Local Skin Anesthesia

See Also

Indications
Local Anesthetic

Infiltration Anesthesia (skin procedures)
Regional Anesthesia or Nerve Block (e.g. Digital Block)
Topical Anesthetic (e.g. tetracaine for ocular procedures)
Spinal Anesthesia
Epidural Anesthesia

Mechanism

Cocaine was the first Local Anesthetic, with subsequent synthetic agents with similar activity
Local Anesthetics Inhibit Nerve Conduction
1. Decrease the Neuron's membrane permeability to Sodium
2. Without Sodium influx when Neurons are activated (depolarization), Action Potentials cannot be generated
Small nerve fibers are most susceptible to penetration and blockade
1. Pain and Temperature sense (A-Delta and C Nerve Fibers) are most susceptible to Local Anesthetic
2. Touch and proprioception are less susceptible
3. Skeletal Muscle innervating Neurons are least susceptible to Local Anesthetic
Images
All synthetic Local Anesthetics have a common structure
1. Hydrophobic region (aromatic ring structure)
2. Linker region (alkyl chain containing either an amide or ester bond)
3. Hydrophilic region (substituted amines)
Synthetic Local Anesthetics are of 2 classes (differentiated by their linker alkyl chain bond)
1. Amides (e.g. Lidocaine)
  1. Less susceptible to enzymatic degradation, clearing more slowly than esters, with longer duration of activity
2. Esters (e.g. Procaine)
  1. Easily hydrolyzed by plasma esterases, resulting in more rapid breakdown, and shorter duration of activity
Adjunctive Agents
1. Bicarbonate
  1. Added to Local Anesthetic to increase the percentage of non-ionized Anesthetic
  2. Nonionized Anesthetic reduces the burning Sensation on injection
  3. Nonionized Anesthetic more easily penetrates the nerve sheath, increasing its Anesthetic activity
    1. Within the Neuron, the Anesthetic re-ionizes to its active form
2. Epinephrine
  1. Epinephrine induces local Vasoconstriction and Hemostasis, keeping Local Anesthetic within the injected region
  2. Epinephrine is added to Local Anesthetic to prolong Anesthetic activity
  3. Epinephrine also reduces systemic absorption (LAST Reaction risk) when added to Local Anesthetic
    1. Maximum safe Anesthetic total dosages are higher when Epinephrine is added

Technique
Helpful hints to make Local Anesthetic more comfortable

Use smaller gauge needles (higher numbers)
1. Elective procedures: 27 to 30 gauge needle
2. Avoid using anything larger than a 25 gauge needle
Prepare the injection solution
1. Warm Lidocaine (Xylocaine) to Body Temperature
2. Use Buffered Lidocaine
  1. Bicarbonate 1 part to Xylocaine 9 part ratio
  2. Stable at room temp for one week
Prepare the site
1. Cool skin before injection
  1. Ice
  2. Quick spray of Liquid Nitrogen, ethyl chloride or frigiderm
  3. Avoid ethyl chloride when using electrocautery
2. Consider Topical Anesthetic use before injection
  1. See topical pre-Anesthetics below
Employ a less painful injection technique
1. Rapidly insert needle (do not allow to needle to linger on surface)
2. Infiltrate skin slowly and steadily while withdrawing
3. Inject through wound edge if possible
4. Inject into subcutaneous tissue (equivalent efficacy to injecting into skin, and less painful)
5. Be generous with Local Anesthetic
  1. However, may distort wound edges
Employ distraction techniques
1. Patient lifts their own leg (Gate theory of pain)
2. Vibrate skin or pinch skin as you inject
3. Have patient keep eyes open (analgous to Lamaze birth)
4. Talk calmly to patient
Mixing Anesthetics offers no benefit
1. Lidocaine (1 hour duration) and Bupivicaine (2-4 duration) have very similar onset of action (3 minutes)
2. Select one agent for Local Anesthetic based on desired duration of Anesthesia
3. Ribotsky (1996) J Am Podiatr Med Assoc 86(10): 487-91 [PubMed]
4. Alhelail (2009) Emerg Med J 26(5): 347-50 [PubMed]

Pharmacokinetics

Amide and Ester Anesthetic metabolism and excretion
1. Metabolized by liver
2. Excreted by Kidney
Dose calculations
1. Solution of 0.5%: 5 mg/ml
2. Solution of 1%: 10 mg/ml
3. Solution of 2%: 20 mg/ml

Preparations
Topical pre-Anesthetics

Ethyl Chloride Spray
1. Do not use with electrocautery
2. Site sprayed for 1-2 seconds
3. Offers 1-2 seconds of Anesthesia
Lidocaine-Epinephrine-Tetracaine (LET)
1. Replaces Tetracaine, Adrenaline, Cocaine (TAC)
2. Indicated for open wound
3. May be especially useful in children
EMLA cream (Eutactic Mixture of Local Anesthetic - Lidocaine 2.5%, Prilocaine 2.5%)
1. Indicated in closed wounds
2. Apply ointment under Occlusion
3. EMLA cream Anesthesia reaches a depth of 3 mm at 1 hour, and 5 mm at 1.5 to 2 hours
4. Use 90 minutes prior to injection
5. ELA-Max (OTC) may offer similar effect (see below)
Iontophoresis
1. Lidocaine sponges applied to intact skin
2. DC current applied to electrodes over Anesthetic
3. Onset within 10 minutes and duration of 15 minutes
4. Penetration depth of 1-2 cm
5. As effective as EMLA cream
Lidocaine 4% in liposomal matrix (LMX or ELA-Max)
1. Apply to skin and occlude for 30 minutes (much faster onset than EMLA cream)
J-Tip (transdermal Lidocaine device)
1. Creates eraser sized wheel of Lidocaine that has been shot through the skin without needle
2. Onset of Anesthesia within 3 minutes of use
3. May also be used for Lumbar Puncture
Buzzy Bee Vibrating Device (effective in ages 3-18 years; not effective in infants)
1. Wings of device are stored in the freezer (use within 10 minutes of removing from freezer)
2. Device and wings are placed on skin 5 cm proximal to venipuncture site (or Immunization site)
3. Cold and vibration prevents pain fibers from transmitting painful stimuli (gate theory)
4. Apply to skin for 1 minute before procedure
Other measures with unknown efficacy in procedures
1. Anesthetic Patch (Lidoderm)

Preparations
Local Anesthetics - Short acting Amide Anesthetics

Local Lidocaine or Xylocaine (0.5%=5 mg/ml, 1%=10 mg/ml, 2%=20 mg/ml)
1. Onset: 2 to 5 minutes
2. Duration: 1 hour (up to 1.5 to 3 hours in some cases)
3. Max dose: 4 mg/kg to 300 mg/dose (up to 15 ml 2%, 30 ml 1% or 60 ml 0.5%)
  1. Epinephrine increases Lidocaine duration or activity and maximal safe dose to 7 mg/kg (see below)
Mepivacaine (Carbocaine) 1%
1. Onset: 3 to 5 minutes
2. Duration: 1.5 to 2 hours
3. Max dose: 4 mg/kg up to 280 mg/dose (28 ml))
Prilocaine (Citanest) 1%
1. Onset: 2 minutes
2. Duration: 1 hour
3. Max dose: 7 mg/kg up to 500 mg/dose (50 ml)

Preparations
Local Anesthetics - Long acting Amide Anesthetics

Lidocaine with Epinephrine 1:100,000 or 1:200,000
1. Onset: 2 minutes
2. Duration: 2 to 6 hours
3. Max dose: 7 mg/kg to 500 mg (up to 25 ml 2%, 50 ml 1%)
4. Epinephrine increases Lidocaine's duration of action and maximal safe dose (decreases systemic absorption)
Bupivacaine or Marcaine or Sensorcaine (0.25%=2.5 mg/ml, or 0.5%=5 mg/ml)
1. Onset: 5 minutes (up to 15-30 minutes)
2. Duration: 2 to 4 hours (up to 6 hours, esp. when combined with Epinephrine)
3. Max bolus dose: 2 mg/kg (some guidelines use maximum of 2.5 mg/kg)
  1. Weight >=75 kg (165 lb): 60 ml of 0.25% (2.5 mg/ml) or 30 ml of 0.5% (5 mg/ml)
4. Avoid in pregnancy (increased Bioavailability, crosses placenta)
5. Always confirm not intravascular (withdraw first) as risk of lethal Arrhythmias (Ventricular Fibrillation)
  1. See LAST Reaction
  2. Less risk of systemic absorption when combined with Epinephrine
Etidocaine (Duranest) 0.5% or 1%
1. Max dose: 4 mg/kg to 280 mg/dose (25 ml 1%, 50 ml 0.5%)
Ropivacaine 0.2 to 0.5%
1. Preferred agent in Regional Anesthesia (decreased LAST toxicity risk)
2. Decrease dose in liver disease, malnourishment, Critical Illness
3. Single bolus
  1. Onset: 10-20 minutes
  2. Duration: 6 to 14 hours
  3. Maximum Dose: 3 mg/kg of 0.5 % (5 mg/ml)
    1. Weight 50 kg (110 lb): Maximum 30 ml of 0.5% (1 bottle of Ropivacaine)
    2. Weight 60 kg (132 lb): Maximum 36 ml of 0.5%
    3. Weight 70 kg (154 lb): Maximum 42 ml of 0.5%
    4. Weight 80 kg (176 lb): Maximum 45-48 ml of 0.5%
    5. Weight 90 kg (198 lb): Maximum 45-54 ml of 0.5%
    6. Weight >=100 kg (220 lb): Maximum 45-60 ml of 0.5% (up to 2 bottles of Ropivacaine)
4. Continuous infusion
  1. Maximum: 0.5 mg/kg/h of 0.2% (2 mg/ml)
    1. Weight 40 kg: Maximum 10 ml/h of 0.2% (2 mg/ml)
    2. Weight 50 kg: Maximum 12 ml/h of 0.2% (2 mg/ml)
    3. Weight >=56 kg: Maximum 14 ml/h of 0.2% (2 mg/ml)
5. References
  1. Reardon and Martel (2020) Regional Anesthesia for Acute Care Conference, attended 12/11/2020
    1. https://stabroom.com/

Preparations
Local Anesthetics - Esters

Background: Mnemonic to distinguish the Ester Anesthetics from the Amide Anesthetics
1. Amides have two "i"
2. Esters have one "i"
Chloroprocaine (Nesacaine)
Procaine (Novocain) 1%, 2%
1. Onset: 2-5 minutes
2. Duration: 15 to 60 minutes
3. Max Dose: 7 mg/kg up to 600 mg/dose (30 ml 2%, 60 ml 1%)
Tetracaine (Pontocaine) 0.5%
1. Onset: 5-10 minutes
2. Duration: 2-3 hours
3. Max Dose: 1.4 mg/kg up to 120 mg/dose (24 ml 0.5%)

Adverse Effects
LAST Reaction

See Local Anesthetic Systemic Toxicity (LAST Reaction)
Prevention
1. Exercise caution to avoid intravascular injection of Local Anesthetic
2. Limit total Anesthetic dose to weight-based maximums established for each agent

Adverse Effects
Allergy to Local Anesthesia

True amide Anesthetic is rare
1. Usually a reaction to preservative methylparaben
Novocaine allergy does not predict Lidocaine allergy
1. Lidocaine is an amide and novocaine an ester
2. Lidocaine may be used if novocain allergy
Options if history of Lidocaine allergy
1. Preservative-free Lidocaine (single use bottles)
2. Select an ester Anesthetic (e.g. Nesacaine, Procaine, Tetracaine)
3. Diphenhydramine Hydrochloride 1%
  1. Dilute 50 mg (1 ml) in 4 ml Normal Saline
  2. Equivalent to Lidocaine 1% activity
  3. Avoid use in Peripheral Nerve Block

Resources

Local Anesthetics: Clinical Pharmacology and Rational Selection
1. http://www.nysora.com/regional-anesthesia/foundations-of-ra/3492-local-anesthetics-clinical-pharmacology-and-rational-selection.html

References

Acker, Koval and Leeper (2017) Crit Dec Emerg Med 31(4): 3-13
Pfenninger (1994) Procedures, Mosby, p. 135-155
Mortiere (1996) Wound Management, p. 27-39
Achar (2002) Am Fam Physician 66(1):91-4 [PubMed]
Latham (2014) Am Fam Physician 89(12): 956-62 [PubMed]