A 52-year-old man with hypertension (on amlodipine) undergoes elective total knee replacement under spinal anesthesia. The procedure lasts 90 minutes. Intraoperatively, his blood pressure drops from 140/90 mmHg to 85/50 mmHg at 45 minutes, and he develops bradycardia (HR 42/min). He is conscious and alert. The anesthesiologist administers oxygen, increases IV fluids, and gives 6 mg ephedrine IV. Despite these measures, his blood pressure remains 90/55 mmHg and heart rate is 48/min. What is the most likely diagnosis?