Supplementary Material for: Excess Conventional Ultrafiltration Volume Increases Risk for Postoperative Cardiac Surgery-Associated Acute Kidney Injury

Introduction
Conventional ultrafiltration (CUF) is employed during cardiopulmonary bypass (CPB) to remove excess fluid and concentrate specific blood components. The study aimed to investigate the impact of CUF volume on postoperative cardiac surgery-associated acute kidney injury (CSA-AKI).
Methods
This single-center, retrospective study included adult patients undergoing on-pump cardiac surgery between 2018 and 2023. Patients were classified into four categories for comparative analysis: the none-CUF group, and three groups according to tertiles of the weight-adjusted CUF volume (tertile 1, 0.1-15.2 ml/Kg; tertile 2, 15.3-25.0 ml/Kg; tertile 3, >25.0 ml/Kg). The primary outcome was postoperative CSA-AKI according to the Kidney Disease Improving Global Outcomes (KDIGO) criteria, and secondary outcomes included perioperative blood transfusion, pulmonary complications, chest drainage volume, urine output, hospital and ICU lengths of stay (LOS), and in-hospital mortality. The association between weight-adjusted CUF volume and patient outcomes was assessed by multivariable logistic regression model.
Results
A total of 22403 patients were included and the incidence of CSA-AKI was 28.1%. Weight-adjusted CUF volume (per 10 ml/kg increase) was independently associated with higher risk of any-stage CSA-AKI (adjusted odds ratio [aOR], 1.25; 95% CI, 1.14-1.38; P < 0.001) and stage 2/3 CSA-AKI (aOR, 1.42; 95% CI, 1.22-1.66; P < 0.001). The restricted cubic splines model illustrated a non-linear relationship between weight-adjusted CUF volume and any-stage CSA-AKI (P for nonlinearity 0.025), while a J-shaped relationship for stage 2/3 CSA-AKI (P for nonlinearity < 0.001). A higher CUF volume was associated with increased odds of prolonged mechanical ventilation (aOR, 1.38; 95% CI, 1.18-1.64; P < 0.001) and perioperative red blood cells (RBCs) transfusion (aOR, 1.13; 95% CI, 1.03-1.25; P = 0.011).
Conclusion
Excess weight-adjusted CUF volume was significantly associated with increasing incidence of CSA-AKI and prolonged mechanical ventilation, but did not reduce the requirement for perioperative RBCs transfusion. These findings highlight the cautious application of CUF in fluid management during cardiac surgery.