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Local Anesthesia Topics

        • 0.15 mL of a 1-mg/mL solution of epinephrine to 30 mL of local anesthetic agent yields a 1:200,000 solution
          • Solutions of epinephrine more concentrated than 1:100,000 provide no additional clinical vasoconstriction, and increase the risk for epinephrine toxicity
          • reverse ischemia with phentolamine
        • Premixed solutions containing epinephrine are acidified to a pH of 3 to 4.5 to stabilize the epinephrine component from spontaneous hydrolysis, which increases its shelf life but also alters the pharmacodynamic properties of the solution (makes it more acidic!, more painful, longer onset)
          • At an acidic pH (as in most local anesthetic solutions or infected tissues), almost all the anesthetic is in the cationic form, which delays the onset of action
        • Relative contraindications:  (consider phenylephrine)
          • history of unstable angina, cardiac dysrhythmias, uncontrolled hypertension
            • limit to 0.04mg (4ml) in pts with cardiac hx
          • uteroplacental insufficiency related to pregnancy
          • hyperthyroidism
          • concurrent use of agents that alter the effects of catecholamines (eg. monoamine oxidase inhibitors or tricyclic antidepressants)
        • Other uses:
          • Adult anaphylactic dose = 0.3-0.5mg
          • Cardiac arrest = 1mg
      • Phenylephrine 
        • 1:20,000 or 1:50,000 dilution often an adequate substitute for epinephrine where there are medical contraindications.
        • must wait 15-20min
        • less effective, shorter duration
        • Mechanism: produces peripheral vasoconstriction without the cardiac effects
      • COCAINE - ONLY anesthetic with vasoconstrictive properties 
        • Mechanism: 
          • blocks sodium channels -> analgesia
          • blocks Norepinephrine reuptake of sympathetics -> vasoconstriction
        • 2-3 mg/kg or 200 mg
        • We use it intranasally 4 ml of 4% (40 mg/ml) = 160 mg
      • Lidocaine 
        • 1% = 1g/100ml = 1000mg/100ml = 10mg/ml
        • 2% = 20mg/ml
        • max dose:
          • 4 mg/kg w/o epi (280mg max for 70kg pt = 28 ml w/o)
          • 7 mg/kg w/ epi (500 mg max for 70kg pt = 50 ml)
      • Marcaine (Bupivacaine)
        • 0.25% w/ 1:200,000 epi = 2.5mg/ml
        • max dose:
          • 3 mg/kg w/ (225 mg max = 90ml) 
          • 2 mg/kg w/o (175mg max = 70ml)
        • *generally not recommended in peds (low max dosing volumes)
      • Benzocaine
        • 1 second spray of 20% benzocaine delivers 0.5 ml (100 mg)
          • very easy to give substantial doses of benzocaine with even conservative use of this drug
          • for this reason, applications are limited
          • general guidelines:  spray <1 second, no more than 2 sprays total
        • Caution:
          • Risk for methemoglobinemia
          • Patients with G-6-PD deficiency, cholinesterase deficiency (succinylcholine sensitivity) or congenital abnormalities of methemoglobin reductase are at greatest risk
          • Methemoglobinemia has been reported with benzocaine doses as small as 150 mg
    • Iontophoresis for myringotomy or ear procedures under local anesthesia

 

 

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