Page Contributor: Luke Sturgill


Overview


Local anesthetics (LA) are commonly used in painful procedures, such as skin surgery and wound repair.


Mechanism

  • Local anesthetics reversibly block sodium channels within the nerve fibers → disrupts depolarization of the nerve.
    • Sensitivity to anesthetic: 
      • Pain >> Temperature > Touch > Pressure & Motor
        • pain fibers are unmyelinated.
        • pressure and motor receptors are heavily myelinated and thus less affected.
          • This is why explaining to the patient that they should not feel sharp pain, but may feel a vague sensation of pressure is highly important. 
  • Local anesthetics are weak organic bases that are composed of charged (ionized) and uncharged forms when in solution. 
    • only the uncharged form can diffuse through the nerve membrane. 
      • infected tissue → ↑ acidity → ↓ uncharged form → more LA required
      • buffering LA w/ Na⁺ HCO₃⁻  → ↑ uncharged form → ↑ analgesic effect 
        • Note: buffering lidocaine decreases the shelf life to 1-3 weeks at room temperature. 





Choice of Local Anesthetic:


Local anesthetics are classified as esters or amides 



Method of application: Topical vs. Infiltrative (injection):

  • Topical:
    • Cocaine
    • Tetracaine 
    • Benzocaine
    • Lidocaine
  • Infiltrative:  
    • Lidocaine 
    • Bupivacaine 
    • Mepivacaine 


Lidocaine and Bupivocaine are the two most commonly used infiltrative local anesthetics. Their key differences and indications are detailed in figure below.


An important take away from the figure above is knowing the max dosages.

  • the max allowable dosage for lidocaine is 5 mg/kg (plain) or 7 mg/kg (w/ epi).
    • Calculating how many mL of lidocaine you can inject: 
      • if using 1% lidocaine = 1g /100 mL = 10 mg/mL 
      • If your patient weighs 70kg, how much 1% lidocaine w/ epi can you inject?
        • (pt weight) x (max dosage) = mg of lidocaine       (mg of lidocaine) / (lidocaine concentration) = how many mL you can inject
        • (70 kg) x (7 mg/kg) = 490 mg of lidocaine        (490 mg) / (10mg/mL) = 49 mL of lidocaine
  • The max total dose of lidocaine regardless of weight is: 
    • plain = 300 mg (30 mL of 1 percent) 
    • w/ epi = 500 mg (50 mL of 1 percent)
  • Note that excess local injection distorts tissue, so a peripheral nerve block may be preferable in situations where a large amount of infiltration will be required or precise alignment of tissue is desired. 



Local anesthetic toxicity


Toxicity is dose dependent, and typically progresses as CNS excitation → Seizures → CNS depression


CNS excitation

  • Generally first sign of toxicity
  • lightheadedness, euphoria, tingling of the lips, tinnitus, bitter/metallic taste  
  • agitation, muscle twitching, and hypertension. 


Seizures

  • can be preceded by a prodrome of slowed speech, jerky tremors, and hallucinations
  • seizures should be controlled with Benzodiazepines 


CNS Depression

  • Occurs with very high blood levels. 
  • Toxic effects on cardiorespiratory system are seen
    • Somnolence, coma, and bradycardia accompany shallow respirations.
  • This is followed by apnea, hypotension, and cardiovascular collapse and arrest.
  • Treatment should be directed toward maintenance of respiration and circulation with intubation, positive pressure ventilation, and cardiovascular support with vasopressors and fluids to restore circulation.


Toxicity can be reduced by:

  • careful calculation and titration of dosage.
  • using solutions w/ epinephrine.
  • avoidance of intravascular injection. 
    • leads to immediate high plasma levels that can cause toxicity with small doses. 
  • Benzodiazepine premedication for its anticonvulsant effect by elevating seizure threshold.


Some specific toxicities to note: 

  • Bupivacaine
    • Has a much higher cardiotoxicity risk than lidocaine, and should be cautioned in elderly and sick patients. 
  • Prilocaine
    • unique in that it can cause methemoglobinemia at doses of 500 mg or more. 
      • tx = IV methylene blue
  • Cocaine
    • Unique among local anesthetic agents for its ability to block the reuptake of norepinephrine at adrenergic nerve endings. 
    • Mydriasis, hypertension, and tachycardia are common side effects. 
    • Can be particularly dangerous in patients with coronary artery or other cardiovascular disease.
    • Avoid in patients on TCAs and MAOIs (these drugs also ↑ norepinephrine and can cause HTN crisis).
    • Combination of ketamine and cocaine should also be avoided as NE release can be ↑ with this combination. 


Epi or No Epi? 


Lidocaine and Bupivacaine solutions with Epinephrine are commonly used.


Epinephrine causes vasoconstriction, leading to the following benefits: 

  • Slows absorptionlonger duration of action & less risk of toxicity
  • ↓ blood loss 


However, additional epinephrine in circulation can exacerbate sympathetic responses.

Situations where epinephrine should NOT be used or used with caution: 

  • Patients with:
    • severe hypertension
    • hyperthyroidism
    • coronary artery disease
    • pheochromocytoma
  • Infiltration of fingers or toes in patients with peripheral arterial disease
  • Periorbital infiltration in patients with narrow angle glaucoma




Materials Needed

  • Povidone-iodine or Chlorhexidine
    • Normal Saline is okay for most laceration repairs
  • Sterile gauze
  • Sterile gloves
  • 27 gauge long (1.5 inch) hypodermic needle for injection
  • 18 gauge larger needle for drawing up anesthetic from vial
  • Syringe (3-10 mL)
  • Local anesthetic agent

"Video: How to draw up LA” - Coming Soon



Techniques


There are various ways that local anesthetic can be administered: 

  • Local Infiltration
    • Direct injection in the area you want to anesthetize. 
  • Nerve Block
    • LA is injected in close proximity to the main nerve trunk supplying the area you want to anesthetize. 
  • Field Block
    • injection into an area bordering the field to be anesthetized. 
  • Topical
    • application of local anesthetic spray, solution, or cream to the skin or mucous membrane. 


Local anesthetics are also used in epidural and spinal anesthesia. 


Local Infiltration: 


  • Ideal for clean lacerations and intact, uninfected skin. 
  • Clean the site you will be injecting with povidone-iodine or chlorhexidine solution. If you are doing a laceration repair, rinsing the wound with normal saline is appropriate. Also make sure the areas distal to the wound show no neurovascular compromise
    • Injecting Anesthetic
      • For open wounds
        • put a few drops of anesthetic into the wound and then rapidly place the needle into the subcutaneous layer by inserting it through the wound margin rather than intact skin.
      • For intact skin: 
        • Place the needle through the skin at a 90º angle into the subcutaneous layer
        • Slowly inject small volumes of anesthetic as the needle is withdrawn. Aspiration is not necessary prior to each infiltration unless the area undergoing local anesthesia is close to major blood vessels.
  • Anesthetize adjacent areas by inserting the needle through the previously injected skin or wound until the entire region requiring anesthesia is infiltrated.
  • After a 2-5 mins, lightly test the skin or wound margins for adequate anesthesia using the injection needle or other sharp object (suture needle, forceps).




Nerve Block


Situations where a regional nerve block may be ideal: 

  • repair of wounds that require precise anatomic alignment, because local infiltration might distort important skin landmarks. 
  • management of large or multiple lacerations where the total dose needed for adequate effect is expected to exceed the maximum dose. 


You must know your anatomy for this technique to be effective! 


Field Block


Topical Anesthetics


Topical anesthetics commonly used in the pediatric population.  They can be applied painlessly without needles and avoid the tissue distortion that can occur with infiltrative anesthetics.  Time to analgesia is the main drawback for most topical anesthetics and often limits their use to non-emergent procedures. 


LET is the most common topical anesthetic used for laceration repair. 

  • It is a combination of:
    • Lidocaine (4%)
    • Epinephrine (0.1%)
    • Tetracaine (0.5%)
  • Available as a solution or gel.
  • Up to 3 mL may be applied to simple lacerations <5 cm, using a cotton-tipped applicator. 
  • Must wait at least 20 minutes before suturing. 
  • Contraindications: 
    • Infants < 1 month old. 
    • Lacerations of mucous membranes.
    • Grossly contaminated wounds. 
    • Patients with allergy to amide or ester anesthetics. 
    • Because it contains epinephrine, should be cautioned for use in digits, penis, nose, ear. 


Other topical anesthetics used for various indications: 

  • lidocaine/prilocaine cream (EMLA)
  • Liposomal lidocaine (LMX)
  • Tetracaine gel (Ametop)
  • Vapocoolant spray




© McGeorge-Sturgill 2016