A Study to Assess the Efficacy, Safety and Pharmacokinetics of Intravenous Conivaptan (Vaprisol®) in Pediatric Subjects With Euvolemic or Hypervolemic Hyponatremia

Inclusion Criteria: * Subject is euvolemic or hypervolemic hyponatremia upon clinical presentation * Subject has serum sodium value ≥ 115 mEq/L (115 mmol/L) and \< 130 mEq/L (130 mmol/L) during the 24 hours preceding inclusion into the study * Female subject of childbearing potential must have a negative serum pregnancy test and must be premenarchal, surgically sterile or must practice a method of birth control Exclusion Criteria: * Female subject is pregnant or lactating * Subject has a body mass index (BMI) \< the 3rd percentile or \> the 97th percentile for their age and stature according to the World Health Organization; Body mass index-for-age percentiles charts for boys and girls ages 2 to 20 * Subject has clinical evidence of volume depletion, dehydration or hypovolemia * Subject with hypovolemic hyponatremia or transient causes of hyponatremia that are likely to resolve during the time of study participation * Subjects with a cause of hyponatremia that is most appropriately corrected by alternative therapies * Subject is expected to receive emergent treatment for hyponatremia during the treatment period of the study * Subject has clinical evidence of hypotension * Subject has uncontrolled hypertension \> the 99th percentile for their age * Subject has uncontrolled bradyarrhythmias or tachyarrhythmias requiring emergent pacemaker placement or treatment * Subject has untreated severe hypothyroidism, hyperthyroidism or adrenal insufficiency * Subject has known urinary outflow obstruction, unless subject is, or can be catheterized during the study * Subject has estimated creatinine clearance \< 30 mL/min during the seven days prior to study drug administration * Subject has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) elevations \> 3 times the upper limit of normal reference range during the seven days prior to study drug administration * Subject has serum albumin ≤ 1.5 g/dL during the seven days prior to study drug administration * Subject has white blood cell count (WBC) \< 3000/micro-liter documented any time during seven days prior to study drug administration or anticipated drop in WBC to \< 3000/micro-liter during the period of the study due to chemotherapy * Subject currently has unstable hepatic function or a history of hepatic encephalopathy, or bleeding esophageal varices within the last 3 months * Subject has acute heart failure. Prior history of heart failure is allowed if there are no current signs/symptoms * Subject has a non-fasting blood glucose value ≥ 275 mg/dL * Subject requires or is suspected to require treatment with potent inhibitors or potent inducers of CYP3A4 * Subject was administered hypertonic saline or oral salt supplement within 24 hours prior to study drug administration * Subject requires the use of medications used in the treatment of Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): including lithium salts, urea or demeclocycline during the week prior to screening and throughout the study drug treatment period * Subject has any condition that may interfere with treatment or evaluation of safety * Subject has received investigational therapy (including placebo) within 28 days or 5 half lives, whichever is longer