From the Guidelines
The risk of local anesthetic systemic toxicity (LAST) in this scenario is low and within safe limits. The total lidocaine administered is 30 mg from the 1% lidocaine with epinephrine (3cc × 10 mg/cc) plus 600 mg from the 2% lidocaine without epinephrine (30cc × 20 mg/cc), totaling 630 mg over 3 hours. For a 72.72 kg patient, this equals approximately 8.7 mg/kg, which is below the maximum recommended dose of 7 mg/kg for lidocaine without epinephrine and 9 mg/kg for lidocaine with epinephrine when administered over several hours, as suggested by guidelines for the use of local anesthesia in office-based dermatologic surgery 1. The presence of epinephrine in the first dose helps reduce systemic absorption, and the extended administration time over 3 hours allows for metabolism of the drug, reducing peak plasma concentrations. Lidocaine has a half-life of approximately 90-120 minutes, so the extended dosing schedule provides time for elimination. However, it's essential to monitor the patient for early signs of LAST, including perioral numbness, metallic taste, lightheadedness, tinnitus, and visual disturbances, which can progress to seizures and cardiovascular collapse if toxicity develops, and be prepared to treat LAST according to guidelines, such as those provided by the American Heart Association 1. Key steps to minimize the risk of LAST include using the lowest effective dose of local anesthetic, aspirating the needle before injection, using incremental injections, and continually assessing the patient for signs of toxicity, as outlined in guidelines for local anesthesia use 1. In case of LAST, treatment may involve the use of lipid emulsion, as suggested by recent guidelines and consensus statements 1. It is also crucial to be aware of the maximum recommended doses for lidocaine and to avoid exceeding these doses, especially in patients with certain comorbidities or when using lidocaine in combination with other local anesthetics or sedatives, as discussed in guidelines for awake tracheal intubation and the use of intravenous lidocaine for postoperative pain and recovery 1. Given the information provided and the guidelines available, the administration of lidocaine in this scenario appears to be safe, but careful monitoring and preparedness for potential toxicity are essential.
From the Research
Local Anesthetic Systemic Toxicity (LAST) Risk Assessment
The patient in question received a total of 3cc's of 1% lidocaine with epinephrine and 6cc's of 2% lidocaine without epinephrine in the first hour, followed by 24cc's of 2% lidocaine without epinephrine over the next 2 hours. To assess the risk of LAST, we need to consider the total dose of lidocaine administered.
Total Lidocaine Dose Calculation
- 3cc's of 1% lidocaine = 30mg of lidocaine
- 6cc's of 2% lidocaine = 120mg of lidocaine
- 24cc's of 2% lidocaine = 480mg of lidocaine
- Total lidocaine dose = 30mg + 120mg + 480mg = 630mg
Risk Factors for LAST
According to the studies 2, 3, 4, 5, 6, the risk of LAST is influenced by several factors, including:
- Total dose of local anesthetic administered
- Site of injection
- Patient comorbidities
- Rate of absorption from the site of injection
- Biodegradation of local anesthetics
Assessment of LAST Risk
Given the patient's weight (72.72 Kg) and the total dose of lidocaine administered (630mg), the risk of LAST cannot be directly quantified without knowing the maximum recommended dose for this patient. However, it is essential to note that the administration of repeated doses and higher doses of local anesthetics increases the risk of developing LAST 2, 3, 5. The use of ultrasound guidance, aspiration prior to injection, and minimal local anesthetic dose can reduce the risk of LAST 2.
Management and Prevention
In case of suspected LAST, early recognition and intervention, including supportive care and intravenous lipid emulsion 20%, are crucial 2, 3, 5. It is also important to be aware of the potential symptoms of LAST, which can range from mild to severe and include neurologic and cardiovascular manifestations 4, 5, 6.