Local Anesthetic Dosing Calculator

Patient Weight

Local Anesthetic Agent

Doses local anesthetics to help avoid toxic doses.

Why Use

LAST can occur with any local anesthetic through any route of administration. Calculating the maximum dose in advance may help reduce risk.

When to Use

Patients undergoing nerve blocks or regional anesthesia.

Formula

Max allowable subQ dose based on weight, mg = max allowable subQ dose for a drug, mg/kg × weight, kg Max allowable subQ dose based on weight, mL = max allowable subQ dose for a drug, mg/kg × (weight, kg / 10) × (1 / concentration, %) Drug Relative potency Duration of action Max allowable subQ dose, mg/kg Bupivacaine High Long 2 Lidocaine Low Medium 4.5 Mepivacaine Intermediate Medium 4.4 Ropivacaine Intermediate Long 3 From Sztajnkrycer 2019 .

Pearls / Pitfalls

Toxicity is based on ideal body weight . Local anesthetic systemic toxicity (LAST) is rare but potentially life-threatening. Symptoms can be divided into two main categories: Neurotoxicity : perioral numbness, metallic taste, mental status changes or anxiety, visual changes, muscle twitching, and ultimately, seizures, coma, and respiratory depression. Cardiovascular toxicity : tachycardia, hypertension, ventricular arrhythmias, and/or asystole. Patient risk factors for LAST include: Extremes of age. Decreased muscle mass. Renal, hepatic, or cardiac disease. Metabolic disturbances (acidosis, hypoxia, hypercarbia). 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). Multiple injections or infusion. Ways to reduce risk of LAST: Calculate patient’s maximum dose. Dilution of anesthetic with normal saline allows for large volume blocks to be performed without risking toxic doses. Vasoconstrictors such as epinephrine can be added to decrease rate of absorption and therefore decrease toxic dose. Use ultrasound to confirm needle placement. Aspirate prior to injecting. Monitor for development of symptoms.

Management

Diagnosing LAST: Local anesthetic toxicity results from sodium channel blockade, which affects the central nervous system and cardiac system. The CNS is more sensitive to the effects of local anesthetics than the cardiac system and will generally manifest signs/symptoms of toxicity first. The presenting symptoms of LAST include perioral numbness, metallic taste, mental status changes or anxiety, visual changes, muscle twitching, and ultimately, seizures, coma, and respiratory depression. Cardiovascular effects include tachycardia, hypertension, ventricular arrhythmias and/or asystole. Management of LAST: Stop the infusion. Provide supportive care (advanced cardiovascular life support, benzodiazepines for seizures, airway management). Consider giving intralipid (see guidelines: American Society of Regional Anesthesia and Pain Medicine , Association of Anaesthetists of Great Britain and Ireland ).

Critical Actions

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.

Advice

If 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 (does not require refrigeration) for easy access if any complications should occur.