A 58-year-old man with severe COPD (FEV₁ 28% predicted) and hypertension is scheduled for transurethral resection of prostate (TURP) under spinal anesthesia. During the procedure, the anesthesiologist administers 3 mL of 0.5% bupivacaine at L3–L4. After 15 minutes, the patient develops sudden onset of dyspnea, hypotension (BP 85/50), and bradycardia (HR 42 bpm). Oxygen saturation drops to 88% despite supplemental oxygen. What is the most likely complication, and what is the immediate next step?