## Distinguishing CNS vs Cardiovascular Toxicity ### Timeline of Local Anesthetic Systemic Toxicity (LAST) **Key Point:** CNS toxicity manifests at LOWER plasma concentrations than cardiovascular toxicity, making seizures the earlier sign. ### Concentration-Dependent Progression | Plasma Concentration | CNS Manifestation | CV Manifestation | |---|---|---| | 5–10 μg/mL | Circumoral numbness, tinnitus | None | | 10–15 μg/mL | Restlessness, tremor | None | | 15–20 μg/mL | **Seizures** | None | | 20–25 μg/mL | Seizures → unconsciousness | Bradycardia, hypotension begin | | >25 μg/mL | Coma, apnea | **Severe arrhythmias, cardiac arrest** | **High-Yield:** The sequence is **CNS first, then CV**. Seizures are the sentinel sign of LAST and occur at lower concentrations than life-threatening arrhythmias. ### Clinical Pearl In a patient receiving local anesthetic who develops seizures, the toxicity has NOT yet reached the cardiovascular system—this is the critical window for immediate intervention (stop injection, secure airway, give lipid emulsion). ### Why This Matters for Exam A common trap is to assume CV collapse is the first sign. In reality, **early recognition of seizures (or prodromal signs like circumoral numbness and tinnitus) allows intervention BEFORE cardiac toxicity develops**. **Mnemonic: LAST Sequence** — **L**ipophilic drugs accumulate in CNS first → **A**ltered consciousness / seizures → **S**evere → **T**oxicity to heart (late).
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