Objective:To investigate the correlation between preoperative incomplete intestinal obstruction and the risk of perioperative reflux and aspiration assessed by preoperative bedside ultrasonography in patients with colon cancer.Methods:A prospective case series study was conducted. A total of 300 patients with colon cancer who underwent elective surgery from March 2020 to January 2022 in the First Hospital of Shanxi Medical University were prospectively selected for the study. According to whether there was incomplete intestinal obstruction before operation, the patients were divided into obstruction group and non-obstruction group. All patients underwent preoperative ultrasound examination of the gastric antrum, measuring the cross-sectional area of the gastric antrum in the right lateral position, and calculating the gastric volume and unit body mass gastric volume (GV/W), which were all expressed as M ( Q1, Q3). The differences in cross-sectional area of the gastric antrum, gastric volume and GV/W in the right lateral position between the two groups were compared, as well as the risk of reflux and aspiration (the risk of reflux and aspiration was extremely low at GV/W<0.8 ml/kg, the risk was low at GV/W 0.8-1.5 ml/kg, and the risk was high at GV/W >1.5 ml/kg). Multivariate logistic regression was used to analyze the influencing factors of high risk of reflux and aspiration. Results:Of 300 patients, 7 patients were excluded because of blurred images on ultrasonography, and a total of 293 patients were finally included. The age [ M ( Q1, Q3)] of obstruction group (146 cases) was 58.0 years old (55.0 years old, 67.0 years old), including 80 males and 66 females; the age of non-obstruction group (147 cases) was 55.0 years old (53.5 years old, 64.0 years old), including 64 males and 83 females. The age and body mass index (BMI) of the patients in the obstructin group were higher than those in the non-obstruction group, and the differences were statistically significant (both P < 0.05), and there were no significant differences in gender, duration of dietary abstinence, comorbid hypertension, comorbid diabetes mellitus, smoking status, sedentariness, and alcoholism between the two groups (all P > 0.05). Compared with the non-obstruction group, patients in the obstruction group had a higher cross-sectional area of the gastric antrum [5.83 cm 2 (5.25 cm 2, 6.70 cm 2) vs. 5.13 cm 2 (4.43 cm 2, 5.79 cm 2), P < 0.001], gastric volume [43.00 ml (37.07 ml, 52.74 ml) vs. 32.78 ml (25.52 ml, 39.85 ml), P < 0.001] and GV/W [0.70 ml/kg (0.65 ml/kg, 0.82 ml/kg) vs. 0.55 ml/kg (0.46 ml/kg, 0.62 ml/kg), P < 0.001] in the right lateral position. The proportion of patients at high risk of reflux and aspiration in the obstruction group was higher than that in the non-obstruction group [22.6% (33/146) vs. 12.2% (18/147), χ2 = 4.59, P = 0.032]. Multivariate logistic regression analysis showed that combination of incomplete intestinal obstruction (combined vs. uncombined, OR = 2.145, 95% CI: 1.096-4.198, P = 0.026), and males (females vs. males, OR = 0.415, 95% CI: 0.199-0.867, P = 0.019) were the independent risk factors for a high risk of perioperative reflux and aspiration assessed by preoperative bedside ultrasonography. Conclusions:Colon cancer patients combined with incomplete intestinal obstruction have significantly larger preoperative cross-sectional area of the gastric antrum, gastric volume and GV/W in the right lateral position, and have gastric retention, which may contribute to a higher risk of perioperative reflux and aspiration.