The purpose of this study is to determine the effect of fluid restriction and the neurohormonal mechanisms in the development of hyponatremia in patients with congestive heart failure and hyponatremia. The hypothesis is that strict fluid restriction leads to a larger increase in plasma sodium than standard treatment in patients with decompensated heart failure associated with hyponatremia. A secondary hypothesis is that the neurohormonal change is greater in patients treated with strict fluid restriction versus standard treatment.

Inclusion Criteria:

• Age > 18 years
• Left Ventricular Ejection Fraction (LVEF) < 40

At least two of the following signs of decompensated heart failure and fluid retention:

• Weight gain > 2 kg
• Pulmonal Congestion
• Jugular vein congestion
• Peripheral oedemas
• Hepatic congestion with ascites
• Radiographic signs of fluid retention
• Increased diuretic dose

And

• New York Heart Association (NYHA) class III-IV
• Plasma sodium < 135 mmol/L
• Symptomatic heart failure and treatment with relevant heart failure medications (beta-blocker, diuretic, digoxin, angiotensin-converting-enzyme inhibitor, angiotensin-II receptor antagonist, spironolactone, hydralazine and/or nitrates)for at least 1 month
• Hospitalization for decompensated heart failure within the last 48 hours
• Given informed consent

Exclusion Criteria:

• Plasma sodium ≥ 135 mmol/L before randomization
• Reduced kidney function (creatinine > 200 μmol/L)
• Severe hematologic disease
• Hypovolemic hyponatremia (volume depletion or dehydration)
• Intolerability to large or fast changes in fluid volume assessed by the investigator
• Plasma sodium < 120 mmol/L accompanied by neurologic symptoms
• Anuria
• Symptomatic systolic blood pressure (supine systolic blood pressure < 90 mmHg)
• Uncontrolled hypertension (systolic blood pressure > 180 mmHg)
• Uncontrolled diabetes diabetes mellitus
• Adrenal insufficiency
• Acute myocardial infarction, sustained ventricular tachycardia or ventricular fibrillation within the last 30 days
• Heart surgery within the last 60 days
• Other severe heart disease: hypertrophic cardiomyopathy, severe heart valve disease, cardiac amyloidosis, active myocarditis, pericardial exudate which is hemodynamically significant
• Left ventricular assist device (LVAD)
• Planned revascularization procedure, electrophysiologic device implantation, mechanic left ventricular assist device, heart transplant or any other heart surgery procedures within the next 30 days
• Cerebrovascular event within the last 6 months
• Comorbidity with an expected survival < 6 months
• Other reasons for hyponatremia: Primary syndrome of inappropriate antidiuretic hormone secretion (SIADH), primary polydipsia, head trauma, uncontrolled hypothyroidism, adrenal insufficiency or other known pharmacologically triggered hyponatremia which is reversible upon discontinuation of the drug, hyperglycemia (pseudohyponatremia), present abuse of alcohol
• Pregnancy
• Pregnant or fertile women who are not using safe contraception
• Dementia
• Unwilling or unable to give informed consent