A 52-year-old man undergoing lower limb orthopedic surgery receives epidural anesthesia at the L3–L4 level. Ten minutes after injection of 15 mL of 0.5% bupivacaine, he develops sudden onset of severe hypotension (BP 80/50 mmHg), bradycardia (HR 42/min), and loss of consciousness. He is breathing spontaneously but weakly. What is the most appropriate immediate next step?