摘要Objective Fluid management in patients with septic shock and coexisting heart failure is a critical challenge,as it requires balancing resuscitation and the risk of fluid overload.This study investigated the potential of the fluid accumulation index(FAI),which is measured serially during the initial 72 h of intensive care unit(ICU)care,to provide dynamic prognostic information to guide fluid management in this high-risk population.Methods Restricted cubic spline(RCS)analysis was used to explore the relationships between FAI levels at different time points within 72 h of ICU admission and ICU mortality.Associations were quantified via multivariate Cox proportional hazards models.Subgroup analyses and Kaplan-Meier survival curves were used to evaluate the consistency of associations and differences in survival between groups.Results A total of 643 patients with septic shock and concurrent heart failure were included,among whom 127 died.The RCS revealed a significant nonlinear relationship between FAI levels at various time points and ICU mortality.The optimal FAI cutoff values decreased over time:the cumulative values were 0.87 at 24 h,0.59 at 48 h,and 0.56 at 72 h.The cutoff values for specific intervals were 0.27 for the 24-48 h period(2-24 h-FAI)and 0.12 for the 48-72 h period(3-24 h-FAI).In the fully adjusted model,FAI values exceeding these time-specific thresholds were significantly associated with increased ICU mortality(24 h-FAI＞0.87,HR=1.96,P=0.0251;2-24 h-FAI＞0.27,HR=2.07,P=0.0051;48 h-FAI＞0.59,HR=2.50,P=0.0005;3-24 h-FAI＞0.12,HR=2.05,P=0.0091;72 h-FAI＞0.56,HR=2.97,P＜0.0001).These associations remained consistent across most predefined subgroups.Conclusion FAI serves as a dynamic and independent prognostic marker for critically ill patients with septic shock and heart failure during the first 72 h of ICU admission.A key finding was the time-dependent decline in the optimal FAI cutoff values(0.87 at 24 h vs.0.12 for the 3-24 h period).This temporal decline supports a shift in fluid management strategy from an initial liberal approach toward a conservative strategy after the first 24 h,which may mitigate mortality risk.