Objective:To investigate the value of plasma syndecan-1 (SDC-1) combined with lung ultrasonography in evaluating the degree of extravascular lung water in patients with acute respiratory distress syndrome (ARDS).Methods:From July 2018 to July 2019, 50 patients with ARDS admitted to the department of intensive care unit of Wuxi People's Hospital Affiliated to Nanjing Medical University were enrolled. After admission, pulse indicator continuous cardiac output (PiCCO) catheter was established for all patients. PiCCO indexes, including extravascular lung water index (EVLWI) and pulmonary vascular permeability index (PVPI) were monitored by one doctor. Another doctor performed lung ultrasound examination, and calculated the sum of the number of B-lines under 10 ultrasound sections of upper blue point, lower blue point, diaphragm point, Plaps point and rear blue point of both lungs. Then the level of plasma SDC-1 was detected by enzyme linked immunosorbent assay (ELISA). Pearson correlation method was used to analyze the correlation between the number of ultrasonic B-lines, plasma SDC-1 level and EVLWI and PVPI. Taking 10 mL/kg EVLWI as the boundary value, the degree of pulmonary edema in patients with ARDS was divided into mild pulmonary edema and severe pulmonary edema. The receiver operator characteristic curve (ROC curve) was drawn, and the number of B-lines, SDC-1 and the predictive value of the combination of the above two indicators on the severity of pulmonary edema in patients with ARDS were analyzed.Results:The cardiac index (CI) and central venous pressure (CVP) of 50 patients with ARDS were (46.84±6.00) mL·s -1·m -2 and (8.12±1.80) mmHg (1 mmHg = 0.133 kPa), cardiogenic pulmonary edema was excluded. In 50 patients with ARDS, EVLWI was (10.82±2.92) mL/kg, PVPI was 3.02±0.69, the number of ultrasound B-lines was 40.90±13.05, and plasma SDC-1 was (568.25±118.14) μg/L. Pearson correlation analysis showed that the number of ultrasound B-lines in patients with ARDS was significantly positively correlated with EVLWI and PVPI ( r1 = 0.802, r2 = 0.799, both P < 0.01). Plasma SDC-1 was also positively correlated with EVLWI and PVPI ( r1 = 0.732, r2 = 0.576, both P < 0.01). ROC curve analysis showed that the number of B-lines and SDC-1 had good predictive value for the severity of pulmonary edema in patients with ARDS. The area under ROC curve (AUC) and 95% confidence interval (95% CI) were 0.891 (0.803-0.979) and 0.875 (0.772-0.978), respectively. When the cut-off of B-lines was 40.50, the sensitivity and specificity were 82.1% and 86.4%, respectively. When the cut-off of SDC-1 was 559.37 μg/L, the sensitivity and specificity were 85.7% and 81.8%, respectively. Combining the number of B-lines with SDC-1 could further improve the predictive value of pulmonary water in patients with ARDS. The AUC (95% CI) was 0.958 (0.890-1.000), and the sensitivity and specificity were 92.9% and 91.8%, respectively. Conclusions:The level of plasma SDC-1 and the number of pulmonary ultrasonic B-lines have a good correlation with the degree of extravascular lung water in patients with ARDS. The combined application of the two noninvasive indexes can be used to evaluate the degree of extravascular lung water in patients with ARDS.