Impact of Fluid Balance on Mortality Is Mediated by Fluid Accumulation Index in Sepsis: A Cohort Study

Fluid balance (FB) is associated with poor sepsis outcomes; however, it cannot accurately reflect the dynamic fluid accumulation status. Here, we explored a new index, the FB to fluid intake ratio (FB/FI), for evaluating dynamic fluid accumulation in sepsis. FB/FI values within 48 hours were recorded. Their association with in-hospital mortality was investigated using logistic regression and mediation analyses of data from 7,839 patients. In extended logistic models, a linear association was found between FB and mortality (odds ratio [OR]: 1.05-1.08, p < 0.001). However, this association became non-significant after the adjustment of FB/FI (OR: 1.00, 95% confidence interval [CI]: 0.98-1.02). For FB/FI and mortality, a cut-off value of 0.25 was defined. In the spline function logistic model, FB/FI > 0.25 was significantly associated with increased mortality (OR: 4.46, 95% CI: 2.92-6.80), whereas FB/FI ≤ 0.25 was not. For the FB/FI > 0.25 subgroup, mediation analysis was used to clarify the relationship between FB, FB/FI, and mortality. We observed that the direct effect of FB was non-significant (adjusted coefficient: -0.001, 95% CI: -0.005 to 0.002) while the indirect effect was significant (adjusted coefficient: 0.009, 95% CI: 0.006-0.011). In the FB/FI ≤ 0.25 subgroup, both the FB volume (0.9 ± 0.7 vs. -2.0 ± 1.9, p < 0.001) and the FB/FI ratio (0.14 ± 0.07 vs. -0.77 ± 1.60, p < 0.001) were significantly higher in patients with FB > 0 than those with FB ≤ 0. However, both the crude and adjusted comparisons of hospital mortality were non-significant. Similar associations were observed in septic shock patients. FB/FI > 0.25 is a significant risk factor for mortality in sepsis, while FB/FI ≤ 0.25 is not. The association between FB and mortality is completely mediated by this new fluid accumulation index. More comprehensive indices are required for evaluating dynamic fluid status in sepsis.