What is the primary risk factor for Local Anesthetic Systemic Toxicity (LAST)?

Risk Factors for Local Anesthetic Systemic Toxicity (LAST)

The primary risk factor for LAST is excessive dosing of local anesthetic, particularly exceeding maximum recommended doses (e.g., >9 mg/kg lean body weight for lidocaine), with inadvertent intravascular injection being the most dangerous mechanism of toxicity. 1, 2

Patient-Specific Risk Factors

High-Risk Populations

• Extremes of age: Infants and elderly patients are at significantly increased risk due to altered pharmacokinetics and reduced drug clearance 3, 4
• Hepatic dysfunction: Patients with severe liver disease cannot metabolize amide-type local anesthetics normally, leading to toxic plasma concentrations 2
• Cardiovascular disease: Patients with impaired cardiac function are less able to compensate for the cardiovascular effects of local anesthetics 2
• Low cardiac output states: Reduced perfusion decreases drug distribution and increases systemic concentration 3

Vulnerable Patient Characteristics

• Debilitated or acutely ill patients require reduced doses commensurate with their physical status 2
• Patients with severe shock or heart block are at higher risk for cardiovascular collapse 2

Drug and Dosing Risk Factors

Excessive Dosing

• Exceeding maximum recommended doses is a critical risk factor: lidocaine >9 mg/kg lean body weight with epinephrine or >4.5 mg/kg without epinephrine 1, 5
• Repeated doses cause significant increases in blood levels with each administration due to slow drug accumulation 2
• Rapid injection increases peak plasma concentrations and toxicity risk 1, 3

High-Potency Agents

• Bupivacaine is the most frequently implicated agent in severe LAST with cardiovascular collapse due to profound sodium channel blockade 6
• Higher concentrations of local anesthetics may provide faster onset but carry greater toxicity risk 1

Technical and Procedural Risk Factors

Injection Technique

• Inadvertent intravascular injection (intra-arterial or intravenous) is the most dangerous mechanism, delivering high concentrations directly into circulation 2, 4
• Head and neck blocks (retrobulbar, dental, stellate ganglion) carry particular risk due to potential retrograde cerebral circulation 2
• Invasive nerve blocks (glossopharyngeal, superior laryngeal) are associated with higher plasma concentrations and increased LAST risk 1

Anatomic Considerations

• Highly vascular injection sites increase systemic absorption rates 3, 4
• Areas with compromised blood supply may lead to delayed absorption followed by sudden release 2

Monitoring and Preparation Deficiencies

Lack of Preparedness

• Absence of lipid emulsion immediately available at the bedside delays life-saving treatment 7
• Inadequate monitoring of cardiovascular and respiratory status during and after injection 2
• Failure to fractionate doses or perform aspiration testing before injection 3

Critical Prevention Strategies

• Calculate maximum safe dose before injection based on lean body weight, not total body weight 1, 7
• Use ultrasound guidance for regional blocks to avoid intravascular injection 1, 4, 8
• Inject slowly with frequent aspiration to detect intravascular placement 1, 3
• Add epinephrine 10-15 μg as an intravascular marker to detect systemic absorption 1
• Consider using lower-toxicity agents such as ropivacaine or levobupivacaine instead of bupivacaine 3
• Have 20% lipid emulsion, resuscitation equipment, and benzodiazepines immediately available before starting any regional anesthetic procedure 7, 3