Objective:To investigate the correlation between lung ultrasound score at the end of spontaneous breathing trial (SBT) and cumulative fluid balance at 48 h and their predictive value of extubation outcome in patients with intro-abdominal infection (IAI) under mechanical ventilation.Methods:A retrospective study was conducted. Patients with IAI under mechanical ventilation for more than 48 h were collected from three hospitals from October 1, 2017 to September 30, 2018. Routine demographic variables and clinical characteristics were recorded. The patients were divided into the successful extubation group and failed extubation group according to whether they could maintain spontaneous breathing for 48 h after extubation. LUS at the end of SBT (before extubation) and cumulative fluid balance at 48 h were compared between the two groups. LUS and cumulative fluid balance at 48 h were analyzed by bivariate correlation analysis, and their correlations with shallow fast breathing index (RSBI) and precursor protein of BNP (Pro-BNP) were analyzed. The predictive value of LUS at the end of SBT (before extubation) and cumulative fluid balance for extubation failure were analyzed by receiver operating characteristic curve (ROC).Results:Totally 207 patients were enrolled. There were significant differences in LUS before extubation [12 (10, 14) vs. 16 (14, 17) points], cumulative fluid balance [-318 (-1 116, 200) mL vs. 1 140 (685, 1 614) mL], RSBI [60 (55, 66) (times/min)/L vs. 70 (65, 78) (times/min)/L], pro-BNP [250 (122, 1 292) pg/mL vs. 1156 (285, 4 346) pg/mL], IAP >15 mmHg [32.8% vs. 46.6%], and ICU stay [8 (6, 12) days vs. 11 (8, 14) days] ( P<0.05). Logistic regression analysis showed that the COPD history, RSBI, LUS at the end of SBT, and cumulative fluid balance at 48 h before extubation were independent risk factors for extubation failure. Correlation analysis showed that LUS was moderately correlated with cumulative fluid balance at 48 h ( r=0.41, P<0.001), weakly correlated with RSBI ( r=0.381, P<0.001), and weakly correlated with pro-BNP ( r=0.220, P<0.001). Cumulative fluid balance at 48 h was weakly correlated with RSBI ( r=0.31, P<0.001), but not with pro-BNP. LUS at the end of SBT and cumulative liquid balance at 48 h had predictive value for extubation failure [AUC=0.87 (95% CI: 0.82-0.91), AUC=0.89 (95% CI: 0.85-0.94), P<0.001]. Conclusions:There is a moderate correlation between LUS at the end of SBT (before extubation) and 48 h cumulative fluid balance in patients with IAI and mechanical ventilation. LUS at the end of SBT (before extubation) and cumulative fluid balance at 48 h have some predictive value for extubation failure.