Effects on Exercise Hemodynamics of Vasopressin Blockade by Conivaptan Infusion in Heart Failure Patients

Inclusion Criteria: * Age \> 18 years * Left ventricular ejection fraction (LVEF) \< 45 % on the baseline echocardiography. * Treatment with beta-blockers and angiotensin-converting-enzyme (ACE) inhibitors for at least 1 month as tolerated * New York Heart Association (NYHA) Functional Class II-III * Given informed consent * Women, who have not yet entered menopause (defined as no menstrual bleeding in the last 12 months), will provide a negative urine HCG before entering the study Exclusion Criteria: * Signs of symptomatic or ongoing myocardial ischemia * Known non-revascularized coronary disease * Presence of hypovolemic hyponatremia (P-Na+ \<130 mmol/l and clinical signs of volume depletion or dehydration as judged by investigator). * Hypernatraemia (P-Na+) \> 145 mmol/L * Chronic obstructive pulmonary disease (FEV1/FVC \< 70 % and/or 30 % \> FEV1 \< 50 %) * Supine systolic blood pressure \< 85 mmHg * Significant orthostatic hypotension * Standing blood pressure \< 80 mmHg or a blood pressure drop \> 20 mmHg when changing from a supine to a standing position * Uncontrolled hypertension evaluated by the investigator * Uncontrolled cardiac arrhythmias evaluated by the investigator * Untreated serious hypothyroidism * Adrenal insufficiency * Poor echocardiographic window * Inability to perform exercise testing * Permanent atrial fibrillation or atrial fluttering * Planned coronary by-pass surgery * Moderate hepatic impairment (ALAT/ASAT \> 3 UNL) * Presence of other diseases affecting treatment with conivaptan or the evaluation of safety as evaluated by the investigator * Severely decreased kidney function (eGFR \< 20 mL/min) * Serum K+\< 3.5 or \> 5.5 mmol/L * Known conivaptan intolerability * Corn allergy * Dextrose Allergy * Treatment with potent CYP3A-inhibitors (ketoconazole, itraconazole, clarithromycin, ritonavir, indinavir) * Treatment with arginine vasopressin, oxytocin, desmopressin and other medications for the treatment of hyponatremia (lithium salts, urea and demeclocycline) * Warfarin treatment * Presence of infection or active bleeding