What is the likelihood of Local Anesthetic Systemic Toxicity (LAST) in a 77.2kg patient administered 36cc's of 2% lidocaine (local anesthetic) without epinephrine (adrenergic receptor agonist) over 3 hours?

From the Guidelines

The likelihood of Local Anesthetic Systemic Toxicity (LAST) in this 77.2kg patient receiving 36cc of 2% lidocaine without epinephrine over 3 hours is low, but careful monitoring is still necessary due to the dose exceeding the recommended maximum. The maximum safe dose of plain lidocaine is 4.5mg/kg, which for this patient would be approximately 347mg, as indicated in the guidelines for local anesthetic agents 1. Since 2% lidocaine contains 20mg/ml, the 36cc (36ml) administered contains 720mg of lidocaine, which exceeds the recommended maximum single dose. However, the extended administration over 3 hours significantly reduces the risk of LAST because the drug is being metabolized during this time rather than reaching peak plasma levels all at once. Some key points to consider:

• Lidocaine has a half-life of approximately 90-120 minutes in most adults, so the extended administration allows for ongoing clearance 1.
• To further minimize risk, it would be prudent to monitor the patient for early signs of LAST including perioral numbness, metallic taste, lightheadedness, tinnitus, and visual disturbances.
• More serious signs include confusion, seizures, and cardiovascular depression.
• If administering this dose is clinically necessary, consider dividing the total dose into smaller increments with aspiration before each injection to avoid intravascular administration, which would dramatically increase LAST risk, as local anesthetic agents are cardiac depressants and may cause central nervous system excitation or depression 1. Key considerations for administration:
• Particular weight-based attention should be paid to cumulative dosage in all patients.
• The maximum allowable safe dosage should be calculated before administration.
• Lower doses should be used when injecting into vascular tissues.
• The initial recording of vital signs should be documented at least every 5 minutes when high doses are used.

From the FDA Drug Label

The maximum recommended dose of 4% Lidocaine Hydrochloride Injection, USP should be such that the dose of lidocaine HCl is kept below 300 mg and in any case should not exceed 4.5 mg/kg (2 mg/lb) body weight. When used without epinephrine, the amount of lidocaine administered should be such that the dose is kept below 300 mg and in any case should not exceed 4.5 mg/kg (2 mg/lb) of body weight.

The patient's weight is 77.2kg. To calculate the maximum dose: 4.5 mg/kg * 77.2 kg = 347.4 mg, which exceeds the recommended maximum of 300 mg. However, for this specific case: 36cc's of 2% lidocaine = 36 cc * 20 mg/cc = 720 mg of lidocaine. Since 2% lidocaine is being used, the actual dose administered is 720 mg, which is more than the maximum recommended dose of 300 mg. Given that the dose administered exceeds the maximum recommended dose, the likelihood of Local Anesthetic Systemic Toxicity (LAST) is increased. Key factors that contribute to this increased risk include:

• Dose: The amount of lidocaine administered (720 mg) exceeds the recommended maximum (300 mg).
• Concentration: The use of 2% lidocaine without epinephrine.
• Duration: The administration over 3 hours. It is essential to monitor the patient closely for signs of LAST, such as central nervous system toxicity and cardiovascular toxicity 2.

From the Research

Local Anesthetic Systemic Toxicity (LAST) Risk Assessment

The administration of 36cc's of 2% lidocaine without epinephrine over 3 hours to a 77.2kg patient poses a risk of Local Anesthetic Systemic Toxicity (LAST). The key factors to consider in assessing this risk include:

• The dose and concentration of the local anesthetic used: 36cc's of 2% lidocaine is a significant dose, and the lack of epinephrine may increase the risk of systemic absorption 3, 4.
• The patient's weight and body composition: A weight of 77.2kg is within the normal range for an adult, but individual factors such as muscle mass and organ function may influence the risk of LAST 4, 5.
• The duration of administration: The administration of lidocaine over 3 hours may increase the risk of systemic accumulation and toxicity 3, 6.

Risk Factors for LAST

Several risk factors for LAST have been identified, including:

• High doses or concentrations of local anesthetics 3, 4.
• Inadvertent intra-arterial or intravenous injection 5, 6.
• Repeated doses or prolonged administration 3, 5.
• Patient factors such as extremes of age, organ dysfunction, or impaired metabolism 4, 5.

Prevention and Management of LAST

To minimize the risk of LAST, it is essential to:

• Use the minimum effective dose of local anesthetic 3, 5.
• Employ techniques such as ultrasound guidance and aspiration to reduce the risk of intra-arterial or intravenous injection 5, 6.
• Monitor patients closely for signs of systemic toxicity, including neurological and cardiovascular manifestations 3, 5.
• Have a plan in place for the management of LAST, including the use of intravenous lipid emulsion and supportive care 4, 6.