Fig. 1

Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) flow diagram: schedule of enrollment, interventions, and assessments. *: mean blood pressure, heart rate, and pulse oximetry, †: examination of the presence of blood on the tube surface or in the oral cavity; T1: before intubation, T2: intubation, T3: emergence period, T4: at post-anesthesia care unit, T5: postoperative 2 h, T6 : postoperative 4 h, T7: postoperative 24 h