A 42-year-old female with severe GERD is undergoing emergency cesarean section under spinal anesthesia for fetal distress. Five minutes after successful spinal injection (0.5% bupivacaine 10 mg), she suddenly becomes hypoxic (SpO₂ 85% on room air) and hypotensive (BP 78/45 mmHg). Auscultation reveals bilateral air entry but the patient reports dyspnea. Capnography shows normal EtCO₂ (38 mmHg). What is the most appropriate immediate next step?