Transcutaneous Electrical Acupoint Stimulation

From 15 minutes before to 15 minutes after anesthesia, Transcutaneous Electrical Acupoint Stimulation(TEAS) at different frequencies will be applied at Neiguan and Jian Shi. The changes in blood pressure during the anesthesia induction period will be compared among the high-frequency group ,the low-frequency group and the control group to explore the better frequency to reduce the incidence of post-induction hypotension(PIH) in the elderly.