Fluid intake-related association between urine output and mortality in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS), a complex response to various insults, has a high mortality rate. As pulmonary edema resulting from increased vascular permeability is a hallmark of ARDS, management of the fluid status, including the urine output (UO) and fluid intake (FI), is essential. However, the relationships between UO, FI, and mortality in ARDS remain unclear. This retrospective study aimed to investigate the interactive associations among UO, FI, and mortality in ARDS.

This was a secondary analysis of a prospective randomized controlled trial performed at 10 centers within the ARDS Network of the National Heart, Lung, and Blood Institute research network. The total UO and FI volumes within the 24-h period preceding the trial, the UO to FI ratio (UO/FI), demographic data, biochemical measurements, and other variables from 835 patients with ARDS, 539 survivors, and 296 non-survivors, were analyzed. The associations among UO, FI, the UO/FI, and mortality were assessed using a multivariable logistic regression.

In all 835 patients, an increased UO was significantly associated with decreased mortality when used as a continuous variable (odds ratio [OR]: 0.98, 95% confidence interval [CI]: 0.98-0.99, P = 0.002) and as a quartile variable (OR of Q2 to Q4: 0.69-0.46, with Q1 as reference). To explore the interaction between UO and FI, the UO/FI was calculated, and a cut-off value of 0.5 was detected for the association with mortality. For patients with a UO/FI ≤0.5, an increased UO/FI was significantly associated with decreased mortality (OR: 0.09, 95% CI: 0.03-0.253, P <  0.001); this association was not significant for patients with UO/FI ratios > 0.5 (OR: 1.04, 95% CI: 0.96-1.14, P = 0.281). A significant interaction was observed between UO and the UO/FI. The association between UO and mortality was significant in the subgroup with a UO/FI ≤0.5 (OR: 0.97, 95% CI: 0.96-0.99, P = 0.006), but not in the subgroup with a UO/FI > 0.5.

The association between UO and mortality was mediated by the UO/FI status, as only patients with low UO/FI ratios benefitted from a higher UO.