A 52-year-old man undergoing elective total knee replacement receives spinal anesthesia with 3 mL of 0.5% bupivacaine at L3–L4 level. Thirty minutes into the procedure, the anesthesiologist notices the patient is becoming increasingly restless, complaining of difficulty breathing, and blood pressure has risen to 180/95 mmHg from baseline 140/85 mmHg. The sensory level is T2. What is the most appropriate immediate next step?