Podcasts · Core EM - Emergency Medicine Podcast
Episode 222: Local Anesthetic Systemic Toxicity (LAST)
· Episode 222
Episode Episode 222: Local Anesthetic Systemic Toxicity (LAST)
We discuss this ominous complication of providing local anesthesia.
Hosts: Elaine Jonas, MD Brian Gilberti, MD
https://media.blubrry.com/coreem/content.blubrry.com/coreem/LAST.mp3
Download Leave a Comment Tags: Critical Care, Toxicology
Show Notes I. Pathophysiology & Mechanisms
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Definition: Systemic toxicity secondary to local anesthetic (LA) via accidental intravascular injection or excessive systemic absorption.
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Threshold: Occurs when plasma concentration exceeds the safety threshold for cardiac and neural tissue.
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Agent Profile: Bupivacaine (High Risk)
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Highly lipophilic with high protein binding.
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“Fast-on, Slow-off” Kinetics: Strong Na+ channel binding with extremely slow dissociation during diastole.
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Myocardial Depression: Direct inhibition of Ca2+ release from the sarcoplasmic reticulum, impairing contractility.
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Low CC:CNS Ratio: The dose required for cardiac collapse is very close to the dose that triggers seizures (narrow safety margin).
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Contributing Factors:
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Acidosis/Hypercapnia: Increases the fraction of free drug and promotes ion trapping in the brain/heart; shifts the LA-binding curve toward higher toxicity.
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Hypoxemia: Exacerbates myocardial depression and lowers seizure threshold.
II. Risk Assessment & Prevention Patient-Specific Risk Factors
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Extremes of Age: Neonates (low α-1-acid glycoprotein) and elderly (reduced clearance).
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Body Composition: Low muscle mass/frailty (decreased volume of distribution).
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Organ Dysfunction:
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Hepatic: Reduced metabolism of amide LAs.
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Renal: Accumulation of metabolites; risk of metabolic acidosis lowering seizure threshold.
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Cardiac: Reduced cardiac output slows hepatic delivery/clearance; heart failure patients are more sensitive to Na+ channel blockade.
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Pregnancy: Increased sensitivity to cardiotoxicity.
Procedural Risk Factors
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Vascularity of Site (Highest to Lowest Risk):
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Intercostal blocks (highest absorption rate).
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Caudal/Epidural.
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Interfascial plane blocks (e.g., TAP block).
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Psoas compartment/Sciatic.
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Brachial plexus.
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Technique: Large volume infiltration, lack of ultrasound, lack of incremental injection.
Prevention Mandates
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Weight-Based Dosing:
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Lidocaine (Plain): Max 4.5 mg/kg.
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Lidocaine (with Epi): Max 7 mg/kg.
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Bupivacaine: Max 2.5–3 mg/kg.
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Incremental Injection: 3–5 mL aliquots with frequent aspiration.
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Intravascular Marker: Use Epinephrine (1:200,000) to detect accidental IV placement (HR increase >10 bpmor SBP increase >15 mmHg).
III. Clinical Presentation Neurologic Phase (Early to Late)
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Subjective: Metallic taste, tinnitus, circumoral numbness/tingling.
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Objective: Visual disturbances, agitation, confusion, tremors.
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Critical: Generalized tonic-clonic seizures, rapid progression to CNS depression, coma, and apnea.
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Note: Early phases are often masked in patients receiving midazolam or propofol.
Cardiovascular Phase
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Initial: Hypertension and tachycardia (if epi used) or transient stimulatory phase.
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Conduction Defects: PR prolongation, QRS widening (classic sign), bundle branch blocks.
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Dysrhythmias: Bradycardia (most common), VT/VF, PEA, asystole.
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Contractility: Profound, refractory hypotension and cardiogenic shock.
IV. Immediate Management Algorithm
Goal: Prevent hypoxia/acidosis and sequester the toxin.
1. Initial Actions
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Stop Injection: Immediately halt all LA administration.
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Call for Help: Specify “LAST Protocol” and “Intralipid Kit.”
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Airway Management:
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100% O2.
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Hyperventilate slightly if needed to counter respiratory acidosis.
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Low threshold for intubation (hypoxia/acidosis rapidly worsen LAST).
2. Seizure Control
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First-line: Benzodiazepines (e.g., Midazolam).
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Avoid: Propofol if hemodynamically unstable (exacerbates cardiac depression).
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Neuromuscular Blockers: May be needed for ventilation, but remember they do not stop CNS seizure activity.
3. Lipid Emulsion Therapy 20%
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Links in the notes 5
- https://coreem.net/podcast/episode-222-local-anesthetic-systemic-toxicity-last/
- https://media.blubrry.com/coreem/content.blubrry.com/coreem/LAST.mp3
- https://coreem.net/podcast/episode-222-local-anesthetic-systemic-toxicity-last/#comments
- https://coreem.net/tag/critical-care/
- https://coreem.net/tag/toxicology/
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