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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2 Responses

  1. Dr. Fox, I enjoyed stumbling onto your article. I am a dentist, but I share your passion for great care. I was motivated by reports of negative outcomes of patients, especially pediatric patients, secondary to over dosing of local anesthetics. Upon investigating further, I have found that nearly 90% of dentists report that they routinely utilize a cocktail of two and at times three different local anesthetics for each case. This only further complicates the calculations and monitoring of total mg of LA as well as epi administered which in nearly every office we interviewed those calculations were left to mental math of the provider and they admitted that they most usually administered local anesthetic by a “feeling” that they were within the parameters of safety. I wanted to provide a solution for all of us so, I created a web app that can be used on any device or computer or phone or operating system. The patients age, weight and height is input (to provide BMI and automatically convert to pediatric guidelines based on age), local anesthetics are selected and doses are entered. They can be entered as cartridge value (for dental providers) or mg or mL. The software does all the normalization calculations and converts all entries to mg. Each dose is time stamped and shown in linear graph form relative to time according to each LA type. When multiple LAs are utilized, each LA has a separate line graph, a running total for each LA is maintained at the far right, but more importantly, the total mg of all LAs is tallied and displayed below all the individual totals. Per pediatric and/or manufacturer’s guidelines coupled with the patient’s reported weight, the software calculates the total percent systemic toxicity as well as states maximum “allowable” amount to be administered for each LA, but upon each dose administered of each LA, the software recalculates every selected LA and modifies the suggested maximum amount to account for that dose having been administered and the total LA to be administered is now less. To really nerd out, we added calculations (actually turned out to be quite complex calculus) to determine the estimated remaining local anesthetic based on half life but reported/calculated every second. This really is only a feature for cases where providers are doing extensive work that requires cases extending beyond three hours. Anyhow, I love your passion and I wanted to share with you. We offer the calculation web app to anyone and everyone for 100% free. What we do at my office is we do all LA entries in my web app on a PC that has my EMR. upon completion of a case, my assistants click the “copy to clipboard” button and then right click on their progress not section and select paste. This inputs all the time stamp of each dose with the mg as well. I’d be honored if you tried it out. You can access it by going to http://www.AnesthesiaCharting.com

    Feel free to reach out to me if you have any questions. Take care!

    Travis Coulter
    AnesthesiaCharting.com
    Travis@AnesthesiaCharting.com

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