What is the likelihood of experiencing Local Anesthetic Systemic Toxicity (LAST) lidocaine toxicity 24 hours after the last injection for local anesthesia?

From the Guidelines

The likelihood of experiencing Local Anesthetic Systemic Toxicity (LAST) or lidocaine toxicity 24 hours after the last injection for local anesthesia is extremely low to virtually nonexistent. Lidocaine has a relatively short half-life of approximately 1.5-2 hours in most adults with normal hepatic function, as discussed in various studies 1. After 24 hours, the drug would have undergone approximately 12 half-lives, meaning that less than 0.01% of the original dose would remain in the bloodstream. This makes systemic toxicity at this point highly improbable. The risk of LAST is highest during administration and within the first few hours afterward when blood levels peak, as noted in a study on the use of intravenous lidocaine for postoperative pain and recovery 1. Symptoms of LAST typically manifest within minutes to hours after administration, not days later. If a patient is experiencing concerning symptoms 24 hours after lidocaine administration, other causes should be investigated rather than attributing them to LAST. The metabolism of lidocaine occurs primarily in the liver through the cytochrome P450 system, and its metabolites are excreted through the kidneys, which efficiently clear the drug from the system well before the 24-hour mark in patients with normal organ function.

Some key factors that may influence lidocaine toxicity include:

• Acid-base status, with acidaemia increasing the dissociation of lidocaine from plasma proteins 1
• Hypoxaemia, which can increase the risk of toxicity
• Hypoalbuminaemia and other conditions where plasma proteins are depleted, increasing the amount of free drug in the plasma and making toxicity more likely
• Drugs that reduce lidocaine metabolism, such as beta-blockers, and clearance, such as amiodarone, which can enhance lidocaine toxicity, especially with prolonged infusions 1
• Inducers and inhibitors of the hepatic enzyme cytochrome P450, which can also have an effect on lidocaine toxicity

It's also important to consider patient factors that may predispose to toxicity, such as low body weight, high body mass index (BMI), and pre-existing systemic factors, as discussed in a study on the use of intravenous lidocaine for postoperative pain and recovery 1. However, the risk of LAST 24 hours after lidocaine administration remains extremely low, and other causes of concerning symptoms should be investigated.

From the Research

Local Anesthetic Systemic Toxicity (LAST) Lidocaine Toxicity

The likelihood of experiencing Local Anesthetic Systemic Toxicity (LAST) lidocaine toxicity 24 hours after the last injection for local anesthesia is low.

• The symptoms of LAST include numbness of the tongue, dizziness, tinnitus, visual disturbances, muscle spasms, convulsions, coma, and respiratory and cardiac arrest 2.
• The exact cause and best treatment of LAST remain unclear, but lipid emulsion is a reasonably well-tolerated and effective treatment 3.
• Lipid emulsion therapy should be a primary element in the treatment of cardiovascular LAST, in addition to ventilation, oxygenation, and chest compressions 3.
• The use of epinephrine and vasopressin should be tailored to specifics of an episode of LAST, and doses should be kept as low as possible while still achieving the desired effects 3.
• The mechanism of action for intravenous lipid emulsion (ILE) in LAST seemingly involves beneficial effects on initial drug distribution and positive cardiotonic and vasoactive effects 4.
• Recent systematic review by collaborating international toxicologic societies have provided reserved endorsement for ILE in bupivacaine-induced toxicity, weak support for ILE use in toxicity from other local anesthetics, and largely neutral recommendation for all other drug poisonings 4.

Risk Factors and Preventive Measures

• Multiple susceptible factors leading to lidocaine toxicity, such as advanced heart failure, can increase the risk of LAST 5.
• The risk factors, preventive measures, and therapeutic approaches to manage this type of complication are discussed in detail 5.
• Dose adjustment of local anesthetic agents in individual patients, especially those with advanced heart failure who undergo cardiovascular procedures, is crucial to prevent LAST 5.
• The importance of standard resuscitation protocol, including ventilation, oxygenation, and chest compressions, in conjunction with lipid emulsion therapy, cannot be overstated 6, 3.

Treatment and Management

• Lipid emulsion remains first-line therapy in LAST, in conjunction with standard resuscitative measures 4.
• Experimental data continue to add to better understanding of LAST and its treatment, including the use of lipid emulsion 6, 3, 4.
• High-quality human data are needed to refine clinical recommendations for the use of lipid emulsion in LAST 4.