Anesthetics| | | | | | Loss of sensation occurs in following order: temperature and pain (C-type fibers), touch, pressure, vibration, proprioception, motor function | | | | | |
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(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 | | | Property | | Factor | | Description | | | 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) | | | | | | | |
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| | | | | | | Table 6-13 Ester and Amide Anesthetics | | | Drug Name | | Potency | | Onset | | Duration | | Important 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 | | | | | | | | | | | | | | |
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| | | | | | | Table 6-14 Reactions to Anesthetics | | | Dose | | Symptoms | | Management | | | 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 | | | | | | | |
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