Ultrasound-Guided Pediatric Nerve Blocks in the Emergency Department: An Evidence-Based Update (Pain Management CME and Pharmacology CME)

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Local Anesthetic Dosing Calculator

• Introduction
• About the Score
• Evidence Appraisal
• Calculator Review Author
• Instructions
• References

Introduction

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About the Score

Local anesthetic systemic toxicity (LAST), which is rare but potentially life threatening, can occur with any local anesthetic through any route of administration. Toxicity is based on ideal body weight, and symptoms of LAST can be divided into 2 main categories: neurotoxicity and cardiovascular toxicity. The symptoms of neurotoxicity include perioral numbness, metallic taste, mental status changes or anxiety, visual changes, muscle twitching, and ultimately, seizures, coma, and respiratory depression. The symptoms of cardiovascular toxicity include tachycardia, hypertension, ventricular arrhythmias, and/or asystole.

Patient risk factors for LAST include extremes of age, decreased muscle mass, renal/hepatic/cardiac disease, metabolic disturbances (eg, acidosis, hypoxia, hypercarbia), and pregnancy. Procedural risk factors for LAST include injection into highly vascular areas (highest incidence in paravertebral injections, followed by upper and lower extremity peripheral nerve blocks) and multiple injections or infusion.

To reduce the risk of LAST, dilute anesthetics with normal saline, allowing for large volume blocks to be performed without risking toxic doses. Vasoconstrictors such as epinephrine can be added to decrease the rate of absorption and therefore decrease toxic doses. Use an ultrasound to confirm needle placement, and aspirate prior to injecting. Monitor the patient for development of symptoms. Using the Local Anesthetic Dosing Calculator on patients undergoing nerve blocks or regional anesthesia to calculate the maximum dose in advance may help reduce the risk of LAST. However, the calculator should not be used as the primary means of dosing. Always double check and err on the side of caution.

If the dose is above the threshold for toxicity, consider lowering the dose of local anesthetic.

Prepare for adverse events by storing a checklist for management as well as intralipid in a nerve block cart (it does not require refrigeration) for easy access if any complications should occur.

When diagnosing LAST, note that it results from sodium channel blockade, which affects the central nervous system and cardiac system. The central nervous system is more sensitive to the effects of local anesthetics than the cardiac system and will generally manifest signs/symptoms of toxicity first.

To manage LAST, stop the infusion of anesthetics, and provide supportive care (advanced cardiovascular life support, benzodiazepines for seizures, airway management). Consider giving intralipid

(see the guidelines from the American Society of Regional Anesthesia and Pain Medicine and the Association of Anaesthetists of Great Britain and Ireland).

Multiple variables may influence a patient's risk of developing LAST. This weight based calculator should be used to generate a rough estimate for toxic doses.

Evidence Appraisal

There are no randomized clinical trials involving LAST. Much of the existing evidence is based on case reports, expert opinion, and retrospective anesthesia databases.

Calculator Review Authors

Nathan Teismann, MD

Instructions

Previous versions of Goldfrank’s Toxicologic Emergencies reported an increased maximum allowable dosage due to the vasoconstrictive effects of epinephrine when added to local anesthetics. The most recent version no longer includes this adjustment. While there likely is a higher safe dose when epinephrine is added, that estimate is no longer included in this calculator. As always, dosages should be double-checked and reviewed using local pharmacy practice and policies.

References

Original/Primary Reference

• Sztajnkrycer MD. Local Anesthetics. In: Hoffman RS, Howland M, Lewin NA, Nelson LS, et al, eds. Goldfrank’s Toxicologic Emergencies. 11th ed. McGraw-Hill; 2019:994-1003.

Other References

• Neal JM, Barrington MJ, Fettiplace MR, et al. The Third American Society of Regional Anesthesia and Pain Medicine Practice Advisory on local anesthetic systemic toxicity: executive summary 2017. Reg Anesth Pain Med. 2018;43(2):113-123. DOI: 10.1097/AAP.0000000000000720
• The Association of Anaesthetists of Great Britain & Ireland. AAGBI Safety Guideline: Management of Severe Local Anaesthetic Toxicity. Association of Anesthetists. 2010. Accessed May 1, 2022. 
• Tetzlaff JE. The pharmacology of local anesthetics. Anesthesiol Clin North Am. 2000;18(2):217-233. DOI: 10.1016/s0889-8537(05)70161-9
• Achar S, Kundu S. Principles of office anesthesia: part I. Infiltrative anesthesia. Am Fam Physician. 2002;66(1):91-94.