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Objectives: Although hyponatremia is a prognostic factor in acute heart failure (AHF), its influence on the acute clinical course of heart failure is unknown. Our objective was to evaluate the association of hyponatremia with diuretic response, renal function, and clinical outcomes in AHF. Methods: A retrospective study included 499 hospitalized AHF patients treated with intravenous loop diuretics for ≥48 h. Patients were grouped by nadir sodium concentrations (normonatremic, NN) ≥135 mEq/l, (mild hyponatremia, MHN) 130-134 mEq/l, and (moderate to severe hyponatremia, MSHN) <130 mEq/l. Association to diuretic response and clinical outcome was assessed. Results: The incidence of hyponatremia was 54% (36% MHN, 18% MSHN). Maximum diuretic dose (furosemide equivalents: NN 84 ± 132 mg/day vs. MHN 114 ± 165 mg/day vs. MSHN 249 ± 450 mg/day, p < 0.001) and incidence …