From the Guidelines
The risk of local anesthetic systemic toxicity (LAST) in this 77.2 kg patient receiving 36 cc of lidocaine without epinephrine over 3 hours is very low. Assuming standard 1% or 2% lidocaine concentrations, this would equate to 360-720 mg total dose. According to the guidelines for monitoring and management of pediatric patients before, during, and after sedation for diagnostic and therapeutic procedures 1, the maximum recommended dose of plain lidocaine is approximately 4.5 mg/kg, giving this patient a maximum safe dose of about 347 mg. However, the extended administration period of 3 hours significantly reduces toxicity risk, as lidocaine has a relatively short half-life (1.5-2 hours) allowing for metabolism and clearance during the procedure.
Key Considerations
- The absence of epinephrine means there's no vasoconstriction to slow absorption, but the gradual administration compensates for this.
- Clinicians should still monitor for early LAST symptoms including perioral numbness, metallic taste, lightheadedness, tinnitus, and visual disturbances, progressing to more serious cardiovascular and central nervous system effects if toxicity develops.
- Factors that could increase risk include liver dysfunction, heart failure, acidosis, or concurrent medications that might compete for metabolism pathways.
Recommendations for Safe Administration
- Use the lowest effective dose of local anesthetic, as recommended by guidelines for the use of local anesthesia in office-based dermatologic surgery 1.
- Aspirate the needle/catheter prior to each injection to avoid introducing the drug directly into a vessel.
- Use incremental injections of anesthetic and continually assess and communicate with the patient to monitor for signs of early toxicity.
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 weighs 77.2 Kg and was administered 36cc's of lidocaine. Assuming a 4% lidocaine solution (40mg/mL), the total dose administered is 36cc * 40mg/mL = 1440mg.
This dose exceeds the maximum recommended dose of 300mg and 4.5mg/kg (4.5mg/kg * 77.2Kg = 347.4mg).
Therefore, the likelihood of local anesthetic systemic toxicity (LAST) is high 2.
From the Research
Local Anesthetic Systemic Toxicity (LAST) Risk Assessment
The administration of 36cc's of lidocaine without epinephrine over 3 hours to a 77.2 Kg patient poses a risk of local anesthetic systemic toxicity (LAST). The likelihood of LAST can be assessed based on several factors, including:
- The dose and volume of lidocaine administered
- The site of injection
- The patient's weight and comorbidities
- The presence of susceptible factors, such as advanced heart failure 3
Factors Influencing LAST Risk
Several studies have identified factors that influence the risk of LAST, including:
- Local anesthetic type, dosage, and volume 4
- Site of injection 4
- Patient comorbidities, such as advanced heart failure 3
- The use of epinephrine, which can help reduce the risk of LAST by slowing the absorption of lidocaine 5
Clinical Presentation of LAST
The clinical presentation of LAST can vary, but common symptoms include:
- Seizures 6, 4
- Cardiac arrhythmias 7
- Refractory cardiac dysfunction 7
- Dysrhythmias 7
- Prearrest situations 7
Prevention and Management of LAST
Prevention and management of LAST are critical to reducing the risk of morbidity and mortality. Strategies for prevention and management include: