Urinary Chloride and Sodium Changes and Residual Congestion in Acute Heart Failure

Inclusion Criteria: * Provision of written informed consent prior to any study-related procedures; * Age ≥ 18 years; * Episode of AHF requiring hospital admission and treatment with intravenous furosemide; * New York Heart Association (NYHA) functional class II-IV; * NT-proBNP \>1000 pg/mL or BNP \>250 pg/mL, measured within a period not exceeding 24 hours prior to inclusion; * Transthoracic echocardiogram performed within the previous 24 months. All LVEF categories will be included: reduced LVEF (\<40%), mildly reduced LVEF (41-49%), and preserved LVEF (≥50%). In patients with preserved LVEF (HFpEF), congruent structural and/or functional echocardiographic abnormalities are required (left ventricular hypertrophy defined as septal or posterior wall thickness ≥11 mm, E/e' \>9, or left atrial volume \>32 mL/m²); * Signs of fluid overload, with at least two of the following: jugular venous distension (at least up to the sternocleidomastoid level, \~10 cm), lower limb edema, ascites, or pleural effusion confirmed by chest radiography or lung ultrasound * Treatment with oral furosemide at a dose of at least 40 mg/day within the previous month. Exclusion Criteria: * Symptomatic hyponatremia or plasma sodium level ≤125 mmol/L; * Hemoglobin \<9 g/dL; * Hypokalemia: serum potassium \<3 mEq/L; * Chronic kidney disease with estimated glomerular filtration rate (eGFR) \<15 mL/min/1.73 m²; * Hemodynamic instability at admission, defined as symptomatic hypotension; * Acute coronary syndrome, cardiogenic shock, or admission to the intensive care unit (ICU); * Severe infection (e.g., pneumonia, sepsis, leukocyte count ≥12,000/μL, C-reactive protein \>50 mg/L, or positive COVID-19 test); * Requirement for inotropic agents; * Life expectancy \<3 months or, in the investigator's judgment, inability to comply with study procedures.