A 52-year-old man with a history of chronic back pain and hypertension (on amlodipine 5 mg daily) is scheduled for elective right knee arthroscopy under spinal anesthesia. During the procedure, his blood pressure drops from 140/90 mmHg to 88/52 mmHg within 8 minutes of spinal injection. He develops mild dyspnea and reports feeling lightheaded. Pulse is 48 bpm. The anesthesiologist notes that the sensory level has reached T4 bilaterally. What is the most likely cause of his hemodynamic instability?