Statistical analysis
The sample size was calculated using PASS software (NCSS, version 2021). In a previous study [Gastric antral area changes on ultrasonography and Levin tube suction amount were evaluated with the Mann-Whitney U test, and surgical grade determined by the surgeon’s judgement was evaluated using the chi-square test.Demographic data, perioperative data, and clinical outcomes between the two groups were examined with the chi-square test for categorical variables and an independent-samples t-test or Mann-Whitney U test for continuous variables. Vital sign changes during the study were analyzed by repeated measures general linear model, and Spearman correlation analysis was performed to evaluate gastric antral CSA changes on ultrasonography, surgeon’s view grade, and Levin tube suction amount. Data were analyzed using SPSS (Version 24; IBM, Chicago, IL, USA). Two-sided alpha of 0.05 was used for all statistical tests.