Calculating Dose for Local Anesthetics

We do complex mathematics every time we evaluate a patient. Risk:Benefit ratios, Odds Ratios and Likelihood Ratios quickly get weighed and measured. Sometimes, however, simple math problems can trip us up (ok… at least trip me up). This is why we know to use tools to help us avoid over or under dosing the important medications. What about, though, when we inject Lidocaine for wound management? Since we manage many types of injuries and wounds (ex, Ear Lacerations, Tongue Laceration, Eyelid Laceration, Abscess, Absorbable Sutures), we appropriately use a lot of Local Anesthetics in the ED. Let us take a brief moment to ensure we know an important aspect of that – Calculating Dose of Local Anesthetics:

Lidocaine Dosage: The Limits

  • Generally speaking, local anesthetics can be used in children as they are in adults.
    • Volume of distribution, initial peak blood concentrations, and clearance of local anesthetic are similar in children and adults. [Gunter, 2002]
  • Some are at greater risk for toxicity, however:
    • Age Matters
      • As always….
      • Infants, unlike older children, do experience different pharmocodyanmics. [Gunter, 2002]
      • Infants have increased volume of distribution for amide anesthetics. [Gunter, 2002]
      • Infants have delayed clearance of amides as well. [Gunter, 2002]
      • Neonates (always at increased risk for misadventures) have decreased plasma concentrations of albumin and other proteins – increases free fraction of local anesthetic.
    • Size Matters
      • Smaller children may make your target for local or regional anesthesia more difficult to distinctly isolate…
      • Increases risk for affecting other structures (like blood vessels).
  • Overall, regional anesthesia is safe when performed correctly!
    • Risk of nerve injury is very low. [Walker, 2018]
    • Risk of severe Local Anesthetic Systemic Toxicity (LAST) is also low – 0.76:10,000 in one study. [Walker, 2018]

Lidocaine Dosage: % to mg/ml

  • Since we want to be safe, before injecting / infiltrating the local anesthetic, it is good practice to calculate the safe “upper limit” dose.
    • Lidocaine w/o Epi – 5 mg/kg of Lidocaine [Berde, 1993]
    • Lidocaine w/ Epi – 7 mg/kg of Lidocaine [Berde, 1993]
    • Bupivacaine – 2 mg/kg [Berde, 1993]
    • Dose range may vary based on reference you use… these are well within the safe range though (I like to avoid risk).
  • Simple enough then… we do weight-based dosing (mg/kg) using mg/ml medication concentrations to determine the actual volume (ml) for pediatrics all of the time…
  • The problem = quickly converting percent (%) of anesthetic to mg/ml.
    • What you have to recall is:
      • % solution is based on (grams of medicine) / 100 ml
      • % solution ~ (1000 mg) / 100 ml
      • % solution ~ 10 mg/ml
    • Examples:
      • Lidocaine 4% = 40 mg/ml of Lidocaine
      • Lidocaine 2% = 20 mg/ml of Lidocaine
      • Lidocaine 1% = 10 mg/ml of Lidocaine
      • Bupivacaine 0.25% = 2.5 mg/ml of Bupivacine

Moral of the Morsel

  • Don’t Inject Willy-Nilly! Generally, local anesthetics are safe to use… but they are safer when you do the math! Especially when dealing with larger wounds!
  • Move the Decimal to the Right! Percentage of Solution represent GRAMS/100ml… not mg/ml or mg/L. 1,000/100 = 10! Multiple the % by 10 to get the mg/ml!

References

Walker BJ1, Long JB, Sathyamoorthy M, Birstler J, Wolf C, Bosenberg AT, Flack SH, Krane EJ, Sethna NF, Suresh S, Taenzer AH, Polaner DM, Martin L, Anderson C, Sunder R, Adams T, Martin L, Pankovich M, Sawardekar A, Birmingham P, Marcelino R, Ramarmurthi RJ, Szmuk P, Ungar GK, Lozano S, Boretsky K, Jain R, Matuszczak M, Petersen TR, Dillow J, Power R, Nguyen K, Lee BH, Chan L, Pineda J, Hutchins J, Mendoza K, Spisak K, Shah A, DelPizzo K, Dong N, Yalamanchili V, Venable C, Williams CA, Chaudahari R, Ohkawa S, Usljebrka H, Bhalla T, Vanzillotta PP, Apiliogullari S, Franklin AD, Ando A, Pestieau SR, Wright C, Rosenbloom J, Anderson T; Pediatric Regional Anesthesia Network Investigators. Complications in Pediatric Regional Anesthesia: An Analysis of More than 100,000 Blocks from the Pediatric Regional Anesthesia Network. Anesthesiology. 2018 Oct;129(4):721-732. PMID: 30074928. [PubMed] [Read by QxMD]
Lönnqvist PA1. Toxicity of local anesthetic drugs: a pediatric perspective. Paediatr Anaesth. 2012 Jan;22(1):39-43. PMID: 21672079. [PubMed] [Read by QxMD]
Gunter JB1. Benefit and risks of local anesthetics in infants and children. Paediatr Drugs. 2002;4(10):649-72. PMID: 12269841. [PubMed] [Read by QxMD]
Berde CB1. Toxicity of local anesthetics in infants and children. J Pediatr. 1993 May;122(5 Pt 2):S14-20. PMID: 8487131. [PubMed] [Read by QxMD]

Sean M. Fox

I enjoy taking care of patients and I finding it endlessly rewarding to help train others to do the same. I trained at the Combined Emergency Medicine and Pediatrics residency program at University of Maryland, where I had the tremendous fortune of learning from world renowned educators and clinicians. Now I have the unbelievable honor of working with an unbelievably gifted group of practitioners at Carolinas Medical Center. I strive every day to inspire my residents as much as they inspire me.

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