Fluid balance trajectories and outcomes in acute heart failure complicated by acute kidney injury: Insights from the MIMIC-IV database

Background: Acute heart failure (AHF) complicated by acute kidney injury (AKI) is associated with poor outcomes. Fluid management is central to treatment but remains challenging due to the bidirectional heart–kidney interaction. While most studies focus on static measures of fluid status, the prognostic value of dynamic fluid balance trajectories has not been fully elucidated. Methods: We performed a retrospective cohort study using the Medical Information Mart for Intensive Care IV (MIMIC-IV) database. Patients with AHF and AKI were identified, and their daily fluid balance during the first week of ICU admission was modeled using the elbow method and silhouette coefficient. Outcomes were assessed using Kaplan–Meier survival curves and Cox proportional hazards regression. Results: A total of 817 patients were included (median age 71.8 years; 52.1% male). Three distinct fluid balance trajectories were identified: sustained negative balance (SNB, 55.7%), positive-to-negative balance (SPNB, 40.6%), and sustained positive balance (SPB, 3.7%). The SPB group had significantly higher 28-day and 365-day mortality compared with the SNB and SPNB groups (30.0% vs. 15.0%; 53.3% vs. 35.1%; both P < 0.05). In multivariate analysis, the SNB group showed a 36% lower risk of death compared with the SPB group (HR 0.64, 95% CI 0.44–0.94; P = 0.021). Independent predictors of mortality included AKI stage 3 (HR 1.55; P = 0.003), lower body weight, hyponatremia, and elevated blood urea nitrogen. Patients with AKI stage 3 had a 28-day mortality of 26.8%, significantly higher than those with stage 1–2 AKI (P < 0.001). Conclusions: Dynamic fluid balance trajectories are strongly associated with outcomes in patients with AHF and AKI. Sustained positive balance confers the highest risk, whereas achieving or maintaining negative balance is linked with improved survival. These findings highlight the potential role of trajectory-based fluid monitoring to guide individualized fluid management.