A 52-year-old man with severe osteoarthritis of the knee is scheduled for total knee replacement under epidural anesthesia. During the procedure, the patient suddenly complains of severe chest pain and becomes hypotensive (BP 80/50). The surgeon notes the patient is pale and diaphoretic. On examination, you find unilateral leg weakness and loss of sensation up to the T4 dermatome bilaterally. What is the most likely complication that has occurred?