A 38-year-old woman undergoes interscalene brachial plexus block for rotator cuff repair. Five minutes after injection of 40 mL of 0.5% bupivacaine, she develops sudden onset seizures, followed by cardiovascular collapse with severe bradycardia and hypotension refractory to initial resuscitation. Which investigation is most appropriate to confirm the diagnosis of local anesthetic systemic toxicity (LAST)?

Explanation

## Diagnosis of Local Anesthetic Systemic Toxicity **Key Point:** Serum bupivacaine concentration is the gold standard confirmatory investigation for LAST, as it directly measures the offending agent and correlates with toxicity severity. ### Clinical Presentation Context The patient presents with the classic biphasic presentation of LAST: 1. **CNS phase** (early): Seizures, perioral numbness, tinnitus, restlessness 2. **Cardiovascular phase** (late): Bradycardia, hypotension, arrhythmias, cardiac arrest The temporal relationship (5 minutes post-injection of high-dose bupivacaine) and the rapid progression from CNS to CV collapse are pathognomonic. ### Why Serum Bupivacaine Concentration is Diagnostic | Investigation | Role in LAST | Sensitivity | Specificity | |---|---|---|---| | **Serum bupivacaine level** | Direct measurement of toxin; confirms diagnosis | High | Very high | | ABG | Supports diagnosis (metabolic acidosis, hypoxia) | Moderate | Low (non-specific) | | ECG | Shows bradycardia, QRS widening, asystole | Moderate | Low (non-specific) | | Plasma cholinesterase | Irrelevant to bupivacaine metabolism | N/A | N/A | **High-Yield:** Bupivacaine is metabolized by hepatic microsomal enzymes (CYP3A4, CYP1A2), NOT by plasma cholinesterase. Serum levels >4 µg/mL are associated with CNS toxicity; >10 µg/mL with cardiovascular toxicity. ### Mechanism of Bupivacaine Toxicity 1. Blocks cardiac sodium channels → bradycardia, conduction delays 2. Inhibits myocardial contractility → hypotension 3. Crosses blood-brain barrier → seizures, loss of consciousness 4. Causes profound cardiovascular depression (more than lidocaine due to high lipophilicity) **Clinical Pearl:** In clinical practice, LAST is diagnosed **clinically** (seizures + CV collapse after local anesthetic injection) and treated immediately with lipid emulsion therapy. Serum levels are obtained **post-hoc** to confirm the diagnosis and guide further management, not to delay treatment. ### Management Timeline - **Immediate:** Stop injection, call for help, begin ACLS + 20% lipid emulsion bolus (1.5 mL/kg IV) - **Concurrent:** Serum bupivacaine level, ABG, ECG, lactate - **Ongoing:** Lipid infusion, prolonged resuscitation (up to 1 hour if needed) **Mnemonic:** **LAST = Local Anesthetic Systemic Toxicity** - **L**ipid emulsion (treatment) - **A**rteries (IV access) - **S**eizures + **S**evere CV collapse (presentation) - **T**oxin level (confirmation)