Pragmatic, multicenter, 2x2 factorial randomized study. Severe patients admitted to the ICU at moderate to high risk for death or acute kidney injury will be randomly allocated to receive a balanced crystalloid solution (Plasma-Lyte®) or 0.9% saline and to receive crystalloids by rapid bolus infusion (999 mL/h) or slow infusion (333 mL/h) whenever plasma expansion is needed.