The goal of this observational study is to learn how changes in urinary sodium and chloride levels relate to fluid overload and short-term outcomes in patients hospitalized with acute heart failure (AHF). The main questions it aims to answer are:

• Do changes over time in urinary sodium and chloride reflect how well excess fluid is being removed during hospitalization?
• Are these changes associated with residual congestion at discharge and with the risk of worsening heart failure or death after discharge?

Participants hospitalized for AHF and treated with intravenous diuretics as part of their usual care will have clinical assessments, blood and urine tests, and echocardiographic evaluations collected at several time points during their hospital stay. Researchers will also record clinical outcomes, including worsening heart failure or death, at 30 days and 3 months after discharge.

This is a prospective, multicenter observational study. A minimum required sample size of n = 223 patients completing the study is planned. This study will include patients hospitalized for AHF and treated with intravenous diuretics. Clinical, biochemical, echocardiographic parameters, and circulating biomarkers (NT-proBNP, CA125, sST2, bioADM, and CD146) will be collected serially during hospitalization. Additionally, clinical events (worsening heart failure and/or all-cause mortality) will be recorded at 30 days and 3 months after discharge. Decongestion will be assessed using a multiparametric approach, incorporating clinical evaluation, echographic parameters, and estimated plasma volume status.

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.

Adult patients of both sexes hospitalized for acute heart failure (AHF) in Internal Medicine and Cardiology departments at Spanish hospitals, presenting with signs of fluid overload and receiving intravenous (IV) diuretic therapy. The clinical diagnosis of heart failure will be established based on the presence of typical signs and symptoms, evidence of underlying structural heart disease on transthoracic echocardiography performed within the previous 24 months, and elevated natriuretic peptides measured at least 24 hours prior to inclusion. Patients across the full spectrum of left ventricular ejection fraction (LVEF) will be included.