Objective To explore the factors associated with delayed defecation in long-term ventilated patients in intensivecare unit (ICU) and their potential effect on prognosis.Methods A prospective observational cohort study was conducted. The patients admitted to general ICU of the Affiliated Hospital of Qingdao University from October 1st in 2013 to September 30th in 2015 who underwent mechanical ventilation (MV) for ≥6 days were enrolled, and they were divided into early defecation group (< 6 days) and late defecation group (≥6 days). At admission, clinical nutritional support were given as usual, and gender, age, acute physiology and chronic health evaluation Ⅱ (APACHEⅡ) score, admission reasons, MV reasons, the usage of morphine and epinephrine/norepinephrine, the highest positive end-expiratory pressure (PEEP), the lowest oxygenation index (PaO2/FiO2) and the lowest systolic blood pressure were collected. Logistic regression analysis was used to analyze the influencing factors of the first defecation time. ICU mortality, the length of ICU stay, central venous catheter (CVC) indwelling time, duration of MV, ICU acquired bacterial infection rate, and the incidence of ventilator associated pneumonia (VAP) were compared between early defecation group and late defecation group. Logisticorgan dysfunction system (LOD) scores and gastric residual volume per day were recorded at the different time of MV.Results Totally 189 patients were enrolled, 39 patients did not satisfied the inclusion criteria and 13 patients gave up treatment or referrals were excluded. Finally 137 patients were enrolled in the analysis, 83 patients in late defecation group and 54 in early defecation group. There were no significant differences in the baseline characteristics such as gender, age, APACHE Ⅱ score, LOD score at 1 day of MV, admission reasons, MV reasons, disgorging and gastric residual volume per day during the first 5 days of MV, enteral nutrition, lactulose treatment in patients with hepatic encephalopathy during the first 5 days of MV, and blood purification treatment between the two groups (allP > 0.05). Compared with the early defecation group, late defecation group had less patients with loose stools or watery stool at first time [15.7% (13/83) vs. 33.3% (18/54)], more patients using morphine and the usage of epinephrine/norepinephrine more than 24 hours [48.2% (40/83) vs. 40.7% (22/54), 42.2%(35/83) vs. 29.6% (16/54)], higher the maximum PEEP level [cmH2O (1 cmH2O = 0.098 kPa): 7.9±3.7 vs. 6.7±3.5], lower the minimal systolic blood pressure [mmHg (1 mmHg = 0.133 kPa): 74.8±28.1 vs. 88.9±30.2] and more severe of hypoxemia [PaO2/FiO2 < 150 mmHg, 54.2% (5/83) vs. 44.4% (24/53)], all of which had significant differences (allP < 0.05). Factors found statistical significances by single factor analysis were enrolled in the multiple regression analysis, which showed that PaO2/FiO2 < 150 mmHg and systolic blood pressure < 90 mmHg were independently associated with a delay in defecation in patients undergoing long-term MV [PaO2/FiO2 < 150 mmHg: adjusted hazard rate: 1.415, 95% confidence interval (95%CI) = 1.061-1.590,P = 0.026; systolic blood pressure 70-89 mmHg:HR = 1.461, 95%CI = 1.164-1.788, P = 0.002; systolic blood pressure ≤ 69 mmHg: adjusted hazard rate= 1.273, 95%CI = 1.010-1.587,P = 0.034). ICU mortality, the length of ICU stay, CVC indwelling time, duration of MV, ICU acquired bacterial infection rate, and the incidence of VAP at 7 days of MV in late defecation group were significantly higher than those of early defecation group [ICU mortality rate: 15.7% (13/83) vs. 7.4% (4/54), the length of ICU stay (day): 17.0 (14.0, 23.0) vs. 15.0 (13.8, 20.0), CVC indwelling time (days): 12.0 (10.0, 14.0) vs. 10.0 (9.0, 11.3), duration of MV (days): 14.0 (10.0, 20.0) vs. 11.0 (9.8, 15.3), ICU acquired bacterial infections rate: 60.2% (50/83) vs. 14.8% (8/54), the incidence of VAP: 32.5% (27/83) vs. 14.8% (8/54); allP < 0.05]. There was no significant difference in LOD score between both groups. The LOD scores at 4 days and 9 days of MV in late defecation group were significantly higher than those of early defecation group (6.41±4.37 vs. 5.21±3.12, 4.33±2.20 vs. 3.50±2.90, bothP < 0.01).Conclusions PaO2/FiO2 < 150 mmHg and systolic blood pressure < 90 mmHg during the first 5 days of MV were independently associated with a delay in defecation in patients undergoing long-term MV. The results suggest that constipation is associated with adverse outcomes in long-term ventilated patients.