## Mechanism of Local Anesthetic-Induced Cardiac Toxicity **Key Point:** Local anesthetics are non-selective sodium channel blockers. At toxic concentrations, they inhibit cardiac sodium channels (Na~v~1.5), disrupting electrical conduction and causing life-threatening arrhythmias. ### Cardiac Sodium Channel Blockade Cascade ```mermaid flowchart TD A[High LA concentration in myocardium]:::outcome --> B[Inhibition of cardiac Na+ channels]:::action B --> C[Slowed conduction velocity]:::outcome C --> D[Prolonged PR, QRS, QT intervals]:::outcome D --> E{Severity?}:::decision E -->|Mild| F[Bradycardia, AV blocks]:::outcome E -->|Severe| G[Re-entrant arrhythmias, VF]:::urgent G --> H[Cardiac arrest]:::urgent B --> I[Decreased cardiac contractility]:::outcome I --> J[Hypotension, cardiogenic shock]:::urgent ``` **High-Yield:** The cardiac toxicity sequence: 1. **Conduction abnormalities** → PR prolongation, QRS widening (hallmark ECG sign) 2. **Re-entrant circuits** → ventricular tachycardia, ventricular fibrillation 3. **Negative inotropism** → reduced contractility, hypotension 4. **Refractory cardiac arrest** → resistant to defibrillation and standard ACLS **Clinical Pearl:** Bupivacaine is particularly cardiotoxic because: - It has high affinity for cardiac Na~v~1.5 channels - It has slow dissociation kinetics (remains bound longer) - It causes profound bradycardia and hypotension before arrhythmias - Standard ACLS drugs (epinephrine, amiodarone) are often ineffective; **lipid emulsion (20% Intralipid®)** is the specific antidote **Mnemonic: LAST (Local Anesthetic Systemic Toxicity)** — CNS then Cardiac - **C**ardiac: Arrhythmias, bradycardia, hypotension, cardiac arrest (Na~+~ channel block) - **N**ervous: Seizures, loss of consciousness (CNS excitation then depression)
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