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)?