What is the likelihood of local anesthetic systemic toxicity (LAST) in a 78kg patient given 36ccs of 2% lidocaine (lidocaine) without epinephrine (adrenaline) over 3 hours?

From the Guidelines

The risk of local anesthetic systemic toxicity (LAST) in a 78kg patient receiving 36cc of 2% lidocaine without epinephrine over 3 hours is very low. This dose equals 720mg of lidocaine (36cc × 20mg/ml), which is well below the maximum recommended dose of 4.5mg/kg/hour for continuous infusions without epinephrine, as suggested by 1. For this patient, the maximum hourly dose would be approximately 351mg (4.5mg/kg × 78kg), and since the 720mg is being administered over 3 hours, the hourly rate is only 240mg/hour. This provides a good safety margin below the toxic threshold. Lidocaine toxicity typically occurs at plasma concentrations above 5μg/ml, and symptoms progress from perioral numbness and metallic taste to seizures and cardiovascular collapse at higher levels. The slow administration rate allows for hepatic metabolism to clear the drug, further reducing toxicity risk. However, caution should still be exercised in patients with liver dysfunction, heart failure, or those taking medications that might interact with lidocaine, as these factors could increase the risk of LAST despite the seemingly safe dosage, as noted in 1. Some key factors to consider in assessing the risk of LAST include:

• Patient factors such as low body weight, high body mass index (BMI), and hypoalbuminaemia, which can increase the risk of toxicity
• The use of other medications that may interact with lidocaine, such as beta-blockers and amiodarone, which can enhance lidocaine toxicity
• The importance of monitoring for signs of toxicity, such as perioral numbness, metallic taste, and seizures, especially in patients with risk factors for LAST. It is also important to note that the pharmacokinetics of lidocaine can be affected by the duration of infusion, with non-linear or time-dependent pharmacokinetics observed for infusion durations greater than 12 hours, as discussed in 1. Overall, while the risk of LAST in this patient is low, careful monitoring and caution are still necessary to minimize the risk of toxicity.

From the Research

Local Anesthetic Systemic Toxicity (LAST) Overview

• Local anesthetic systemic toxicity (LAST) is a life-threatening condition that can occur after the administration of local anesthetics, such as lidocaine 2, 3, 4, 5.
• The risk of LAST is increased when local anesthetics are administered without epinephrine, as epinephrine helps to reduce the systemic absorption of the anesthetic 4.

Factors Influencing the Risk of LAST

• The dose and concentration of the local anesthetic, as well as the duration of administration, can influence the risk of LAST 3, 4.
• Patient factors, such as weight and muscle mass, can also affect the risk of LAST, with pediatric patients and those with reduced muscle mass being at increased risk 2.
• In this case, the patient received 36ccs of 2% lidocaine without epinephrine over 3 hours, which is a relatively large dose for a 78kg patient.

Calculation of Lidocaine Dose

• The total dose of lidocaine administered can be calculated by multiplying the volume of lidocaine (36ccs) by the concentration (2% or 20mg/cc), resulting in a total dose of 720mg.
• The maximum recommended dose of lidocaine without epinephrine is 4-4.5mg/kg, which would be approximately 312-351mg for a 78kg patient 4.

Likelihood of LAST

• Based on the calculated dose, the patient received approximately 2-2.3 times the maximum recommended dose of lidocaine without epinephrine.
• Given the large dose and the lack of epinephrine, the likelihood of LAST in this patient is increased, and close monitoring for signs of toxicity, such as seizures, cardiac dysrhythmias, and cardiovascular collapse, is necessary 2, 3, 4, 5.