[Association between early fluid balance and outcomes in patients receiving continuous renal replacement therapy for cardiac surgery-associated acute kidney injury].

To evaluate the association between fluid balance within 48 hours after initiating continuous renal replacement therapy (CRRT) and 90-day mortality of patients with cardiac surgery-associated acute kidney injury (CSA-AKI). Adult patients who received CRRT for CSA-AKI for more than 24 hours between January 2016 and December 2021 in the First Affiliated Hospital of Nanjing Medical University were prospectively included. Exposures were fluid input, fluid output, fluid balance and percentage of fluid overload change (defined as weight-standardized fluid input/output difference) within 48 hours after CRRT initiation. The primary outcome was 90-day mortality. The relationship between the exposures and 90-day mortality was determined by restricted cubic spline and segmented Cox regression. The study enrolled 262 patients, and 90-day mortality was 60.3% (158/262). There were 171 males and 91 females, with a median age of 64 (54, 71) years. The dead patients (=158) had more fluid input [6.43 (5.62, 7.34) L vs 6.07 (5.09, 6.90) L, 0.013], greater fluid balance [-0.01 (-1.36, 1.34) L vs -0.65 (-1.71, 0.42) L, =0.005] and higher percentage of fluid overload change [-0.01% (-2.17%, 2.01%) vs -0.85% (-2.49%, 0.52%), 0.013] within 48 hours after CRRT initiation than the survived patients (=104). The cumulative fluid balance and fluid overload changes have a "J" curve relationship with 90-day mortality. Higher positive fluid balance (adjusted per 1 L increase above 0.5 L=1.33, 95%: 1.17-1.50) and greater proportion of fluid overload changes (adjusted per 1% increase above 0.7%=1.20, 90%: 1.11-1.30) were associated with an increased risk of death. Cumulative fluid input was linearly related to the 90-day mortality. Higher input (adjusted per 1 L increase=1.13, 95%: 1.03-1.24) was associated with an increased risk of death. The cumulative fluid output showed a U-shaped relationship with risk of death. Both lower output (adjusted per 1 L increase below 6.7 L=0.73, 95%: 0.58-0.90) and higher output (adjusted per 1 L increase above 6.7 L=1.24, 95%: 1.01-1.53) were associated with increased risk of death. In patients with CSA-AKI, excessive fluid input, insufficient or excessive output and a positive fluid balance or percentage of fluid overload change within the first 48 hours after CRRT initiation are associated with increased risk of 90-day mortality.