Case Scenario: Roqaiya, a 61-year-old woman with no known drug allergies is hospitalized with a chief complaint of increasing shortness of breath (SOB) and orthopnea over the past week. She has been treated previously for heart failure and has not taken any medication over the past 2 weeks. Roqaiya has severe (4+) pedal edema and is in respiratory distress. Laboratory tests were ordered and reported back as follows: • sodium (Na), 123 mEq/L (normal, 135-145) • potassium (K), 4.1 mEq/L (normal, 3.5-5.0) • chloride (Cl), 90 mEq/L (normal, 95-105) • carbon dioxide (CO2), 28 mEq/L (normal, 22 to 28) • blood urea nitrogen (BUN), 20 mg/dL (normal, 8-18) • serum creatinine (SCr), 1.3 mg/dL (normal, 0.6-1.2) • fasting glucose, 100 mg/dL (normal, 70-110). SI units: Na, 123 mmol/L; K, 4.1 mmol/L; Cl, 90 mmol/L; CO2, 28 mmol/L; BUN, 10.7 mmol/L; SCr, 115 µmol/L; glucose, 5.5 mmol/L]
Question: Why should Roqaiya not be given sodium chloride to return her serum sodium concentration to a normal value? Why is the BUN abnormal for Roqaiya?