Episode 222: Local Anesthetic Systemic Toxicity (LAST)

We discuss this ominous complication of providing local anesthesia. Hosts: Elaine Jonas, MD Brian Gilberti, MD Download Leave a Comment Tags: Critical Care , Toxicology Show Notes I. Pathophysiology & Mechanisms Definition: Systemic toxicity secondary to local anesthetic (LA) via accidental intravascular injection or excessive systemic absorption. Threshold: Occurs when plasma concentration exceeds the safety threshold for cardiac and neural tissue. Agent Profile: Bupivacaine (High Risk) Highly lipophilic with high protein binding. “Fast-on, Slow-off” Kinetics: Strong N a + channel binding with extremely slow dissociation during diastole. Myocardial Depression: Direct inhibition of C a 2+ release from the sarcoplasmic reticulum, impairing contractility. Low CC:CNS Ratio: The dose required for cardiac collapse is very close to the dose that triggers seizures (narrow safety margin). Contributing Factors: Acidosis/Hypercapnia: Increases the fraction of free drug and promotes ion trapping in the brain/heart; shifts the LA-binding curve toward higher toxicity. Hypoxemia: Exacerbates myocardial depression and lowers seizure threshold. II. Risk Assessment & Prevention Patient-Specific Risk Factors Extremes of Age: Neonates (low α -1-acid glycoprotein) and elderly (reduced clearance). Body Composition: Low muscle mass/frailty (decreased volume of distribution). Organ Dysfunction: Hepatic: Reduced metabolism of amide LAs. Renal: Accumulation of metabolites; risk of metabolic acidosis lowering seizure threshold. Cardiac: Reduced cardiac output slows hepatic delivery/clearance; heart failure patients are more sensitive to N a + channel blockade. Pregnancy: Increased sensitivity to cardiotoxicity. Procedural Risk Factors Vascularity of Site (Highest to Lowest Risk): Intercostal blocks (highest absorption rate). Caudal/Epidural. Interfascial plane blocks (e.g., TAP block). Psoas compartment/Sciatic. Brachial plexus. Technique: Large volume infiltration, lack of ultrasound, lack of incremental injection. Prevention Mandates Weight-Based Dosing: Lidocaine (Plain): Max 4.5 mg/kg . Lidocaine (with Epi): Max 7 mg/kg . Bupivacaine: Max 2.5 – 3 mg/kg . Incremental Injection: 3 – 5 mL aliquots with frequent aspiration. Intravascular Marker: Use Epinephrine ( 1 : 200 , 000 ) to detect accidental IV placement (HR increase > 10 bpm or SBP increase > 15 mmHg ). III. Clinical Presentation Neurologic Phase (Early to Late) Subjective: Metallic taste, tinnitus, circumoral numbness/tingling. Objective: Visual disturbances, agitation, confusion, tremors. Critical: Generalized tonic-clonic seizures, rapid progression to CNS depression, coma, and apnea. Note: Early phases are often masked in patients receiving midazolam or propofol. Cardiovascular Phase Initial: Hypertension and tachycardia (if epi used) or transient stimulatory phase. Conduction Defects: PR prolongation, QRS widening (classic sign), bundle branch blocks. Dysrhythmias: Bradycardia (most common), VT/VF, PEA, asystole. Contractility: Profound, refractory hypotension and cardiogenic shock. IV. Immediate Management Algorithm Goal: Prevent hypoxia/acidosis and sequester the toxin. 1. Initial Actions Stop Injection: Immediately halt all LA administration. Call for Help: Specify “LAST Protocol” and “Intralipid Kit.” Airway Management: 100% O 2 ​ . Hyperventilate slightly if needed to counter respiratory acidosis. Low threshold for intubation (hypoxia/acidosis rapidly worsen LAST). 2. Seizure Control First-line: Benzodiazepines (e.g., Midazolam). Avoid: Propofol if hemodynamically unstable (exacerbates cardiac depression). Neuromuscular Blockers: May be needed for ventilation, but remember they do not stop CNS seizure activity. 3. Lipid Emulsion Therapy 20% Indications: Start at first sign of serious toxicity (airway compromise, seizures, or CV instability). Bolus: 1.5 mL/kg IV over 1 minute . Infusion: 0.25 mL/kg/min immediately following bolus. If Instability Persists: Repeat bolus (up to 2 times). Increase infusion to 0.5 mL/kg/min . Upper Limit: ≈ 12 mL/kg total dose. 4. Modified ACLS Epinephrine: Use low doses ( < 1 mcg/kg ) to avoid worsening arrhythmias and interfering with lipid rescue. Antiarrhythmics: Amiodarone is preferred. CONTRAINDICATED: Lidocaine: (Class Ib antiarrhythmic—will worsen toxicity). Vasopressin: Associated with poor outcomes in animal LAST models. Calcium Channel Blockers / Beta Blockers: Exacerbate myocardial depression. Refractory Arrest: Early consultation for ECMO or Cardiopulmonary Bypass (CPB). V. Differential Diagnosis for the Peri-Procedural Patient High Spinal: Ascending sensory/motor block, profound sympathectomy (hypotension/bradycardia). Anaphylaxis: Urticaria, wheezing (rare with amides, more common with esters). Air/Gas Embolism: Sudden dyspnea, “mill-wheel” murmur, acute right heart strain. Vasovagal Syncope: Bradycardia/hypotension, usually lacks the QRS widening or seizure activity. VI. Post-Resuscitation & Complications Observation: At least 2 hours after a CNS-only event. At least 4–6 hours after a CV event. Lipid Complications: Lab Interference: Lipemia interferes with hemoglobin, creatinine, and electrolyte measurements (draw labs before ILE if possible). Pancreatitis: Rare, delayed complication of high-dose ILE. Fat Embolism/Overload: Rare pulmonary complications. VII. Clinical “Red Flags” for Toxicity Unexpected Agitation: In a patient who just received a block, don’t assume “anxiety.” Wide QRS: Any widening of the QRS complex post-injection is LAST until proven otherwise. Refractory Arrest: Standard ACLS failing in a patient who received LA. Lipid must be given. Critical Note: LAST is a clinical diagnosis. Do not wait for serum lidocaine levels or laboratory confirmation to initiate Lipid Emulsion Therapy. Immediate correction of pH and P a CO 2 ​ is as vital as the lipid itself. Read More

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Episode 222: Local Anesthetic Systemic Toxicity (LAST) is an episode from Core EM Podcast by Core EM.

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This episode was published on Apr 7, 2026.

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