Management of Local Anesthetic Systemic Toxicity

LAST: Definition & Pathophysiology - Uh Oh, Overdose!

• Definition: LAST: Severe systemic reaction to high plasma LA levels; affects CNS & CVS.
• Primary Cause: Accidental intravascular injection; also excessive dose, rapid absorption, or ↓metabolism.
• Mechanism:
  • Dose-dependent blockade of voltage-gated $Na^+$ channels.
  • CNS: Initial excitation (perioral numbness, tinnitus, seizures) via blockade of inhibitory pathways, then generalized depression (coma, respiratory arrest).
  • CVS: Myocardial depression (↓contractility, bradycardia, hypotension), arrhythmias (VT/VF, asystole).

  ⭐ Highly lipid-soluble LAs (e.g., bupivacaine) have ↑cardiotoxicity due to slower dissociation from cardiac $Na^+$ channels.

LAST: Clinical Features - Systemic Storm Signals

• CNS Manifestations (Often Biphasic):
  • Early Excitation: Metallic taste, perioral numbness, tinnitus, agitation, dizziness, visual disturbances, muscle twitching, tremors.
  • Advanced Excitation: Slurred speech, seizures (tonic-clonic).
  • Depression (Late): Drowsiness, unconsciousness, coma, respiratory depression/apnea.
• Cardiovascular Manifestations (Can precede, follow, or occur with CNS signs):
  • Early (Hyperdynamic): Hypertension, tachycardia, ventricular ectopy.
  • Late (Depressant): Hypotension, bradycardia, ↓contractility, conduction blocks (↑PR, ↑QRS duration), ventricular arrhythmias (VT/VF), asystole.
• 📌 Mnemonic: SAMS (Slurred speech, Altered mental status, Muscle twitching, Seizures) for key CNS signs.

⭐ Bupivacaine is notorious for severe cardiotoxicity that can be refractory to standard resuscitation and may occur before or concurrently with CNS signs, often with rapid progression to cardiovascular collapse.

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LAST: Prevention Strategies - Dodging Danger

• Risk Assessment: Identify high-risk patients (elderly, organ dysfunction, pregnancy).
• Dose Minimization: Use lowest effective LA volume & concentration.
  • Strictly adhere to maximum recommended doses.
• Safe Technique:
  • Frequent aspiration before & during injection.
  • Incremental injections: 3-5 mL aliquots.
  • Ultrasound guidance (USG) strongly advised.
  • Consider epinephrine test dose.
• Preparedness: Ensure immediate availability of lipid emulsion & resuscitation equipment.

⭐ Ultrasound guidance significantly reduces LAST risk by allowing real-time visualization of needle and injectate spread, minimizing intravascular injection or excessive local spread near sensitive structures.

LAST: Management Protocol - Rescue Mission

Key Steps:

• STOP LA INJECTION & Call for Help.
• Airway: 100% O₂. Intubate if needed.
• Seizures: Benzodiazepines (Midazolam 0.05-0.1 mg/kg). Avoid large propofol doses.

Lipid Emulsion (20%):

• Bolus: 1.5 mL/kg IV over 1 min.
• Infusion: 0.25 mL/kg/min.
• Repeat bolus (1-2x) if unstable. Max: 10-12 mL/kg (first 30 min).

Modified ACLS (if arrest):

• Epinephrine: Small doses (≤1 mcg/kg).
• Amiodarone for VT/VF.
• Avoid: Vasopressin, CCBs, β-blockers.
• Prolonged resuscitation.

⭐ Lipid emulsion therapy is the cornerstone of LAST management, acting as a "lipid sink".

High-Yield Points - ⚡ Biggest Takeaways

• Stop LA injection immediately at first signs of toxicity.
• Secure airway, give 100% O2; manage seizures with benzodiazepines.
• Administer 20% lipid emulsion: 1.5 mL/kg IV bolus, then 0.25 mL/kg/min infusion.
• For cardiac arrest, use modified ACLS: small epinephrine doses (<1 mcg/kg), avoid vasopressin.
• CNS symptoms (e.g., tinnitus, metallic taste, seizures) often precede cardiovascular collapse.
• Avoid propofol for seizures if cardiovascular instability is present; lipid emulsion is preferred treatment for LAST-induced seizures or cardiac arrest.