Study population
Between November 2021 and July 2022, all consecutive patients scheduled for major abdominal surgery taking 1 h or more under propofol-remifentanil intravenous anesthesia, were screened, and patients who met the inclusion criteria were contracted by primary investigators a day before the surgery to obtain written informed consent.The inclusion criteria were age between 19 and 75 years and American Society of Anesthesiologists physical status I or II. The exclusion criteria were as follows: emergency surgery; body mass index < 18.5 kg m