Fluid and Electrolyte Balance — Surgical

A 52-year-old man undergoes elective open cholecystectomy for symptomatic cholelithiasis. Intraoperatively, 800 mL of blood is lost and replaced with 2 L of normal saline. On postoperative day 2, he develops tachycardia (HR 110/min), hypotension (BP 95/58 mmHg), and oliguria (urine output 180 mL in 24 hours). Serum sodium is 128 mEq/L, serum osmolality 265 mOsm/kg, and urine osmolality 580 mOsm/kg. What is the primary mechanism underlying this patient's hyponatremia?

A 68-year-old woman with a history of chronic kidney disease (baseline creatinine 2.2 mg/dL) undergoes emergency repair of a perforated duodenal ulcer. Intraoperatively, she receives 3 L of normal saline. On postoperative day 1, her serum potassium is 6.8 mEq/L (normal 3.5–5.0), and an ECG shows peaked T waves and prolonged PR interval. Her urine output is 150 mL in 24 hours. What is the most appropriate initial management?

A 58-year-old man undergoes elective open cholecystectomy for symptomatic cholelithiasis. Intraoperatively, 800 mL of blood is lost. He receives 2 L of normal saline (0.9% NaCl) and 1 unit of packed RBCs. On postoperative day 2, he develops abdominal distension, weakness, and confusion. Serum sodium is 128 mEq/L (normal 135–145), serum osmolality is 260 mOsm/kg (normal 280–295), and urine osmolality is 580 mOsm/kg. Urine sodium is 45 mEq/L. What is the most appropriate next step in management?

A 72-year-old woman is admitted with a 3-day history of vomiting and abdominal pain. She has a history of gastric outlet obstruction from peptic ulcer disease. On examination, she is lethargic, with dry mucous membranes and poor skin turgor. Vital signs: HR 110/min, BP 95/60 mmHg. Laboratory findings: Na⁺ 132 mEq/L, K⁺ 2.8 mEq/L, Cl⁻ 88 mEq/L, HCO₃⁻ 38 mEq/L, pH 7.52. Urine chloride is 5 mEq/L. What is the primary acid–base disturbance and the most appropriate initial fluid for resuscitation?

A 58-year-old man undergoes elective open cholecystectomy for chronic cholecystitis. Intraoperatively, 800 mL of blood is lost and replaced with 2 L of normal saline. Postoperatively on day 2, he develops respiratory distress, bilateral crackles on auscultation, and chest X-ray shows bilateral pulmonary infiltrates. Serum sodium is 128 mEq/L, potassium 3.8 mEq/L, and albumin 2.1 g/dL. What is the primary mechanism underlying his pulmonary edema?