A 42-year-old woman undergoes femoral nerve block for knee arthroscopy under regional anesthesia. Within 5 minutes of local anesthetic injection, she develops circumoral numbness, tinnitus, and tremors. Which is the most common cause of local anesthetic toxicity in this clinical scenario?

Explanation

## Mechanism of Local Anesthetic Toxicity **Key Point:** Intravascular injection is the most common cause of acute systemic local anesthetic toxicity (SLAT), particularly when rapid onset of CNS and cardiac manifestations occurs within minutes of injection. ### Why Intravascular Injection Is Most Common Intravascular injection delivers high plasma concentrations of local anesthetic directly into the circulation, bypassing the slow absorption phase. This results in: - Rapid achievement of toxic plasma levels - Acute onset of symptoms (within seconds to minutes) - Predominantly CNS toxicity initially (tremors, circumoral numbness, tinnitus, seizures) - Potential progression to cardiac toxicity (arrhythmias, cardiovascular collapse) ### Clinical Presentation Timeline | Plasma Concentration | CNS Manifestations | Cardiac Manifestations | |---|---|---| | 1–5 µg/mL | Lightheadedness, tinnitus, circumoral numbness | None | | 5–10 µg/mL | Tremors, visual disturbances, muscle twitching | Mild bradycardia | | >10 µg/mL | Seizures, loss of consciousness | Hypotension, arrhythmias, cardiac arrest | **High-Yield:** The rapid onset (within 5 minutes) in this case strongly suggests intravascular injection rather than systemic absorption from tissue, which would cause delayed toxicity over 10–20 minutes. ### Prevention Strategies 1. **Aspiration test** — Always aspirate before and during injection 2. **Incremental dosing** — Inject in small aliquots with frequent aspiration 3. **Use of vasoconstrictors** — Epinephrine (1:200,000) slows absorption and provides a "test dose" (transient tachycardia if intravascular) 4. **Ultrasound guidance** — Visualize needle tip and local anesthetic spread **Clinical Pearl:** A test dose of 3 mL of 1.5% lidocaine with 1:200,000 epinephrine will produce a heart rate increase of ≥10 bpm within 15–45 seconds if injected intravascularly — a useful safety maneuver in awake patients. ### Management of SLAT 1. Stop injection immediately 2. Secure airway and hyperventilate with 100% O₂ 3. Seizure control: benzodiazepines (midazolam 2–4 mg IV) or propofol (1–2 mg/kg IV) 4. Lipid emulsion therapy: 20% lipid emulsion 1.5 mL/kg IV bolus, then infusion at 15 mL/kg/min for cardiac toxicity 5. ACLS protocol for cardiac arrest [cite:Gupta & Pratihar, Textbook of Regional Anesthesia Ch 8]