blocks: use minimal anesthetic with epi, should not exceed 1.5 ml per side (3 ml total per finger)
  • Topical anesthetics
    • EMLA: eutectic mixture of 2.5% lidocaine, 2.5% prilocaine,←(Do not use in infants as it can cause methemoglobinemia) under occlusion
    • Ela-Max (LMX): 4 or 5% lidocaine, no occlusion necessary
   
 
Table 6-12 Anesthetic Properties
 PropertyFactorDescription
 
Onset of action
pKa level
↓ pKa → more rapid onset (closer to 7.4 means more uncharged base form, which can pass through neuronal cell membrane)
 
Duration of action
Protein binding
↑ Ability of binding plasma proteins → increased duration of action (lipid solubility less important)
 
Potency
Lipid solubility
↑ Lipid solubility → ↑ potency (↑ penetration of hydrophobic environment)
      
 
   

   
 
Table 6-13 Ester and Amide Anesthetics
 Drug NamePotencyOnsetDurationImportant Features
AMIDES
 
Prilocaine
++
Rapid
+
Shortest acting; risk of methemoglobinemia (risk ↑ with infants and G6PD deficiency)
 
Lidocaine

{1% lidocaine = 10 mg/ml: for 70 kg person give 50 ml or 500 mg max}
++
Rapid
++ (1–6 h)

{Class B pregnancy}
Adult max dose: 4.5 mg/kg without epi; 7.0 mg/kg (500 mg) w/ epi

{1:100,000 epinephrine}
 
Pediatric max dose: 3–4 mg/kg with epi
 
Mepivacaine
 
++
   
++
  
 
Etidocaine
++++
Rapid
+++
 
Bupivacaine
++++
Slow
++++ (3–7 h)
Longest acting; ↑ cardiac arrhythmias
ESTERS
 
Procaine
+
+ (30 min)
 
Chloroprocaine
+
Rapid
+
pKa high but fast onset due to ↑ concentration
 
Tetracaine
++++
Slow
+++
     
     
  Cocaine vasoconstrictive ester; others vasodilating  
     
     
          
 
   

   
 
Table 6-14 Reactions to Anesthetics
 DoseSymptomsManagement
 Lidocaine Overdose
 
1–6µg/ml
Paresthesias (circumoral, tongue, digital), euphoria lightheadedness, restlessness, talkativeness, metallic taste
Observation
 
6–9µg/ml
Nausea, vomiting, tremors, blurred vision, tinnitus, muscle twitching, confusion, excitement, psychosis, slurred speech
Maintain airway, ± diazepam
 
9–12µg/ml
Seizures and cardiopulmonary depression
Respiratory support
 
>12µg/ml
 
Coma, cardiopulmonary arrest
 
CPR, life support
Vasovagal Reaction
 
Excess parasympathetic tone (↓ pulse, ↓ BP): diaphoresis, nausea, vagal-induced bradycardia, and hypotension

{Most common side effect seen}
Cold compresses, Trendelenburg
Epinephrine Reaction
 
Self limited: palpitations, ↑ pulse, ↑ BP, anxiety, diaphoresis, tremor, HA

{Serious effects (rare): cardiac arrhythmias, cardiac arrest}
Monitor pulse/BP, short-lived typically
Anaphylactic Reaction
 
(↑ Pulse, ↓ BP): tachycardia, angioedema, stridor, bronchospasm
SubQ epinephrine, maintain airway