Understanding Local Anesthetic Systemic Toxicity (LAST)
Local Anesthetic Systemic Toxicity (LAST) is a medical emergency caused by an excessive concentration of a local anesthetic agent in the bloodstream. It can significantly affect the central nervous system (CNS) and cardiovascular system (CVS). While the classic presentation of LAST is progressive, atypical presentations are also common, and symptoms can appear immediately or be delayed.
Symptoms of a Local Anesthetic Overdose: Central Nervous System
CNS effects typically occur first, presenting with both excitatory and later depressive symptoms.
Initial (Excitatory) Phase Early signs can be subjective, including auditory or visual changes, a metallic taste, numbness or tingling around the mouth or tongue, lightheadedness, dizziness, confusion, agitation, anxiety, restlessness, and slurred speech.
Later (Depressive) Phase As toxicity progresses, muscle twitching, tremors, and seizures may occur. Severe CNS depression can lead to drowsiness, unconsciousness, coma, and respiratory arrest.
Symptoms of a Local Anesthetic Overdose: Cardiovascular System
Cardiovascular toxicity usually appears at higher drug concentrations and can be life-threatening. In some cases, cardiovascular symptoms may occur with minimal or no preceding CNS signs.
Initial Phase Initial signs may include increased heart rate (tachycardia) and increased blood pressure (hypertension) as a compensatory response.
Later Phase As toxicity worsens, dangerous symptoms can develop, including decreased heart rate (bradycardia), low blood pressure (hypotension), and various heart rhythm abnormalities (arrhythmias) like a widened QRS complex, ventricular tachycardia (VT), and ventricular fibrillation (VF). The most severe outcome is cardiac arrest.
Risk Factors and Management Considerations
Several factors can increase the risk or alter the presentation of LAST, including extremes of age, pre-existing heart, liver, or kidney disease, pregnancy, metabolic conditions like acidosis and hypoxia, the type of local anesthetic used, and the injection site.
Comparison of Early CNS vs. Cardiovascular Symptoms
The classic understanding of LAST, where CNS signs always precede cardiovascular collapse, is not always the case. The table below highlights potential differences in presentation:
| Symptom Category | Mild/Early CNS Presentation | Severe/Late CNS Presentation | Mild/Early Cardiovascular Presentation | Severe/Late Cardiovascular Presentation |
|---|---|---|---|---|
| Mental Status | Lightheadedness, dizziness | Confusion, seizures, coma | Anxiety, agitation | Unconsciousness, collapse |
| Sensory | Metallic taste, tinnitus, perioral numbness | Visual changes, hallucinations | Palpitations, chest pain | Absent (masked) |
| Motor | Restlessness, agitation, tremors | Muscle twitching, generalized convulsions | Tachycardia, hypertension | Bradycardia, arrhythmias, asystole |
| Respiratory | None or anxiety-induced hyperventilation | Respiratory arrest | Shortness of breath | Respiratory arrest |
Emergency Management of Local Anesthetic Overdose
Immediate action following a specific protocol is essential for suspected local anesthetic overdose.
Immediate Steps:
- Stop Injection: Immediately cease further administration of the anesthetic.
- Call for Help: Alert emergency services and summon experienced staff.
- Manage Airway and Breathing: Ensure a patent airway and provide 100% oxygen. Intubate if needed.
- Control Seizures: Administer benzodiazepines like midazolam or lorazepam. Use other agents like propofol cautiously due to potential cardiovascular effects.
- Administer Lipid Emulsion Therapy: For refractory seizures or any sign of cardiovascular toxicity, administer a 20% lipid emulsion infusion according to guidelines.
- Manage Cardiovascular Collapse: Perform a modified advanced cardiovascular life support (ACLS) protocol for cardiac arrest. Use smaller doses of epinephrine (≤1 mcg/kg), avoid vasopressin, calcium channel blockers, and beta-blockers. Amiodarone is preferred for arrhythmias; avoid lidocaine. Prolonged CPR may be required, and extracorporeal life support (ECMO) should be considered in severe cases.
Conclusion
Understanding the potential manifestations of local anesthetic systemic toxicity is crucial for patient safety. Symptoms range from subtle initial CNS signs to severe cardiovascular collapse. Early recognition and a rapid, coordinated emergency response are key to a positive outcome. Preventative measures such as proper dosing, incremental injection with aspiration, and ultrasound guidance are vital. Healthcare professionals must be trained in recognizing these signs and implementing specific resuscitation protocols. For detailed emergency protocols, refer to the American Society of Regional Anesthesia and Pain Medicine (ASRA) guidelines.
