The goal of this clinical trial is to determine the ED50 and ED95 of prophylactic norepinephrine infusion for preventing post-induction hypotension in elderly patients undergoing major abdominal surgery using up-and-down sequential method . The main question it aims to answer is: What is the effective concentration of prophylactic norepinephrine infusion for preventing post-induction hypotension in elderly patients. Participants will receive different concentrations of norepinephrine infusion at the beginning of anesthesia induction until 15 minutes after intubation.
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Incidence of hypotension
Timeframe: 15 minutes after intubation